(5) Rocco Iafigliola

Transcription

1 BETA DECAY ENERGIES AND STRENGTH FUNCTION DETERMINATIONS OF NEUTRON RICH ISOTOPES IN THE A = REGION by 5) Rocco Iafigliola A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy Foster Radiation Laboratory McGill University, Montreal Quebec, Canada c November, 1985

2 To my wife Maria and To my children Josie and Dominic

3 -i- ABSTRACT A solid-state DE-E telescope beta spectrometer has been built and used to study beta-minus spectrr. The system c nsists of a 15 mm x 500 E-detector and a 300 ~m x 200 special "keyhole" design. 2 mm hyperpure germanium HPGe) 2 mm Si DE-detettor with a The response function of the system ha~ been determined experimentally using mono-energetic electron brams with incident energies ranging from 1 to 12 MeV. The beta end point energies of 25 nuclei, nam~ly, 32p, 88 Rb 9l-99 Rb 9l-96 Sr 99-l00 Sr, 92-96y and 100y h~ve been measured. The Q-beta values and mass excesses for A ';' have been determined. The results for 99Rb, 99-l00Sr and looy have been obtained for the first time. The beta strength functions for odd mass Rb isotopes, namely Rb have been measured for the first time from direct beta spectroscopy with a solid-state telescope system. The experimental results have been compared to calculations using the Brown-Bolsterli B-B) model.

4 ., r. -ii- q RESUMÉ Un spectrom~tre b~ta constitué d'un télescope d0 jonctions DE-E a été construit et utilisé pour l'êtude dps, spectres b~ta-moins. Le système consiste en une Joncll:>n l "E" au Germanium de grande pureté HPGe) de 15 mm x ')00 ltilti~ ') et une jonction "DE" au Silicium de 300 ~ m x 200 mm- et cl' lin dessin en "trou de serrure" spécialement étuclié. La fonction de réponse de ce système a été détermin&e expérimentalement à l'aide d'électrons monocinétiques dont l'énergie variait entre 1 et 12 MeV. Les énergies maximum de désintégration bêta ont &té mesurées pour 25 noyeuse Sr, 92-96y et 100y Les valeurs de Q et d'excès de masse ont été déterminées pour A = C'est la première fois que des résultats sur sont obtenus. La fonction de force bêta pour les isotopes du Rb de masse impaire 91-99Rb ) a pour la première fois été measurée directement par spectroscopy b~ta à l'aide d'un télescope de jonction semi-conductrices. Les résultats expérimentaux ont été comparés aux predictions du modèl Brown-Bolsterli model B-B)....

5 -ii1- ACKNOWLEDGEMENTS l would like to begln by thanking Dr. J.K.P. Lee, my thesis supervisor and director of the Foster Radiation Laboratory. Without hls constant guidance and support this research project would never have been completed. l owe my greatest debt ta Dr. H. Dautet. He was responsible for the design and construction of the beta spectrometer and was involved in the planning and running of the experiments. Special thanks are due to Dr. S.W. Xu who developed the computer programs for the theoretical beta strength function calculations and for his he!p in running sorne of the experiments. l would also like to thank Dr. M. Chatterjee, Dr. R.C. Shang, Dr. V. Raut, S. Burgida, N. Campeau, and R. Turcotte for assisting me in performing some of the experiments. Dr. K.S. Kuchela and C. Jorgensen have maintained the computer systems in excellent working conditions and for this they are sincerely thanked. l am grateful to Dr. L. Lessard, director of the Laboratoire de Physique Nucléaire at the Université de Montréal. Dr. H. Jeremie, G. Beaudoin and ail the technical staff there. The use of their mono-energetic beta spectrometer facility made it possible to ob tain experimental response functions.

6 , L~_ -ivl am grateful to Dr. R.E. Cnrien for provldlng the bedm time at TRISTAN Brookhaven Natlonal Laboratory). l would also like to thank Dr. R. Gill, Dr. M. Schmid and the technical staff for their assistance in the expvrlments conducted there. The remainlng professiona] and technical staff of lhl' Fos ter Rad i a t ion La b 0 rat 0 r y i sai sot 0 b eth an k e d for t Il l' 1 r help and encouragement durlng the course of this work. Special thanks are due to Dr. K. Oxorn who proofread the final text of this the&is and to M. Bodnar fur preparlng the numerous diagrams. The financial support of a Carl Reinhardt fellowship has been greatly appreciated. On a more personal basis, 1 would like to thank my family for their encouragement and understandlng. My children, Josie and Dominic, have made the many years spent on this project that much more bearable. My wlfe, MarIa, has constantly supported me in my work and it would not have been possible without her help. Her strongest show of support has been in her quick mastery of the mlcrocomputer and typing the many drafts of this thesis. Finally, l would like to pay tribute to the lale Dr. J.E. Kitching whose work has been an inspiration to us a11 r... at the Foster Radiation Laboratory. forever with us. His memory will remaln

7 -v- { TABLE OF CONTENTS Page ABSTRACT RESUMÉ ACKNOWLEDGEMENTS LIST OF FIGURES LIST OF TABLES i ii iii viii xi CHAPTER l - INTRODUCTION 1 CHAPTER 2 - THE BETA SPECTROMETER 2.1 The Magnetic Beta Spectrometer 2.2 Energy Deposition Detectors 2.3 Gamma-ray Suppression 2.4 Description of the DE-E Telescope System 2.5 The DE-E Electronic System 2.6 Gain-Matching and Energy Calibrations 2.7 System Energy Resolution and Window Loss 2.8 Detector Dead Layer Loss CHAPTER 3 - SOURCE PRODUCTION FACILITIES 3.1 The McGill ISOL Facility 3.2 The Tristan Facility CHAPTER 4 - THE DE-E SYSTEM RESPONSE FUNCTION 4.1 The Nature of the Response Function 4.2 Methods of Determining the Response Function 4.3 The University of Montreal Facility Paramptrization of the Response Function Comparison with Monte Carlo Calculations CHAPTER 5 - METHODS OF ANALYSIS 5.1 The Theoretical Spectrum 5.2 The Beta Analysis Program 5.3 Application of the Response Function 5.4 Parabolic Interpolation

8 -vi Il Fermi-Kurie Plots Background EstImation 4) The Program PILUP 0& Experimental Determination of Secondary Actlvities Test Spectra - Endpoint Energy DetermInatIons 101 Determination of the Total Uncertainty Sample Spectra 88 Rh 32 p 94 y 92 Rb Test Spectra - 88 Rb 95 Rb Beta Intensity DistrIbutions Organization of the Experimental Results 4S \12 \14 Il h Il H 1 2.: 127 CHAPTER Preamble Rb Sr Rb Sr y Rb Sr y Rb Sr y Rb Sr y Rb Sr y Rb EXPERIMENTAL RESULTS AND DISCUSSIONS Qfj MEASUREMENTS 131 l ' R '55 l'56 11) l 7 )

9 -vii Rb Rb Sr Sr y 6.25 General Comments on Q-beta Results 6.26 Mass Excesses and Comparison with Direct Mass Measurements 6.27 Comparlson with Theoretical Mass Calculations 6.28 S2n) Values CHAPTER 7 - EXPERIMENTAL RESULTS AND DISCUSSIONS BETA STRENGTH FUNCTION DETERMINATIONS Preamble Method of Analysis 91 Rb Rb Rb Rb Rb Theoretical Calculations Comparison of Experimental and Theoretical Results Half-life Calculations General Comments on Determination of S-beta 253 CHAPTER 8 - SUMMARY AND CONCLUSIONS 255 REFERENCES 260 APPENDIX 1: BE TA STRENGTH FUNCTION CALCULATIONS APPENDIX II: REPRINT - Precise Q-beta Measurements for A = 91 to 93 Mass Chains APPENDIX III: REPRINT - AMCO 7 - Beta Decay Energies of 98,99 Rb, 99,100 ;:,r c an d 100 y APPE~DIX IV: LIST OF PUBLICATIONS AND CONFERENCE PRESENTATIONS

10 -viii- LIST OF FrGURES Figure Descflption Page Cutaway view of the telescope spectrometer Photograph of the DE-E telescope Schematic view of the DE-E telescope Schematlc of the electronics system 207 Bi Spectra HAVAR window absorption curve ' Layout of the McGill rsol facility Layout of the Tristan ISOL facility Photograph of run at BNL University of Montreal mono-energetic electron facility Electron response functions 1, 2, 3 t 4 MeV) Electron response fùnctions 5, 6, 7, 8 MeV) Electron response functions 9, 10, ll, 12 MeV) Sketch of response function Fits of tails of response functl.ons 1, 2 MeV) Fits of taiis of response functions 3, 4 MeV) Fits of tails of response functions 5, 6 MeV) Fits of tal.ls of response functions 7, 8 MeV) Fits of tails of response functlons 9, 10 MeV) Fits of tails of response functions 11, Parameter H and fit Parameters A, B and flts Parameters C, ms, ss and fits Parameters sr, mr and fits Comparison of Monte-Carlo and experimental response functl.ons at 2 and 8 MeV 12 MeV) Rb beta spectrum - no background subtraction 105

11 r -ix Rb beta spectrum - no background subtraction 88 Rb data - showing effect of background subtractlon and pileup corrections 88 Square root plot of Rb data and fit BNL) 88Rb Fermi-Kurie ~lot BNL) 88Rb Fermi-Kurie plot McGill) 32p Fermi-Kurie plot 94y Fermi-Kurie plot 92 Rb Fermi-Kurie plot 88Rb multibranch fit 95 Rb intensity distribution 95 Rb multibranch fit Rb Fermi-Kurie plot 91 Sr Fermi-Kurie plot 92y Fermi-Kurie plot and 93 Rb Fermi-Kurie plot 93Sr Fermi-Kurie plot 93 y Fermi Kurie plot 94Rb Fermi Kurie plot 94Sr Fermi-Kurie plot 95Rb Fermi-Kurie plot 95Sr Fermi-Kurie plot 95y Fermi-Kurie plot 96Rb Fermi-Kurie plot 96Sr Fermi-Kurie plot 96 y Fermi-Kurie plot 97 Rb Fermi-Kurie plot 98Rb Fermi-Kurie plot 99Rb Fermi-Kurie plot 99Sr Fermi-Kurie plot 100Sr Fermi-Kurie plot 100y Fermi-Kurie plot 92Sr square root plot 6.21a) Plot of differences between Rb mass excesses from this work and those from direct mass

12 , l '~,II'.. -xmeasurements : b) Plot of S2n) values Rb multibranch fit a) 91 Rb beta cranch fractions b) 91 Rb beta strength function this work) c) 91 Rb beta strength function reported) Rb multibranch fit a) 93 Rb beta branch fractions 2' b) Rb beta strength function this work) c) 93 Rb beta strength function reported) a) Rb beta branch fractions 2~n 7.5b) 95 Rb beta strength function this work) c) 95 Rb oeta strength funct10n reported) 23J Rb multibranch fit a) 97 Rb beta branch fractions b) 97 Rb beta strength function this work) c) 97 Rb beta strength function reported) Rb multibranch fit a) 99 Rb beta branch fractions b) 99 Rb beta strength function this work) a) 91 Rb theoretical beta strength functionthis work) b) 91 Rb theoretical beta strength functionreported) a) 93 Rb theoretical beta strength functionthis work) b) 93 Rb theoretical beta strength functionreported) c) 93 Rb theoretical beta strength functionreported) a) 95 Rb theoretical beta strength functionthis work) b) 95 Rb theoretical beta strength functionreported) c) 95 Rb theoretical beta strength functionreported) a) 97 Rb theoreti.cal beta strength functionthis work) b) 97 Rb theoretical beta strength functionreported) c) 97 Rb theoretical beta strength functionreported) a) 99 Rb theoretical beta strength functionthis work) b) 99 Rb theoretical beta strength functionreporteè) 2:> Half-lives of odd-a Rb isotopes 252 ~ i r 1 L

13 -xi- LIST OF TABLES Table Description Page 2. 1 Gamma-ray calibration sources Response function electron energies, magnetic field, total electron and background spectrum counts and accumulated charge 4.2 Polynomial coefficients for fits to response function parameters H, A, Band C 4.3 Polynomial coefficients for fits to response function parameters ms, ss, mr and sr Rb beta branch feeding intensities Rb Q-beta values and mass excesses Sr Q-beta values and mass excesses y Q-beta values and mass ex cesses Rb experimental and predicted mass excesses Sr experimental and predicted mass excesses y experimental and predicted mass excesses

14 -1- CHAPTER 1 INTRODUCTION The mystery of the continuous beta spectrum was solved fifty-five years ago with the introduction of the elusive neutrino by Wolfgang Pauli see Wu, 1966). Only a few years later, Enrico Fermi 1934) developed his famous theory of beta-decay which predicted with remarkable accuracy the shape of beta spectra, the energy of disintegration and the rate of beta-decay. These early revelations of beta-decay were understandably surprising. Yet no less surpris~ng or significant were the discoveries that ensued, unravelling even more mysteries of the nucleus camouflaged by nature in the beta-decay process. In fact beta-decay remains to date one of the most fruitful sources of information on both the weak and nuclear interactions. The acquiring and subsequent analysis of beta spectra is commonly referred to as beta spectroscopy. The present work i5 concerned with the measurement of beta end point energies and beta branch intensities. Beta endpoint energies enable the determination of total beta-decay energies Q-beta values) which are a means of obtaining nuclear masses. Beta strength functions can be obtalned if beta branch intensities are known. These determinations have played an important raie in the study of nuclear phenomena. In the region close ta the line of stability,

15 -2- the se studies are more or less complete. However, the region far from beta stability is still largely unknown. As a result this region has been the foeus of attention for some time now. Importance of Mass Measurements Nuclear mass measurements have played a major raie in nuclear phys1cs research. They provide information that is essential in nucl~ar structure studies. Such studies lead to a better understandlng of the nuclear interaction. So fart the masses of nuclei close ta beta stability as weil as the masses of stable nuclei) have been determined and are well-known. However, far from beta stability, there still exist discrepancies between the various mass measurements. If we are ta extend our knowledge of nuclear structure to this region of more campi ex nuclei, then more precise information on ground-state masses must be obtained. Precise nuclear masses are required for several reasons. Theoretical mass formulae which are used ta predict properties of nuclides far from stability are based mostly on data obtained from stable or long-lived nuclei. Hence it i8 essential that additional information be provided to refine these mass formulae 50 that extrapolations are more reliable. Mass formulae parameters are used in calculations that have both fundamental and practical application. For example, they are relevant ta theories of nucleosynthesis

16 1-3- and to predictions about superheavy e1ements K1apdor et al., 1979; Meldper et al., 1976). Also, measurements of the two-neutron separation energies S2n) ) can be used tu study nuc1ear deformations. Shell closures or nuelear sh.lpe t ra n s i t ion s are exp e c t e d in the ver y n eut r 0 n rie h rl~ è\ Ion Sheline, 1976; Wollnik et al., 1977; Pfeiffer et al 1984) Ina d dit ion, the e val u a t 1. 0 n 0 f nue 1 e a r ln il S S e x C l' :, ~-, e.. can guide searches for delayed neutron emitters Reeder 0t al., 1983). Finally, total deeay energies are requlrcd in refining reactor decay-heat predictions, sinee lt possible to stop the fission proeess in a reactor rapldly but it is impossible to stop the decay of the fission products Aleklett et al., 1982). ~ignificance of Beta Strength Function Determinations The beta strength function SaCE) S-beta) corresponds to the energy distribution of the reeiproca1 ft value the mathematical expression is given in Chapter 7). As sueh lt i5 directly related to the reduced beta-transition probability Kratz, 1984). The beta-strength functlon was initially expected ta vary smoothly with Z and N Hansen. 1973) and early experimental findings from low-resolution total absorption spectroscopy seemed to support this general behaviour Duke et al., 1970; Johansen et al., 1973; Hansen, 1973). Nuclear structure in S-beta has, however, more recently been verified in a number of high-resolution 1

17 -4- beta-decay experiments Kratz, 1984). A knowledge of the structure in S-beta for nuclei far off stability is essential for several reasons. In predicting branching ratios for beta-delayed phenomena as well as beta-decay half-lives, structures within the Q-beta window must be considered. Most extrapolations are based on structureless assumptions for the S-beta such as the Takahashi et al., 1969). "gross theory of beta-decay" AIso, the absolute magnitude of S-beta is of great importance in astrophysical calculations the r-process), in reactor physics and when analyzing beta-delayed fission experiments Klapdor et al., 1980). The beta-decay properties of the nuclides between the beta-stability line and the r-process path will be determined by the position of the low-lying structures in S-beta. In determining average beta-ray energy for nuclear reactor fission products, extrapolated beta strength functions could be used in cases for which no experimental data exist Aleklett et al., 1982). Experimental Methods Nuclear masses can be determined either directly or indirectly. The direct mass spectrometrie technique, for the most part, has been applied stable nuclei w~th high precision. has also been applied to ta obtain the masses of Recently, this method obtain the masses of

18 ' -5- neutron-deficient and neutron-rich isotopes far from betu stability Epherre, 1979). Indirectly, the masses of radioactive nuclei can be determined from nuclear reaction Q-values, Q-alpha values and from beta endpolnt energy measurements. The most commonly used method for the determina tion of masses 0 f nuclel 1 yi ng far f rom the lillt;\ 01 stability is the measurement of beta endpoint ell'rglc"~., 'l- Coupled with decay scheme information, the binding energy difference or Q-beta value between two neighborlng element5 with the same A) can be obtained. Usually. the Q-beta values for a mass chain are obtained. Then starting with d known mass value the subsequent masses or mass excesses) can be evaluated. The beta spectra of the nuclei to be studied must be obtained with an appropriate detector system or beta spectrometer. In general there are two types of beta spectrometers. The first type is the magnetic spectrometer which has been used to study the beta spectra of long-llved isotopes close to beta-stability. However, far from beta-stability the activities are short-lived and must be studied on-line. As a result, magnetic spectrometers, whjch require long counting times, are unsuitable for such studies see Chapter 2 for more discussion). The second type is the energy deposition beta spectrometer which can measure the - complete beta spectrum and thus reduces significantly the... required accumulation time One of the more wlde1y used

19 -6- ones for on-line studies is the plastic scintillator beta detector. This detector offers relatively high efficiency for beta detection, i5 inexpensive and can easily be obtained in any desired size and shape. The main disadvantage of this system is its poor energy resolution which in turn requires calibration techniques that can lead to inaccurate endpoint energy determinations Clifford et al., 1973). More recently, the high purity germanium HPGe) detector has become popular in beta spectrometers designed for high precision end point energy measurements Decker et al., 1980; Brenner et al., 1982; IafigliQla et al., 1983; Bloennigen et al., 1984). These detectors, still offering reasonable efficiency, have significantly better energy resolution. Since these detectors have also a high photopeak efficiency for gamma-rays, they can be calibrated with high precision using known gamma-ra y sources. Thick detectors 15 mm) are available and enable the study of beta-rays of up to about 15 MeV. The main disadvar.tage of HPGe detectors is their low full energy peak efficiency for incident beta particles Munnich et al., 1984). This is due mainly to the higher cross-sections for backscattering and bremsstrahlung due to the high Z-value of Ge. The earlier beta strength function studies vere carried out using total absorption spectroscopy techniques Hansen, 1973). Such a system employs low-resolution NaITl)

20 -7- detectors and hence cannot be used to study the detailej structure of S-beta. Also, in addition to beta decav, neutron rich nuclei far off the stability line cao,lecd\ through beta-delayed neutron emission. H e n ce, w 1 t. h t 11 e advent of high resolution GeLi) gamma-ray detectors élnil '1 Ik neutron spectrometers, it has been poss~ble to Investlg.\le the structure of S-beta up to the highly excited stclte'l in the daughter nucleus within the Q-beta window. However. this method has several disadvantages. First, there pre the difficulties associated with calibrating the efficiencies ot the different detectors. Second v neutron emission may lead to excited levels in the residual nucleus. This requlres unfolding the single neutron spectra into partial spectra ta each final state by measuring delayed neutrons 1n coincidence with gamma-rays depopulating the excited final states. Alternatively, the beta branch intensities, required for the evaluation of S-beta, could be obtained from beta spectra. The HPGe spectrometer i5, therefore, a viable system for beta strength function studies. Such a system offers high resolution and the data obtained do not require complex analysis techniques. However, sinee HrGe detectors are sensitive to gamma-rays, a method must be employed to discriminate the gamma radiation when acquiring beta spectra. ".'

21 -8- The DE-E Telescope Detector In order to obtain precise beta endpoint energies and to extract reliable beta-branch intensities a unique detector telescope system has been employed for the first time. This system consists of a 300 um x 200 mm 2 Si surface 2 barrier DE-detector placed in front of a 15 mm x 500 mm HPGe E-detector. Since the DE-detector is insensitive to gamma-rays, they may be rejected from beta spectra electronica11y. The e1ectronic system which has been used a1so allows the option of recording the gamma-rays which are used for precise energy calibrations. Tc add to the system's versatility the thin DE-detector is specially designed to eliminate the need for external collimators this unique design is discllssed in Chapter 2). The beta spectra obtained with this system are of superior quality in the sense of high statistics, low background beyond the end point energy and the absence of gamma-ray contamination. The System Response Function The major drawback of a HPGe detector is the i~complete energy deposition in the detector of the incident beta-rays. For an incident electron of energy E, o a significant fraction of the pulses generated by the detector do not correspond to energy E This results in a response o function with a peak at E and a tait that extends down to o

22 -9- "-' zero energy. lt is essential then, that the respons~ function of the detector system be well-known before the beta spectra can be analyzed. There are two approaches to determing the response function. The first 18 L0 use d Monte Carlo simulation routine and then modlfy the reslllt~ to fit well-known spectra. ThlS method works we Il Hetherington, 1984) if well-known spectra are ~v~lln~\e that bracket the region of interest. At present tilts le; possible on1y for end points of up to about 6 MeV. Thp second approach, and the one taken in this work, is Lo experimentally determine the response function u~ing mono-energetic electrons. In this way response functions up to the highest beta energy studied could be obtained. ThIS, therefore, eliminates the need for modificatlons which, If made up to about 6 MeV, are unreliable at, say, 10 MeV. Reliable response function determinations are essential if feedings to highly excited states are ta be obtalned. ln this work electron response functions, for the study of beta-minus spectra of up to about 12 MeV, were obtained for the DE-E telescope detector system. Nuclei Studied in this Work The DE-E telescope system has been used to study the beta spectra of neutron rich Rb, Sr and Y isotopes ln the mass region A = 91 ta 100. This region has been studied by several groups. In fact much of the work undertaken in this

23 -10- thesis has been carried out concurrently with that of other groups. There are severaj reasons for the continued interest in this mass region. First, despite the many independent Q-beta experiments that have been performed to date, there still remain significant discrepancies in the Q-beta values reported. In order to provide a reliable database for theoretical mass formulae, it is imperative that these discrepancies be resolved. Alternate methods of measurement and analysis are, therefore, required to investigate the nature of these discrepancies and to hopefully arrive at a reliable set of results. Second, for very neutron rich nuclei near A = 100, there is little or no information on the Q-beta values. This is due to the fact that the half-lives of these isotopes are short and, therefore, are difficult to obtain. They must be studied on-line at an isotope separator facility I80L) where there is sufficient production to perform the experiments within a reasonable amount of time. Another reason for studying this mass region is to investigate the transition from shell closure sphericity) t d f S o e ormat1on. pectroscop1c stu do 1es 0 f 99 Sr, 100Sr an d looy have revealed the onset of deformation Pfeiffer et al., 1984; Azuma et al., 1979). From Q-beta measurements the two-neutron binding energy S2n) as a function of neutron number N can be obtained. A sharp increase in the two-neutron binding energy would indicate the onset of

24 -11- deformation. Finally, there has been considerable interest in studying the structure of S-beta in this mass region. ln particular, the beta decay studies of 95 Rb have revealed prominent resonances within the Q-beta window and significant structures h a v e ais 0 b e e n se e n for 9 l - ) 3 1{ li,1 n d 97Rb Kratz et al., 1981). However, due ta the ~lll)rt half-lives and large beta delayed neutron-emlss1,)[1 probabilities for these isotopes, the beta strength function experiments so far have involved complex detection and analysis techniques and few have been carried out. Hence, further investigation into the low-lying structures of S-beta i8 merited and extension of such studies ta the A = 100 region is valuable for further investigation of these structures. In addition, the use of a simpler detectlon system and more straightforward analysis techniques, as used in this work, make these studies more accessible and, therefore, more relevant in obtaining nuclear structure information. Conventions Used in the Thesis The terms Q-beta value or Q-value used in the thesis are understood to mean the same es % or % -value which are frequently used in the literature and on occasion ln thls thesis. A "beta-particle" or "beta-ray" refers to an electron or positron of nuclear origin emitted in

25 -12- beta-decay. An electron refers to a particle of nuclear or atomic orgin. The beta strength function S6E) will be written as S-beta in the texte Summary of Objectives and Organization of the Thesis The aim of this work is ta study the beta spectra of sorne neutron rich nuclei with the view of obtaining precise beta end point energies to deterrnine Q-beta values) and of extracting beta branch intensities to determine beta strength functions). The high precision desired in this study has necessitated the accurate determination of the system response function over a wide dynamic energy range. The following is a brief summary of the contents of the thesis. Chapter 2 describes the beta spectrometer, the electronic system used, and the calibration techniques. Chapter 3 deals with the source production facilities at McGi11 University and at Brookhaven National Laboratory as weil as the data acquisition systems and cycling procedures. Chapter 4 concentrates on the experimental determination of the system response function; including the analysis of the experimental results and the method of parametrization of the response functions. Chapter 5 details the analysis Methode The reliability

26 -13- of this method is demonstrated through the determinat~on of endpoint energies and beta branch intensities of some well-known nuclei. Chapter 6 presents and discusses the results of the beta endpoint energy measurements and the deduced Q-beLa values of Rb, Sr and Y isotopes in the energy range A = ~l ta 100. Chapter 7 deals with the beta strength functl.oll determinations of Rb isotopes with A = 91, 93, 95, 97,?ud 99. In addition to the experimental results, theoretical calculations based on the Brown-Bolsterli 8-8) model are outlined and compared ta the experimental results. Chapter 8 contains the conclusions drawn from this work. r ~....

27 -14- CHAPTER 2 THE BE TA SPECTROMETER The study of the beta-ray energy or momentum distribution requires an appropriate deteeting system or, to use more common terminology, a beta spectrometer. Throughout the history of beta decay studies various systems have been used, each designed and built according to the specifie experimental requirements. The Most eommonly used beta spectrometers fall under two general categories. They are the magnetic spectrometer systems and the energy deposition beta-ray detectors. 2.1 The Magnetic Beta Spectrometer In studies of the beta-ray spectra of nuclei close to the line of stability where the half-lives are typically long, the magnetic beta spectrometer has been widely used Siegbahn, 1965; Mladjinovie, 1976). In these systems the beta speetra are obtained in a single channel mode eovering only a narrow momentum band at a time. Henee long counting times and strong sources sometimes severa! mci) are required. In addition, the data must be properly corrected to aceount for the source decay during accumulation and if more than one source is used, accu rate norma!ization to the same source strength is necessary. The most attractive feature of the magnetic spectrometer is its superior system

28 -15- resolution. The reso1ution, defined as dp/p, attainable by a magnetic spectrometer, can be of the order of 0.01% Siegbahn, 1965). The magnetic beta spectrometer, however, 1s not suited for the study of beta-ray spectra of nuclei far from the line of stability. The half-lives of these nuclei are short and their production rates are relatively low. Hpoce sources with high activities are not possible and long counting times cannat be sustained. If numerous sources are accumulated to achieve the desired statistics, then the individual source strengths must be accurately normalized, a task next activity. to impossible to perform with short-lived 2.2 Energy Deposition Detectors On the other hand, with energy deposition detectors the difficulties associated activities are easi1y overcome. with the study of short-lived As a result these detectors have come into widespread use as beta spectrometer systems. These systems can be operated in multi-channel mode. This means that the complete beta spectrum can be accumulated without need for scanning across the energy or momentum) distribution in discrete intervals. Hence the accumulation time required ta attain a certain level of counting stat1stics 1s greatly reduced. corrections need be applied and Furthermcre, no source decay if more than one source is

29 -16- accumulated, as is usually the case, no source strength normalization is required. There are several types of energy deposition beta-ray detectors; the most widely used ones being the solid organic plastic) detectors and the more recent solid state diodes. Plastic Detectors Plastic detectors or scintillators are relatively inexpensive and can easi1y be obtained in any desired size and shape. These factors, coupled with the overal1 simplicity of the beta spectrometer system, have made them very popu1ar for on-1ine studies of short-lived nuclei Stipp1er, 1978; Keyser, 1981; Pahlmann, 1984). Their rletection in matter. properties are based on the scintillation process Essentially the incoming beta particle interacts with the detector material and deposits ail or some of its energy through the process of fluorescence Knoll, 1979). The fluorescence decay time is short ns) and hence these detectors exhibit fast response time. On the other hand the energy resolution that cao be achieved with plastic scinti1lators is quite poor. The output pulse amplitude, for a given incident particle energy, will f1uctuate due to the statistical variations. The energy resolution, defined as de/e, is to a good approximation

30 -17- inversely proportional to the square root of the incident. energy E- 1 / 2 ). This is about 9% de = 180 kev) FWHM) for a 2 MeV incident electron energy and about 4% de = 400 kev at FWHM) at 10 MeV Munnich, 1984). Measurements of beta endpoint ene4gies with very high precision tens of kev) are very difficult with plastic detectors. Also, plastic detectors cannot be conveniently calibrated us1ng, say, standard gamma-ray sources. This is due mainly to the poor resolution of the system and the low photopeak efficiency, characteristic of low Z-value materials. Therefore, for the calibration procedure, beta-ray spectra with well-known end point energies are often used. Also, ta ensure the stability of the detector system, sorne sophisticated and complex calibration methods are often emp1oyed. Another important feature, and one that i5 common to ail energy-deposition type detectors and must be taken into account when analyzing beta spectra, is the incomplete deposition of the incident particle energy into the detector. ln other words these detectors have a full energy peak efficiency FEPE) Munnich, 1984) of less than 100%. This is due to the fact that the incident electron may undergo backscattering and bremsstrahlung processes upon interacting with the detector, resulting in an energy deposition less than the incident energy. The detectors are then said to have a response function for a particular incoming radiation. For a scintillator, the type of

31 -18- { material and detector size are the basic characteristics that determine its response function. Due to the low Z-value of plastic, the FEPE is relatively high 75% for 10 MeV incident electrons) and the response function can be described by a simple, smooth mathematical function that can be easily incorporated in a beta analysis program Beaudoin, 1984). German~um Detectors More recently high purity germanium HPGe) diodes have been developed. Due to the higher Z-value of germanium over plastic, these detectors offer higher electron stopping power. This has meant even smaller, more compact beta spectrometer systems 5 cm of plastic versus l cm of Ge is required ta stop 10 MeV electrons Pages et al., 1970)). Most important, however, is the superior energy resolution offered by these detectors. This is due mainly to the large number of electron-hole pairs that are created when a beta particle interacts with the detector. Hence the statistical variations are small, resulting ~n ~ output signal pulses with correspondingly small fluctuations for the same energy deposition in the detector. Nominally the energy resolution FWHM) is less than 10 kev at 1 MeV electron energy and about 20 kev at 10 MeV. This represents a factor ~f about 20 better than the plastic detectors. HPGe detectors capable of stopping up to about 15 MeV electrons are now

32 -19- commercially available. Another important feature of the HPGe diode is its high gamma-ra y photopeak efficiency. This again is due to the high Z-value of germanium. In fact HPGe detectors to date offer superior energy resolution for gamma-rays as well und are used widely in that field of study. This feature mnkes possible high precision energy calibrations using well-known gamma-ray lines. On-line experiments involving short-lived nue lei often require long running times. Hence a quick and easy calibration is also usefui to check on the stabillty of the system. The main disadvantage of a HPGe detector i5 the low FEPE which is about 10% for 10 MeV e1ectrons incident on a 15 mm thick detector. This is due ta the much higher probability for backscattering at low incident energies) and bremsstrahlung production at higher incident energies). The result is a much more complex response function for thls spectrometer as compared with the plastic scintillator. However, if the response functions can be fairly weil determined, their effects can then be accounted for when analyzing the experimentai data. The study of the response function is an important aspect of systems employing HPGe detectors for high precision measurements. The response function for the beta spectrometer used in this work js discussed in detail in Chapter 4.

33 Gamma-ra) Suppression The gamma-ray sensitivity of HPGe detectors i5 use fui for convenient and precise energy calibration. However, the gamma-ray response spectrum, when superimposed on the beta-ray continuum, can interfere with the analysis of the beta-ray spectrum. Usually, this interference 1s MOSt important in the lower energy region. If the high energy region of the beta spectrum is free of this interference, the end point energy of the highest beta branch can still be determined precisely. However, if detailed beta branch feedings to higher excited states are required, it is essential that the gamma-ray contamination of the beta-ray spectrum be minimized. Hence a beta spectrometer utilizing a single HPGe diode and accumulating data in a singles mode, i.e. accepting limited usage. ail the radiation incident on it, has very This means that the inclusion of sorne method to differentiate as much as possible between the gamma- and beta-ray radiations is necessary. Several techniques have been employed. One method, developed at the Foster Radiation Laboratory, is to place both the beta emitting source and the HPGe detector inside the bore of a superconducting solenoid AI-Alousi, 1984). produced inside the bore electrons along the bore axis detector while at the same The high magnetic field will effectively focus the on to the surface of the time leaving the gamma-ray

34 -21- radiation unaffected. In this way, the beta detector may attain an effective solid angle for beta particles close to 2~ at the source but a much lower solid angle for gamma-rays. This system has been used successfully for off-line endpoint energy determinations and shape fnctor measurements Hetherington, 1984). HOWeVE!r, due to limitations on the magnitude of the magnetl.c field attalllpd inside the solenoid, only beta-ray energies of up to about 7 MeV cou Id he focussed with the same large solld angle to achieve the required enhancement factor. Also, unless such a system is used on line, it is not suitable for studies invoiving short-lived activity. Another method to suppress the gamma-ray radiation when ohtaining beta-ray spectra is to use a multi-detector system where one of the detectors is insensitive to gamma-rays. If a very thin Si surface barrier detector is placed in front of the HPGe detector, only the beta-particles will deposit sorne of their energy in the Si detector. The gamma-rays effectively pass through it without detection and hence can be discriminated electronically. This type of system is commonly referred to as a "telescope" beta spectrometer. Since the thin detector nominally accepts only a smaii fraction of the heta-particle energy, it is referred to as a DE-detect')r. The Iarger detector is the E-detector. :r ~ " - Recently such systems have gained popularity with beta-ray studies of short-lived nuciei Born, 1983; Trzaska et al.,

35 ------~ ; Bloennigen et al., 1984). During the early stages of this work, a telescope system consisting of a standard 300 um x 300 mm 2 Si DE-detector and a 15 mm x 500 mm 2 HPGe E-detector was used. A 10 mm thick brass collimator with a 200 mm 2 circular opening was placed in front of the telescope to ensure that the beta particles pass through the central region of the dp.tector assembly, and to reduce scattering from the sides of the DE-detector itself. While this telescope rejects gamma-rays effectively, it has the disadvantage that the scattering of beta particles by the collimator itself introduces large distortions in the lower energy region of the spectrum. In particular, this scattering effect varies with the relative source-detector geometry and, therefore, is not suitable for extracting detailed beta branch feedings to the highly excited states. This telescope was later improved by the introduction of a custom built DE-detector that has a weil defined active area. This innovation eliminates the need of a physical collimator, resulting in a detector response function that is relatively independent of the source-detector geometry. The main features of the present telescope beta spectrometer are: a) high resolution state-of-the-art solid state detectors, b) an E-detector capable of stopping beta-rays in the energy range of up ta about 1S MeV, c) a DE-detector that is specially built ta eliminate the need

36 " -23- for additional co'limators between source and detectors, d) an electronic system capable of reducing pulse pile-up thdt can distort significantly the resion near the endpolnt energy and e) a self-enclosed cryostat containlng th~ telescope system that can be placed on-line at a ~ourc~ facility independently. The following section descrlbes ln detail the DE-E beta spectrometer together with the electronic components needed for its operation. 2.4 Description of the DE-E Telescope System A schematic diagram of the detector assembly is shown in fig The photograph of fig. 2.2 reveals the actual components of the detector system. The DE-detector is li surface barrier transmission type Si wafer with a deplet~on depth of 300 um and an active surface area of 200 mm 2 The E-detector is a HPGe diode with a 15 mm depletion depth and an active surface area of mm The deteetors make thermal contact 50 that they can both be eooled down ta liquid nitragen temperature in vacuum. The thickness of the E-detector is such that eleetrons of ~p ta about 15 MeV can be stapped. This is weil beyond the range of energies studied in this work. The DE-deteetor is of special design by Ortee. Its active area is "weil defined". This 1s achieved by evaporating both sides with a precise circular area definition and such that the contact o tabs are set at 90 to

37 Fig_ 2.1 Cutaway view of the telescope semiconductor beta-ray spectrometer. '

38 .- '~ CRYOSTAT BARREL COlD FINGER PREAM P CONNEC TIONS E-DETECTOR 6.E-DETEC TOR

39 -25- Fig. 2.2 Photograph of the DE-E telescope components. The insert shows the face of the DE-detector.

40 1 The DE-Dûtector

41 -26- each other as shown in fig This "keyhole" design ensures that the tabs do not contribute to the active region of the detector. Furthermore, the active region is weil within the Si wafer and away from the mounting ring. Hence, only those particles passing through the central region are detected. The detector, therefore, perform~ as if a collimator were present. However, scattering effects, which wouid be present wlth an externa1 collimator, are eliminated. The DE- and E-detectors are mounted at the end of the cold flnger of a modified GeLi) cryostat assembly. An anodized aluminum cover fits over the detector assembly. A th in HAVAR 1) foil 10 ~m thick, sandwiched between two annular rings and bolted onto the cover, forms the entrance window. Once assembled the system can be pumped down via a valve at the back of the detector housing. After a vacuum of about 10-6 Torr is reached the detectors can be cooled down to liquid nitrogen temperature 77 0 K). The pumping action of Zeolite in the cold finger housing helps maintain good vacuum over extended periods of time. Temperature re-cycling can be achieved for both detectors. Hence the system can be conveniently dismantled and stored at room temperature. It can a1so be easi1y transported to other facilities for experiments with no special transportation requirements. It should a1so be pointed out that once the telescope cryostat has been pumped down and cooled, it can

42 , r -27- Fig. 2.3 Schematic view of the DE-E telescope detector system illustrating the special DE "keyhole" design. 1"'"" " '.~

43 ~ ~ E L\E to cold finge'i 7 1- r-~1 '' havar wi ndow ~E Detector Si Surface Barrier "Keyhole" Type ACTIVE AREA

44 -28- be integrated into any source line regardless of the vacuum quality of the l~ne. This is important if the system is to be use fui for measurements at different faci1ities. At most ISOL installations the vacuum at the source port may he much -6 higher than 10 Torr and may also be contaminated wlth pump oi1. These are not conditions conducive to proper detectoi operation. 2.5 The DE-E Electronic System The thin DE-detector is sensitive to beta partic1es passing through its active region since these particles will deposit sorne of their energy in the detector. However, it is virtually insensitive to gamma-rays. On the other hand the E-detector is sensitive to both beta and gamma radiations. Hence to suppress the gamma-ray activity in beta-ray spectra and to perform gamma-ray calibrations, appropriate electronic components must be employed to process the detector signais. The electronic system must be able to discriminate the gamma radiation except when gamma-ray calibration spectra are required. It must also be able to produce a voltage output proportional to the sum of the two detector outputs. In this way spectra representing the incoming beta-ray energy distribution, free cf gamma -,, radiation, are produced. A block diagram of the electronic circuit for the DE-E telescope is shown in fig The

45 -29- ~ Fig. 2.4 Schematic of the electronics set-up. AlI instrument numbers refer to Ortec model numbers. TFA - Timing Filter Amplifier CFD = Constant Fraction Discriminator GDG - Gate and Delay Generator TAC Time to Amplitude Converter

46 H.V. BIAS " VOLTS LOGIC SIGNAL FROM PILE-UP VETO TO PILE-UP VETO CIRCUll l'../'\. ~ UNEAR GATE... 2 /'-. SUMAMP ~33 J\..,...n.. >>- ""'- r--, ÂMP 572 TO AOC TO PILE-UP VETO CIRCUIT H.V. BIAS "S9 1 loo VOLTS A E DETECTOR f PREA,,",P.sld PULSE R l TFA, 74 -y- "'REAMP TFA.74 -,...-- E DETECTOR CFO.73A J1. GOG 16A..JL 1 f TAC "fi? -f'"l. CFO.73A..n.. GOG.. 16A AE E INH INH. PILE-UP VETO.n.. TO LINEAR GATE

47 -30- detectors are electrically connected to two BNC feed-throughs. Short cab les from the terminais lead ta the respective preamplifiers. The DE-detector is biased at +100 V are and the E-detector at V. The preampltfier outputs routed to a conventional fast-slow coincidence circuit. Logic pulses are produced at the TAC output when both deteetors have been triggered by events. Renee, when a beta-partic1e is incident on the DE-detector and reaches the E-deteetor, both detectors will register events and a TAC output is produced. However, a gamma-ray following a similar path will trigger on1y the E-detector and no TAC output is produced. The resolving time of the circuit is about 50 ns. The output pulses from the linear amp1ifiers with gains adjusted for energy matching as described later) are added using a summing amplifier. The output pulse of this amplifier is proportiona1 to the sum of the input pulses. AlI events detected by either the DE-detector or the E-detector will produce an output at this stage. To reject events corresponding to gamma-ray detection the output of the summing amplifier is fed to a linear gate and the TAC output is then used to open the linear gate. The analog output of the linear gate is then processed by an ADC. The linear amplifier Ortec 572) also has a Duilt in pileup rejection circuit. Pileup events, if not praperly accounted for, can distort the high energy region of a beta

48 -31- spectrum. The amplifier is inhibited and no output is produced when the interna1 pi1eup rejection logie deteets a distortion of the input signal. This can be caused when two events are deteeted within a time interval less than the resolving time of the system. Pi1eup can occur whelher the coincidence requirement is present or note 'l'he lnhihlt pulses of the 1inear amp1ifiers are routed to an OR/NOR velo unit a10ng with the TAC output. Pileup pulses will veto lhe TAC signal and hence no gating pulse arrives at the linear gate. The pileup rejection circuit is not the only means of reducing pulse pileup. Proper choice of amplifier time constant and maintaining a beta counting rate weil below 1000 counts per second will a1so ensure minimal pileup effects. The DE-E telescope eircuitry just described is very versatile and off ers several accumu1ating options whieh may be demanded by the user. For the most part, ta 0 btain beta-ray spectra, the telescope is operated in sum coincidence mode i.e. the DE+E summed output triggered by the DE-E coincidences). However, if a very low beta branch «1 MeV endpoint energy) must be reeorded, then the DE+E summed output is triggered by the DE-deteetor alone. Hence events that correspond to low energy beta-rays stopped ln the DE-detector will be recorded. For the gamma-ray energy calibrations, the DE-E output is triggered in anti-coincidence mode, yielding events that correspond to

49 -32- detection in the E-detector and not both. The electronic system just described forms the basic configuration for the proper operation of the DE-E telescope. Depending on the experiment that is performed there May be additional electronic requirements. For example, often high resolution gamma-ray spectra are also required and hence a GeLi) gamma-ray detector, and accompanying electronic units must be employed. An additional GeLi) gamma-ray detector has been employed in this work in singles mode as weil as in coincidence mode for beta-gamma coincidence measurements). Singles, high resolution gamma-ray spectra, when accumulated successively in time multispectruming), enables half-life determinations wnich in some cases are required for isotopie identification. When the GeLi) detector is placed in coincidence mode with the DE-E telescope, additional information can be value in some cases. obtained to help establish the Q-beta lt should be pointed out that the Q-beta results obtained in this work have been obtained through careful analysis of the singles DE-E) beta spectra which usually have very good statistics. On the other hand the coincidence spectra have very poor statistics and at most could be used to help verify the results of the singles analysis. In fact for the runs involving masses in the A = 100 region, prohibitive run times would have been required to accumulate equivalent statistics in the coincidence

50 -33- spectra as compared to the singles spectra. In one case for the decay of 93Sr see Chapter 6) beta- gamma coincidence results helped establish a ground-state beta branch that had not been reported in the more r~cent decay scheme studies. 2.6 Gain-Matching and Energy Calibrations For proper pulse height addition by the sum amplifier the gains of the two linear amplifiers must be properly matched. This is accomplished as follows. A source, with electron conversion lines of energy 481.7, and kev Lederer et al., 1978) is placed in front of the HAVAR window. The E-detector is then disconnected from the sum amplifier, the linear gate is placed in ungated mode and a spectrum is accumulated. The peak positions of ail the electron conversion lines are recorded. Then with both detectors connected to the summing amplifier and the linear gate in gated mode, a second 207 Bi electron spectrum ~s obtained. The gain of the E-detector amplifier is adjusted until the peak positions of the electron lines seen in the second spectrum DE+E) are exactly the same as the corresponding peaks seen in the first spectrum DE). The gains of the two amplifiers are then matched. A spectrum taken in ungated mode with bath detectors connected is used ta calcula te the gain. Here the gamma-ray lines of 569.7, and kev are seen along with the electron

51 E -34- conversion peaks. Typically, the gain is about 4 kev per channel, ensuring a maximum energy range of about 16 MeV in 4096 channels. Also, the relative positions of the electron and gamma-ray peaks in the ungated DE-E spectrum can serve as an additional check of proper gain matching. It should be pointed out that since the energy deposited in the DE-detector is very nearly constant for beta particles from 1 MeV to 10 MeV and is of the order of about 100 kev, a gain mismatch few kev. of a few percent results in an absolute error of a This error is accounted for as a systematic error and is presented with the discussion of error determination 'Ch 5 F' 2 5 h 207B,,. ln apter 19.. s ows 1 spectra uslng varlous accumulation modes. The pro pert y of the system ta detect gamma-ray radiation in the ungated mode enables precise energy calibrations to be made. Standard gamma-ray sources of long half-lives are easily available for calibration. Often, however, the maximum energy of gamma-ray lines of these sources is not sufficiently high. In this case on-line production of well-known gamma-ray half-lives is required. For example, sources of short 90 a Rb gamma-ray spectrum has a prominent gamma-ray line at kev. AIso, at bath cyclotron and reactor facilities, neutron background activity produces abundantly very high energy gamma-rays through n,y) reactions on surrounding material. A list of sorne useful gamma-rays and their sources used in

52 , r -35- Fig. 2.5 Th 207B' b. d. h h 1 e 1 spectra 0 ta1ne W1t tete escope beta spectrometer under various accumulation modes. The top spectrum was obtained with the DE-E coincidence requirement and clearly demonstrates the suppression of y-ray lines. The bot tom spectrum was obtained using the ungated mode and reveals both the electron conversion lines as weil as the y-rays.

53 Cf) 1- Z 3 U K 207 Si K E".GE) -COINCIOENCE L L. SPECTRUM 't KeV) Cf) 1- Z :::> o U '. " SINGLES SPECTRUM K o CHANNEL NUMBER

54 -36- this work for calibration purposes is given table 2.1. Frequent accumulation of gamma-ray calibration spectra during a run is also a useful way to monitor possible gain shifts in the system. 2.7 System Energy Resolution and Window Loss The entrance window for the detector telescope is ~ thin 8.0 mg/cm 2 10 um ) HAVAR foi1. The energy loss for electrons is determined experimentally. An additional foi1 of equal thickness is placed in front of the window and a 207Bi electron conversion spectrum is obtained. The positions of the electron peaks are then compared to those obtained without the additional foil. The energy 10ss ls calculated from the difference' in peak position. The experimental results along with the values calculated from the tables of Pages et al. 1970) are plotted in fig Above about 1 MeV a straight line relation i5 obtained. This correction is applied to the energy calibration in the analysis program. The uncertainty est1mated for th1s correction is also taken into account see discusskon ln Chapter 5). The effect of the w1ndow thicknes5 on the energy resolution 1s minimal and is estimated to be less than 0.5 kev. The overall system energy resolution for e1ectrons is," ~ l... found ta be about 12 kev for the 976 kev peak. For gamma-rays it is found to be about 8 kev for the 1063 kev

55 -37- Table Gamma-rays and their sources used in this work ta calibrate the DE-E detector. SOURCE GAMMA-RAY LINES MeV) 207 Bi , Rb , , , , , , Ga , , , Alnj' ) escape peaks Fen,Y ) , , escape peaks

56 , Fig. 2.6 The energy absorption in the HAVAR entrance window. The experimental points are shown by dots and error bars and are obtained usjng a 207Bi source. The sol id line is the theoretical curve obtained from the tables of Pages 1970). The linear extrapolation above about 1.7 MeV is based on the theoretical calculation.

57 - > ~ 14 ~ 12 -li) li) 0...J 10 w E M 2V ) electron energy)

58 -39- peak. 2.8 Detector Dead Layer Loss The thickness of the dead layer near the front surface for a HPGe detector ls about 400 nm and for a Sl detector il ls about 150 nm Knoll, 1979). The energy deposlted in O.S ~m of Ge by a 1 MeV e1ectron ls about 0.3 kev Pages et al., 1970). Hence the energy 10ss through the detector dead layers ls negligible and ls not accounted for separately. 1) HAVAR is a registered trade name of a coba1t-based alloy, manufactured by Hamilton Techno10gy Incorported. It 15 composed of Co42.5%), Ni13.0%), Cr20.0%), Fe17.9%), Mo2.0%), W2.80%), Mn1.60%) and trace amounts of Be and C. 3 It has a density of 8.3 g/cm

59 E -40- CHAPTER 3 SOURCE PRODUCTION FACILITIES The present study covers a wide mass range of neutron rich isotopes far from the line of stabi1ity. Isotopes with mass numbers from A = 91 to 100 were studied. In order to obtain experimental beta-minus spectra with the best possible statistics, it was necessary to conduct the experiments at two isotope separator facilities. At the McGil1 ISOL isotope separator on-line) the maximum production is at about mass A ~ 91 and there is no production of isotopes with mass number around A On the other hand the ISOL faeility TRISTAN at the High Flux Bearn Reactor HFBR) at the Brookhaven National Laboratory BNL) has a production peak several rnass nurnbers higher A z 94) with production, a1though low, for masses around A The McGil1 ISOL Faci1ity The layout of the McGil1 150L faei1ity is shown in fig The beam 1ine of the McGil1 synchrocyclotron is coup1ed to the ion source chamber of the isotope separator located in the external beam hall. Radioactive Rb isotopes are produced by bombardment, vith 100 MeV protons or 50 MeV deuterons), of a 238U target inside the source chamber. The target assembly has been described extensively by Pilar

60 , -41-, Fig. 3.1 Layout of the McGill ISOL facility...

61 ~~ c;.,l tl. ~ :)'~, o ~ '~{ ;3 '),,... ':'\ ~; e.,)" J., -s... D D DE D~ Dl ~ <: w co..j... <:..J z::> ffi~... x LU U ~~~ë::::=:=j\::.~... ~ j)j) QW u:;/) -t-t:mr t-z uw lij..j..j W o W OZ Cl'~ 4: 2: ~ G.

62 -42- \ i 1974), Nikkinen 1977) and Clara 1978). The radioactive atoms are then ionized by surface ionization on rhenium or tantalum foil and accelerated through a potential difference of 5 kv. The ions are focussed by an ion-optica1 system, consisting of an extraction plate and two half-plates, onto the entrance slit of a 90 0 sector magnet. The 100S are imaged at the exit slit of the sector magnet and enter into the ion-beam transport line. The Ion Beam Line A series of parallel-plate electrostatic deflectors and einzel lenses guide the ions along the ion-heam line over a distance of approximately 3 m. The ions are then deflected by a 45 0 electrostatic deflector into a second length of beam line approximately 4 m) that leads through a concrete shielding wall to the source chamber and detector area. The positioning of the isotope separator at large distances from the experimental area is necessary to reduce as much as possible the background activity during the experiments. The ion beam optics are optimized by focussing the ions onto a Channeltron electron multiplier which can be inserted into the ion beam line at various strategie positions. A LeCroy 32-channel high voltage programmable power suppl y is used to set the voltages on ail the e1ectrostatic deflectors and r. ~ 1enses. Several turbomolecular and mechanical pumps instal1ed at different positions along the line ensure a

63 vacuum of 10 Torr or better. Source Preparation The radioactive ions are focussed at the end of the ion-line on thin aluminum foils 0.7 mg/cm um). The foils are placed on aluminum rings fastened at the end of a plunger rad. Ta reduce built-up activity the foils are periodically changed. The DE-E telescope is p05itioned 50 that it faces the aluminum foil opposite the side on which the sources are collected. The energy 105s for electron5 through the fa il is less than 1 kev at 1 MeV incident energy. This 10ss is added to the window thickness correction. Cycling Procedures and Data Acquisition The production of a particular isotope in a given mass chain is enhanced by fo11owing an appropriate cyc1ing procedure when collecting the source. An Elesta c10ck unit capable of producing up to five time-sequenced pulses is employed to control the cyclotron bombarding time interva1, the cooling-off period, the source collection time and the accumu1ar ion starting time. In addition the data accumulation program used on the PDP-l5 computer enables the setting of the accumulation time interval for which the ADe is enabled. The data ls always collected with the cyclotron bombarding beam and ion beam off. A possible run sequence

64 J -44- is as follows: o Tl T2 T3 T4 CYCLOTRON ON OFF OFF OFF ION BEAM ON ON OFF OFF AOC OFF OFF OFF ON The data acquisition system revolves around a PDP-lS computer with DEC-tape and magnetic tape for data storage. Up to three ADC's can be used for simu1taneous accumulation of beta and gamma-ray spectra with the ADC's gated by appropriate sequencing pulses. AlI spectra are recorded ln 4096 channels. For analysis, the data is transferred to an off-line POP 11/34 multi-user computer with a 160 megabyte Winchester disk for data storage. This system can be coup1ed to the McGi11 MUSIC interactive computing) system and several graphies terminais are available for high reso1ution plotting. 3.2 The TRISTAN Facility The floor layout of the TRISTAN ISOL at the HFBR at BNL is shown in fig An extensive description of the ISOL and the accompanying computer system for data acquisitlon,:... and ana1ysis is given by Gill et al. 1981) and Brenner et al. 1980). The ion source is a Nielsen-type with posit~ve

65 -45- Fig. 3.2 Layout of the Tristan ISOL facility.

66 -'"-"~ t. 3 \- '" N o 1 METER '" o ELECTROSTATIC LENSES ~E-E DETEC TOR MOVING TAPE COLLECTM -DETECTOR a:: co 1.L I ~ L -1 ", ',,~ /'. ~ " \'-.~ "-" /'.) L L~';;~ SPECTROSCOPY J

67 -46- ion surface ionization. The target material is enriched 235U embedded into a graphite cloth matrix and positioned in a neutron beam flux of about 1010 n/cm 2 s. The ion beam is guided, by means of electrostatic lenses, onto the entrance of a 90 0 sector magnet. The beam is then directed, by means of additional electrostatic lenses, into the collector box. A switching magnet just beyond the collector box allows the beam ta enter the desired experimental area. The beta spectrascopy station is located at the 45 0 port where a moving tape collector unit MTC) is installed. Source Preparation The DE-E telescope system is integrated into the beam line just above the moving tape unit. The sources are collected on aluminized tape in front of the detector and can be removed at the discretion of the experimenter using appropriate cycling commands to the MTC. The detector is typically pasitioned about 6-10 cm from the source. Opposite to the DE-E system can be positioned another detector for simultaneous gamma-ray counting and half-life measurements. The photograph of fig. 3.3 shows the set-up during one of the runs. Cycling Procedures and Data Acquisition Time sequencing is achieved by an external clock that can be programmed according to the experimental

68 -47- Fig. 3.3 Photograph of the DE-E telescope detector integrated into the ion beam line of the Tristan ISOL. -

69 .