IN-BEAM COMMISSIONING OF A RECOIL MASS SPECTROMETER

Transcription

1 NUCLEAR PHYSICS IN-BEAM COMMISSIONING OF A RECOIL MASS SPECTROMETER T.B. SAVA 1,3, D. BUCURESCU 1, G. CĂTA-DANIL 1,2, I. CĂTA-DANIL 1, D. DELEANU 1, D. FILIPESCU 1, D. GHIŢĂ 1, T. GLODARIU 1, M. IVAŞCU 1, N. MĂRGINEAN 1, R. MĂRGINEAN 1, C. MIHAI 1, V. MOŞU 1, A. NEGREŢ 1, G. PASCOVICI 4, S. PASCU 1, L. STROE 1, N. V. ZAMFIR 1 1 Department of Nuclear Physics, National Institute for Physics and Nuclear Engineering Horia Hulubei, Atomiştilor 47, RO-77125, POB-MG6, Măgurele-Bucharest, Romania, EU tiberiu@tandem.nipne.ro 2 Faculty of Applied Sciences, University Politehnica of Bucharest, Splaiul Independenţei 313, RO-642,Bucharest, Romania, EU 3 Faculty of Physics, University of Bucharest, Atomiştilor 45, RO-77125, Măgurele-Bucharest, Romania, EU 4 Institute for Nuclear Physics, University of Cologne, DE-5937 Cologne, Germany, EU Received February 2, 211 A review of electromagnetic separation for the low-energy fusion-evaporation recoils produced on a thin 12 C target by a 44 MeV beam of 19 F delivered by the IFIN- HH tandem accelerator is presented. The mass, velocity, energy and angular distributions of these recoils were studied using a spectrometer composed of a velocity selector (Wien filter), a 6 analyzing dipole magnet and two magnetic quadrupole doublets. Focal plane secondary beam diagnostic was performed using a 4 2 matrix of photovoltaic Si-PIN detectors and a pseudo-time of flight (ToF) technique using 5 MHz RF from the beam pulsing system. A complete description of the experiment preparations, running and the results interpretation is made. Key words: recoil spectrometer, electromagnetic separation, low energy recoils. 1. INTRODUCTION Studying nuclei away from the valley of stability is a central theme in present nuclear structure physics. For proton-rich nuclei, fusion-evaporation reaction involving stable beams and targets is a usual technique to access isotopes towards the proton drip line. For a proper selection and identification of such isotopes a residue detector is very advantageous. The present paper presents a type of instrument that is used for separating and analyzing recoils resulted from fusion-evaporation reactions produced with ion beams delivered by the Bucharest FN Tandem accelerator [1]. For these recoils we were interested in determining their mass distribution, evaluating velocity distribution using a Wien filter [2], energy spectra and some aspects of angular distribution properties. The test case is represented by the slightly inverse kinematics reaction of 19 F (44 MeV) bombarding a 2 µg/cm 2 12 C foil. The aim of the experiment (c) Rom. RJP Journ. 56(Nos. Phys., Vol. 9-1) 56, Nos. 9-1, P , 211 Bucharest, 211

2 2 In-beam commissioning of a recoil mass spectrometer 195 was to separate the main outgoing fusion-evaporation recoils, which in this case have A=29 ( 29 Si) and A=26 ( 26 Al, 26 Mg) from the primary beam, and to fully diagnose the separated reaction products. For secondary beam diagnosis we used photovoltaic Si-PIN detectors [3] placed in the focal plane of the spectrometer combined with a pseudo-tof measurement between the timing signals of the Si-PIN detectors and a signal having a constant frequency of 5 MHz from the beam pulsing system. The electric field of the Wien filter was adjusted in order to minimize the contribution of the primary scattered beam in the focal plane. Other elements that had a significant influence on the Si-PIN diodes spectra were the horizontal slits position of the Wien filter and of the 6 analyzing dipole magnet, the magnetic field gradient of the collecting quadrupole doublet and the magnetic field strength of the analyzing dipole. 2. THE RECOIL SPECTROMETER The spectrometer, shown in Fig.1, was designed for separation of the recoils produced by fusion-evaporation reactions induced on solid targets. The incident beam delivered by the FN tandem accelerator is focused on the target by the Q 1 magnetic quadrupole doublet. The first element of the spectrometer is the thin, solid, self-supporting target mounted in front of another magnetic quadrupole doublet (Q 2 ) which has a key role in collecting the emerging products. The quadrupole doublets Q 2 and Q 3 have a 4 mm circular aperture and a maximum magnetic field gradient of 24 T/m. The next ion optical element is a Wien filter with a 21 mm gap between the electrostatic plates, which has the function of separating the primary beam from the reaction products. The separation slits (F 1 ) are mounted in the horizontal plane at a distance of 1.9 m from the Wien filter exit. For this experiment the electric and magnetic fields were set in order to stop the primary beam on the right side slit. The 6 dipole magnet provides deflection of the recoils as a function of their magnetic rigidity (Bρ) and also has a focusing function in the horizontal plane due to the pole face rotation angle of 3 [4]. The dipole is provided with analyzing slits mounted in the focal plane at 1.47 m from the pole exit. Finally, quadrupole doublet (Q 3 ) makes the secondary beam focalization on the Si-PIN particle detectors arranged in a 4 2 matrix. The magnetic field values together with nine beam currents along the spectrometer beam-line are real-time monitored, while the power supply parameters are remote controlled. The preparation of the spectrometer for in-beam commissioning implied diverse activities as providing a vacuum level to the order of 1 7 mbar, a proper beam line alignment, extension of the cooling water and compressed air circuits. During the rebuilding phase some in-beam intermediate checks were per-

3 196 T.B. Sava et al. 3 Tandem beam Q1 Switching dipole Wien filter Target Q2 6 degree dipole magnet F1 F2 Q3 Implanting material (detector) Fig. 1 Schematic layout of the spectrometer. formed in order to test and calibrate the electric, magnetic and beam measuring elements. 3. EXPERIMENT PREPARATIONS A proper reaction to test the separator in-beam had to meet the following selection criteria: - narrow angular and energy distributions for the fusion-evaporation recoils emerging from the reaction; - for an easy detection, production cross section should be as large as possible for one of these recoils; - the primary beam should have a high intensity for the chosen projectile; - a self supported target should be used. Following this requirements the chosen reaction was: 19 F(44 MeV) + 12 C (2 µg/cm 2 ) as the 19 F beam had previously proved to be intense and 12 C self supported targets were available. A series of calculations and simulations were performed in order to estimate the recoil species, production cross sections, energy and angular distributions of these particles and their flight through the spectrometer. These simulations implied the use of two main codes, LISE [5], and SIMION 3D 7. [6]. The simulation sequence consisted in using LISE++ output as input data for ion-optics program SIMION 3D. LISE++ package was used for determining the parameters of the ion beam-target interaction like primary beam energy loss in target,

4 4 In-beam commissioning of a recoil mass spectrometer 197 Table 1. Nuclear residuals and production cross-sections calculated with PACE code Z(amu) N(amu) Nucleus Percent Cross section (mb) P.63% Si.63% P.21% Si 49.2% Al 1.71% Si.1% Al 1.62% Al.76% Al 2% Mg 16.3% Mg 8.51% Na.45% Ne.2%.162 energy, angular and mass distributions of the recoils and SIMION 3D used this data to simulate the flight of the ions through the spectrometer. For the projectile energy of 44 MeV the main production cross-section of the residual nuclei goes to 29 Si (398 mb) in the pn reaction channel, as Table 1 shows, while 26 Al and 26 Mg have 162 mb and 132 mb, respectively, in the αn and αp channels. The code used for these calculations was PACE which is included in the LISE++ package. The high cross section together with the narrow energy and angular distributions favor 29 Si for the transport through the separator system. Beside this conclusion, the simulations showed that: - primary beam energy loss in the target is approximately 1.6 MeV; - fusion-evaporation recoil energy ranges between 5 MeV and 36 MeV. Depending on the reaction channel, the energy distributions are centered on different values and have different amplitudes, as presented in Fig. 2 for 29 Si and 26 Al residues; - the recoils emerge at angles smaller than 17, but for different species the angular distribution caries particular signatures as can be seen in Fig. 3; - the angular acceptance of the magnetic quadrupole doublet does not exceed ± 9, Fig. 3; - the Wien filter must allow 29 Si passage and also provide a good rejection of the

5 198 T.B. Sava et al Y ie ld (p p s /(M e V )) S i C h a rg e s ta te s Y ie ld (p p s /(M e V )) A l C h a rg e s ta te s E (M e V ) E (M e V ) S i C h a rg e s ta te s A l C h a rg e s ta te s Y ie ld (p p s /(M e V )) Y ie ld (p p s /(M e V )) V e lo c ity (% c ) V e lo c ity (% c ) Fig. 2 (Color online) Energy/velocity distributions for 29 Si and 26 Al after the production target. In case of 26 Al the distribution is wider compared to 29 Si. primary beam. As a starting point, the fields in the Wien filter must be set such as the allowed central velocity is: v = E B = 1.3MV/m.79T =.43c (1) which represents the velocity of the compound nucleus resulted from the reaction. For this electric field the corresponding voltage is U=21.6 kv (±1.8 kv on each electrostatic plate). In the case of this ratio (β=4.3%) the 29 Si of 23 MeV and 19 F of 42.4 MeV are separated by a 6 cm distance at the slits point, which is enough to physically separate them. The value v=.43c set on the Wien filter allows 29 Si to pass while the low values of both terms E and B avoid overspreading the recoils and therefore affect the total transmission. The magnetic field strength B will be kept constant during the experiment to the value of.79 T, the variation of the central velocity being possible by adjusting the electric field. Eight particle detectors (photovoltaic Si-PIN diodes, PDB-C69-3 model, pro-

6 6 In-beam commissioning of a recoil mass spectrometer S i ( M e V ) ( M e V ) 3 C o u n ts A n g le (d e g ) A l (< 1 4 M e V ) ( M e V ) ( M e V ) C o u n ts A n g le (d e g ) Fig. 3 (Color online) Angular distribution for the first two fusion-evaporation recoils judging after the yield, 29 Si and 26 Al. The highlighted area is the angular acceptance of the collecting quadrupole doublet. Fig matrix of photovoltaic Si-PIN diodes used for particle detection. duced by API) arranged in a 4 2 matrix were mounted in the focal plane with the four position side in the horizontal plane, Fig. 4. The detectors were calibrated on the low energy part of the spectrum with an alpha particle double source, 239 Pu and 241 Am, having the energies of MeV and MeV respectively, and with the primary beam of 42.4 MeV on the high energy part. For the gamma radiation detection we used four LaBr 3 (Ce) scintillation detectors [7], model 51S51, produced by Saint Gobain that were placed near the production target. The mounting angle for these detectors was 9 with respect to the beam axis in order to eliminate the Doppler shift of the gamma rays. We used three detectors with crystals of 2 diame-

7 11 T.B. Sava et al. 7 ter and one with a crystal of 1.5 diameter. The spectra collected with these detectors were energy calibrated using three calibration sources of 152 Eu, 137 Cs and 6 Co. The DAQ setup made use of the energy signals coming from the Si-PIN diodes and also of the time measurement estimated with a time to amplitude converter (TAC). The start of this TAC was given by the OR function applied to the time signals of the particle detectors, the stop being given by the signal having a constant frequency of 5 MHz coming from the tandem beam pulsing system. The relevant part of the electronic scheme is presented in Fig. 5. Q(t) Preamplifier 1μs Diff. to Ground Signals 1μs Spec. Amp. Shaping Time.5 μs Energy signals to ADC 8X 8X Photo Cell 2V Input HV Bias Out A Out B 16X In A In B Splitter 8X CFD Gate generator 8X NIM to ECL HV Power supply 1μs Timing signals TFA Delayed OR (4xLaBr3 detectors) Internal Delays ~4 ns 16X Stop TDC Start Stop TAC Logic Unit 1X to ADC Start 1X Out (OR) 8 X DAQ Trigger Fig. 5 (Color online) Main part of the electronic scheme used in the experiment. 4. IN-BEAM COMMISSIONING AND INTERPRETATIONS The incident beam current was approximately 1 na, which means approximately 2x1 1 pps for 19 F (7+) and all the obtained statistics will be related to this value. The beam repetition rate was a 2 ns beam pulse at every 2 ns. The spectrum displayed in Fig. 6 corresponds to the following settings of the spectrometer: β=4.3% in the Wien filter, B=.463 T in the magnetic analyzer, about 7 T/m in the quadrupole lens and Wien filter and dipole horizontal slits opened at ±15 mm. The dependence between 1/ E and time is linear. This type of plot allows the analysis of different recoil slopes specific to distinct masses. This plot is the result of summing all the eight particle detectors individual spectra. The formation of the various clouds of particles is characteristic to different charge states in which the particles are found. The reason of being detected in the

8 8 In-beam commissioning of a recoil mass spectrometer 111 Fig. 6 (Color online) 1/ E(t) dependence for β=4.3% in the Wien filter. focal plane by the particle detectors is that all the particles found in these charge states have the same magnetic rigidity (Bρ). As the charge state is varying the only parameter which permits the same magnetic rigidity to be achieved is the particle velocity. This shows that the velocity acceptance of the Wien filter considering the inner fields is around ±55% with respect to the central velocity. A zoom in the lowerright corner of the Fig. 6 shows the existence of at least two types of peaks, one with a better energy resolution but with lower statistics and one with a larger dispersion but with significantly better statistics, Fig. 7. The first group of peaks belongs to the scattered beam of 19 F and the other group of peaks is associated with fusionevaporation recoils. The acquisition time for the spectrum in Fig. 6 was 3 minutes during which we measured a total number of counts N T = The ratio between recoils and scattered beam events is in this case R f/sb =5.2. In order to clean the upper plot for better analysis, the velocity in the Wien filter was increased to β=4.6% by raising the electric field, the other parameters being kept constant. 1/ E(t) dependence is presented in Fig. 8, in which the total number of counts, during 3 minutes, was N T =7138 and the ratio between recoils and scattered beam events was R f/sb =5.17. Two different masses are clearly visible in Fig. 8. For these two masses, having several charge states, the relative slope ratio can be calculated. This corresponds to the ratio of the mass 26 and mass 29: r 1 = a 1 = = A(26 Al) a 2 A( 29 Si) = (2)

9 112 T.B. Sava et al. 9 Fig. 7 (Color online) Fusion-evaporation recoils and scattered beam formations observed in the lower-right corner of the Fig. 6. Fig. 8 (Color online) Different masses grouped in several charge states having different slopes in 1/ E(t) dependence. The attempt to calibrate the mass spectra with the known mass of the primary beam failed due to the very large difference in ToF between the recoils and the scattered beam considering the reference frequency of 5 MHz. Therefore, we decided to reject the primary scattered beam from the focal plane by varying the electric field in the Wien filter and thus adjusting its central velocity. This measure simultaneously led to the observation of the reaction products velocity spectra. Fusion-evaporation/scattered beam events ratio was measured for different cen-

10 1 In-beam commissioning of a recoil mass spectrometer R e c o ils (c n ts.) / S c a tte re d b e a m (c n ts.) β(% c ) Fig. 9 Fusion-evaporation recoil events vs. scattered beam events ratio as a function of the central velocity in the Wien filter. tral velocities in the Wien filter. A maximum ratio of almost 16:1 was obtained for the central velocity β=5.1% as presented in Fig. 9 where N T =1786 during 3 min. For this point the 1/ E(t) dependence reveals a modified picture as the lighter mass gains abundance compared to the heavy one, as illustrated in Fig. 1. This fact agrees very well with PACE calculation which shows that the lighter masses are likely to be found traveling with higher velocities compared with mass 29 (also see Fig. 4). Fig. 1 (Color online) β=5.1% - point of the second test related to the observed masses and the most favourable velocity in the Wien filter relative to the recoils vs. scattered beam ratio. Keeping β=5.1% we performed an additional test in order to verify that the

11 114 T.B. Sava et al. 11 observed masses are 26 and 29 corresponding to 26 Al, 26 Mg and 29 Si, respectively. The test consisted in calculating the energy ratio for the two masses being in the same charge state as one can see in the central position of the Fig. 1, which additionaly have the same time signature. For these two masses the ratio of the energies would roughly be the same as the ratio of the masses because of the same velocity they travel: r 2 = 19.4MeV 21.5MeV = = A(26 Al) A( 29 Si) = amu amu (3) Fig. 11 displays the total number of recoils and scattered beam events for different velocities set in the Wien filter. This sum was normalized with the LaBr 3 detectors counting rate as a current measurement. Two maximum values are revealed in this measurement. The first one corresponds to the compound nucleus velocity (β=4.3%) in which the largest contribution is from 29 Si, second one is centered at β=5.3% in which predominant are 26 Al and 26 Mg. 7 6 R e c o ils + S c a tte re d b e a m (c o u n ts ) β(% c ) Fig. 11 The sum of fusion evaporation and scattered beam events measured for different β values. For β=4.3% in the Wien filter and B=.498 T in the analyzing dipole we approximated the recoil transmission through the spectrometer considering the yield of the fusion residual nuclei as given by: N = N A d target I beam A target (Ze) σ f (4) where N A is Avogadro s number [mol 1 ], d target represents target thickness [g/cm 2 ],

12 12 In-beam commissioning of a recoil mass spectrometer 115 A target is the atomic mass number of the target [a.m.u.], I beam is the beam intensity [ea], Z represents the particle ionization state [elem.charge], e is the elementary charge [C] and σ f is the fusion reaction cross section [barn]. Considering the angular distribution of all the outgoing recoils, the angular acceptance of the collecting quadrupole doublet and the 89 mbarn total cross section of the fusion-evaporation reaction estimated by PACE (in good agreement with Ref.[8]), about 8.8% from the recoils emitted in the qudrupole doublet accepted solid angle are transmitted trough the spectrometer. The LaBr 3 (Ce) gamma detectors were used for measuring the prompt gamma rays as a signature of the residues that were produced. In order to have a higher counting rate a thicker target (1.75 mg/cm 2 ) was used. The observed peaks were associated with the main residues produced in the reaction. The results are presented in Fig. 12. Fig. 12 Prompt gamma-ray spectrum taken with LaBr 3 (Ce) detectors near the target. The significant gamma lines are associated with the main reaction products. Further, we adjusted various parameters of the spectrometer in order to obtain a better understanding of the functioning regimes. For 7.1 T/m magnetic gradient in the collecting lens we obtained a maximum of transmission through the spectrometer with the Wien filter set on β=5.1%. The modification of the horizontal slits positions for the Wien filter and analyzing dipole from ±15 mm to ±3 mm provided a better separation of the recoils in the spectrometer. This change can be observed in Fig. 13 where the splitting of each of the broader peaks into two parts is noticeable, one for mass 26 and one for mass 29. Another observation was the increasing counts of the scattered beam when the slits gap was reduced. Estimation of the energy resolution in this two cases shows that we obtained 68 KeV instead of 9 KeV when reducing the slits gap. We further varied the magnetic field strength in the 6 analyzing dipole which centered

13 116 T.B. Sava et al. 13 Fig. 13 (Color online) The effects of reducing the slits gap from 3 mm to 6 mm is the splitting of the each of the broader peaks in two components, corresponding to masses 26 ( 26 Al, 26 Mg) respectively 29 ( 29 Si), and also the effect of increasing the scattered beam. the recoils energy distribution in different values, Fig 14. For B=.498T and β=4.3% we obtain a maximum in the count rate of the particle detectors which was explained by centering the 23.7 MeV 29 Si through the spectrometer (see also Fig. 4). Fig. 14 Dependence of the recoil and scattered beam events sum by the magnetic flux density (B) in the analyzing dipole. For B.5 T we obtain the highest particles counting rate as the distribution is centered in 23.7 MeV, see Fig 3.

14 14 In-beam commissioning of a recoil mass spectrometer CONCLUSIONS The in-beam behavior of the mass spectrometer was determined and well understood. Various parameters were adjusted in order to observe the influence on the transmitted particles. Nuclear species resulted from reaction, their abundance, energy and angular distributions observed in the experiment proved to be in a good agreement with the preliminary simulations made with LISE++ and SIMION 3D codes. The pseudo-tof method used in this experiment represents a powerful tool for rapid identification of the fusion-evaporation recoil products in this type of spectrometers. With a proper beam pulsing frequency the 1/ E(t) dependence can be mass calibrated if scattered beam reaches the particle detectors. The spectrometer transmission in this case was estimated to be around 8.8% from the fusion-evaporation recoils entering the collecting quadrupole doublet. Minor improvements and upgrades can be foreseen, as appropriate shape of the slits and probably a new position for them. This will lead to a reduction of the scattered beam presence in the focal plane. For a higher transmission rate to be achieved the replacement of the collecting quadrupole doublet either with a solenoid or with a quadrupole doublet having a wider aperture would be essential. A complex diagnosis of the secondary beam using ToF and position sensitive detectors, like micro channel plates detectors (already in development), and an E- E telescope for atomic number identification will be further possible. REFERENCES 1. V. Ceausescu et al. - Nucl. Instr. and Meth. in Phys. Res. A 244, Issue 1-2, (1986). 2. C. Pierret et al. - Nucl. Instr. and Meth. in Phys. Res. B 266, (28). 3. A. Simon et al. - Nucl. Instr. and Meth. in Phys. Res.B 26, (27). 4. E.A. Heighway - Nucl. Instr. and Meth., 123, Issue 3, (1 Feb. 1975). 5. O.B. Tarasov, D. Bazin - Nucl. Instr. and Meth. in Phys. Res. B 24, (23). 6. D.A. Dahl, Int. J. Mass Spectrom. 2, 3 (2). 7. R.Pani et al. - Nucl. Instr. and Meth. in Phys. Res. A 567, (26). 8. P.Sperr et al. - Phys. Rev. Lett. 36, 653 (1976).