Measurement and simulation of proton induced activation of LaBr 3 :Ce

Transcription

1 Nuclear Instruments and Methods in Physics Research A 578 (7) Measurement and simulation of proton induced activation of LaBr 3 :Ce E.J. Buis a,, H. Beijers b, S. Brandenburg b, A.J.J. Bos c, C. Dathy e, P. Dorenbos c, W. Drozdowski c,d, S. Kraft a, E. Maddox a, R.W. Ostendorf b, A. Owens f, F. Quarati f a Cosine Research BV, Niels Bohrweg 11, 2333 CA Leiden, The Netherlands b Kernfysisch Versneller Instituut, Zernikelaan 25, 9747 AA Groningen, The Netherlands c Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands d Institute of Physics, N. Copernicus University, Torun, Poland 1 e Saint-Gobain Cristaux 104 Route de Larchant BP 521, Nemours Cedex, France f European Space Agency, ESTEC-SCI-A, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands Received 6 April 7; received in revised form 9 May 7; accepted 10 May 7 Available online 18 May 7 Abstract To assess the suitability of LaBr 3 :Ce scintillators for space mission applications, proton induced activation of LaBr 3 has been investigated. The crystals were irradiated using proton beams at several different energies to mimic the spectrum of a solar flare. We have measured the activation both internally and externally by measuring the count rate in the scintillator itself and with a germanium detector. The setup allowed to identify the dominant radioactive isotopes before and after irradiation. At lower energies, we have seen that the dominant source of activation originates from proton induced activation of bromine. At energies above 2 MeV the activation is several orders of magnitude less and the spectrum shows no distinct features. In addition, we discuss results of a simulation of the activation using Monte Carlo methods in combination with state-of-the-art calculations of nuclear cross-sections. Results from the Monte Carlo simulations and the measurements agree within a factor of 2. r 7 Elsevier B.V. All rights reserved. PACS: Ka; Lx Keywords: Scintillators; LaBr 3 ; Activation 1. Introduction Recent advances in crystal growth technology have resulted in the availability of large volume of lanthanum bromide crystals with energy resolutions comparable to that of compound semiconductor detectors. Coupled with room temperature operation and its large density, Ce doped LaBr 3 seems an attractive alternative for space based gamma-ray spectroscopy. Applications may be found in remote planetary sensing or in nuclear astrophysics. General instrument requirements for space based gamma-ray spectroscopy are good performance, in terms of energy resolution and stopping power, coupled with Corresponding author. Tel.: ; fax: address: ebuis@cosine.nl (E.J. Buis). 1 Author on leave. robustness, simplicity and above all radiation hardness. Moreover, the material should have a low activation potential to ensure a low background so as not to spoil the sensitivity of the instrument, because, once in space, material activation is unavoidable even for instruments with a low activation potential. In order to assess the propensity of LaBr 3 to become activated, we have irradiated several crystal samples using a proton beam and measured the resulting activation. The results are presented in this paper. Radiation damage effects are discussed elsewhere [1], with the main conclusion that LaBr 3 shows no measurable degradation effects when exposed to proton fluences up to protons cm 2. The outline of this paper is as follows. In the next two sections we describe the proton irradiation beam and the experimental procedure. Then, in Section 4 we present the results from the activation analysis. A comparison of /$ - see front matter r 7 Elsevier B.V. All rights reserved. doi: /j.nima

2 240 ARTICLE IN PRESS E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research A 578 (7) the measured activation with Monte Carlo simulations is given in Section Proton irradiation at KVI Irradiation of the LaBr 3 :Ce(5%) crystals was carried out at the Kernfysisch Versneller Instituut (KVI) in Groningen, The Netherlands. Protons were accelerated to an energy of 190 MeV and subsequently degraded with copper and polystyrene absorber plates to approximate a solar flare with an energy spectrum between 60 and 184 MeV. A Monte Carlo simulation using Geant [2] confirms that the beam energy with its initial value of 184 MeV is properly tuned as shown in Fig. 1(a). Moreover, it was verified that the moderated beam did not contain a significant amount of neutrons (at the percent level), since this could lead to an underestimation of the total deposited dose in the irradiated sample. More details about the proton beam are found in Ref. [1]. The total integrated fluence was varied in four runs between 10 9 and protons/cm 2 as indicated in Fig. 1(b). Note, however, that these doses are rare in space at 1 AU, but the primary purpose was to investigate the breakdown of the scintillator due to radiation damage. A typical solar proton event has a total fluence of protons with energy above 10 MeV at 1 AU. For planned missions to Mercury and the Sun fluences will be 1 2 orders of magnitude higher. Given the long mission periods the fluences applied allow us to assess the suitability of the material for these missions. 3. Experimental setup In total four LaBr 3 packaged samples were irradiated; one test run and four irradiation runs were made. In addition, spare packaging material was irradiated. Table 1 provides a list of all irradiated materials. Special assemblies were procured from Saint Gobain, that allowed the photomultiplier tube to be detached from the crystal. The crystals, being hygroscopic, were housed in aluminum cans of 0.5 mm thickness. A BK7 glass window was used to optically couple the crystal to the photomultiplier tube. To properly determine the activation effects, the activation of the sample was determined both internally by using the scintillator itself and the externally with a germanium detector. Internally measured activation is defined here as the decay of isotopes which leave their decay products, such as electrons and gammas, in the irradiated sample. It is this activation that gives rise to background. On the other hand, in the externally measured activation, the emitted radiation, consisting mostly of photons, leaves the sample and is less problematic Internally measured activation A setup for the internal measurement of the activation included a crystal, attached to a demountable PMT and a HV base. The crystal was detached from the PMT during irradiation. The signals generated in the PMT were shaped Table 1 List of irradiated samples and their dimensions Material Size/weight Dose (p/cm 2 ) LaBr 3 :Ce 1 in. cylinder 10 9 LaBr 3 :Ce 1 in. cylinder LaBr 3 :Ce 1 in. cylinder LaBr 3 :Ce 1 in. cylinder Al housing 29.5 g BK7 glass window 1 in., 4.7 g A 1 in. LaBr 3 assembly including housing, glass and crystal weighs 98.9 g KVI Beam Aug solar protons entries [cnts/mev/proton] fluence [cm -2 ] Oct solar protons Fig. 1. Simulations of the beam energies after degrading (a). Six discrete beam energies are visible, the seventh energy (at 184 MeV) is obtained from the unperturbed beam. In (b) the beam energy spectrum in comparison to measured fluxes is shown (adapted from Ref. [3]).

3 E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research A 578 (7) and processed through an Amptek MCA8000A multichannel analyzer. Several sources ( 60 Co, 137 Cs) were used to calibrate the multi-channel analyzer. After being exposed to the proton beam, the crystals were re-attached to their PMT and were left to decay, while the count rate was monitored. Immediately after beam exposure, all crystals (apart from the ones that received the lowest dose) were too active to handle and were left to cool down. For the sample, that received the highest dose, the dead time of the MCA was still around 10% after 50 h. Hence, the short-lived isotopes were not considered Externally measured activation As the energy resolution of the scintillators is not good enough to resolve the individual lines of decaying isotopes produced, the activation was measured externally using a germanium detector. The facilities at KVI include a lead castle, which consists of a well-calibrated germanium detector inside a low activity lead and copper shield. The germanium detector measured gamma-rays in the energy range kev in ADC channels. Because of the excellent energy resolution of the germanium detector, most lines found in the sample were clearly resolved. Lines were analyzed by fitting the line peak and the Compton continuum at various times after irradiation. The identification is subsequently carried out by using the combined information of the peak position (energy) and the decay time of the isotope. In addition, since the relative intensity of the line for each isotope is known from the table of isotopes, the relative peak height provides a third handle. To find peaks above the Compton continuum, a peak finding algorithm was applied to the data, based on the ROOT analysis package [4]. This algorithm found the bulk of the lines. Small and not well-separated peaks were analyzed separately. 4. Results Activation was determined both internally and externally before irradiation. Natural occurring lanthanum contains 0.09% unstable 138 La, with gamma lines at 789 and 1436 kev. From the abundance of 138 La, its lifetime and the relative line intensities one can infer that the intensity in the lines should be around 0.5 and 1.0 emissions/s/cm 3 for the 789 and 1436 kev lines, respectively. The spectrum was measured in the lead castle and after correction for the acceptance of the lead castle (see Section 5) we measured an intensity of 0.64 for the 789 kev line and 0.93 for the 1436 kev line for a 1 in. crystal. Milbrath et al. [5] have studied the background in LaCl 3 scintillators due to alpha decay of the contaminant 227 Ac. The measured background in the LaBr 3 due to alpha decay was 0.45 Bq/cm 3 or Bq/g, while in LaCl 3 scintillators [5] count rates of 1.6, 0.37 and 1.8 Bq/g were found in three samples. In addition to the 138 La lines, more than 10 lines in the range from 100 to 500 kev from the Actinium contamination are found in the sample of which the dominant lines are: the 236 and 256 kev lines from 227 Th decay, the 269 kev line from 223 Ra decay and the 271 kev line from 219 Rn decay. Other lines in the spectrum originate from 22 Na, 40 K and 24 Na Internally measured activation after irradiation The detector background count rate measurements were started one day after the irradiations. For this measurement, two samples were attached to their PMT and data collected for two days. One of the two samples had received a total dose of p=cm 2, while the other was irradiated at the highest dose p=cm 2. The gain was adjusted such that the covered energy range of the samples differed. internal activation [cnts/s/cm 3 /kev] Dose = cm Dose = cm hrs after irradiation 23 hrs after irradiation 43 hrs after irradiation 43 hrs after irradiation hrs after irradiation 63 hrs after irradiation 83 hrs after irradiation 83 hrs after irradiation before irradiation 1 internal activation [cnts/s/cm 3 /kev] Fig. 2. Several spectra of internally measured activation of the sample which had received p=cm 2 for energies up to 700 kev in (a) and in (b) for a sample for energies up to 7000 kev which received a dose of p=cm 2.

4 242 ARTICLE IN PRESS E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research A 578 (7) activation [cnts/s/kev] internal 10 3 external, total assembly external, Al housing only entries/bin/hr Fig. 3. Activation measured approximately 90 h after irradiation with a dose of p=cm 2. The externally measured activation shows the clearly resolved lines above a background continuum (background noise is negligible). In addition, the externally measured activation of the irradiated spare aluminum housing is shown. In (b) a snippet of the spectrum is plotted with the identified isotopes. The sample had received a dose of p=cm 2. entries [cnts/hr] kev; I(t)= exp( *t) 239 kev; I(t)= exp( *t) 261 kev; I(t)= exp( *t) elapsed time since irradiation [hrs] Table 2 Most abundant isotopes produced in LaBr 3 :Ce Isotope Half life Main lines (kev) Na y Kr h 261.4, Na h ; La 19.5 h As h ; La 1: y As d La 40.3 h Se d 136.0; Ce 17.7 h Br 16.2 h 559.1; Ce 34.4 h Br h 239.0; 249.8; 297.2; Ce 9.0 h Br h 554.4; Ce d Fig. 4. Lifetime fit of lines found at 511, 239 and 261 kev. The fit results in half-lives of 38.5, 63.0, 36.5 h, respectively. The latter two lines are attributed to 77 Br and 79 Kr. The sample a gain setting was used to detect photons from the low X-ray photon energy ðo50 kevþ to energies of about 1 MeV. The sample had a higher gain and operated in the range 150 kev to 7 MeV. Spectra of the internally measured activation at specific times after irradiation for both samples are plotted in Fig. 2. While the spectrum at lower energies is characterized by the many (line) features, the activation at higher energies is more or less featureless: no lines were found. In Fig. 3, both the internally and externally recorded activation approximately 90 h after irradiation are overlayed. From the plot, it is clear that the peaks seen in the internally measured activation spectra are mainly due to several lines instead of single lines Externally measured activation after irradiation In addition to the LaBr 3 assemblies, spare housings and glass pieces were irradiated as well. In Fig. 3, the activation of the crystal after irradiation is shown together with the contribution from the housing. The aluminum housing contributes 1 10% to the activation at the lower energies,

5 E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research A 578 (7) average acceptance E γ = 239 kev E γ = 657 kev depth in crystal [cm] depth in crystal [cm] Fig. 5. Simulation of the development of the beam inside the crystal (a) and acceptance (see text) of the germanium detector in the lead castle for gammarays that originate from the irradiated crystal. which corresponds to the weight fraction of the housing to the total weight of the crystal and housing. In Fig. 3(b) a snippet of the spectrum is shown with all the lines identified. The analysis and isotope identification yielded a fit of the peaks and background. In this way, the lifetime of the lines could be obtained and the relevant isotopes identified. In Fig. 4, a lifetime fit of three dominating lines is shown. In the energy range shown in Fig. 3(b) nearly all the lines are identified, while over the entire energy (1 3 MeV) range only about 60% of the lines were identified. This rather low number is mainly due to the fact that at higher energies peaks are due to (double) escape peaks or sum peaks. The observed isotopes are listed in Table 2 with their lifetimes and principle decay lines. In nuclear astrophysics applications, the 511 kev is highly significant, so preferably the material should contain a small number of beta emitters. The fitted half-life after about 100 h is 38.5 h. The half-life of the annihilation line is mainly determined by the isotopes 79 Kr, 76 Br and 77 Br, with lifetimes of, respectively, 35, 16 and 57 h. We also determined the 511 kev entries in the other samples as well. While after 100 h the 511 kev entries for the total assembly is 0.47 emissions/s/g, the count rate for the housing is emissions/s/g and the count rate for the glass is 0.07 emissions/s/g. This means that the glass and the housing contribute about 20% to the total count rate of the 511 kev line. 5. Comparison to Monte Carlo simulations It is important for the use of a scintillator in space to understand activation and to be able to predict the background that is generated. We have carried out extensive simulations to reproduce the line fluxes that were found in the samples. The count rate (N meas ) of the product of a given process can be determined from the cross-section s and the incoming flux N beam : N meas ¼ n dxs (1) N beam where n is the number of parent (or target) particles per volume and dx is an infinitesimal thin slab of target material. Because we did not have a monoenergetic beam during the irradiation and the fact that the irradiated sample was thick, 2 it was not possible to do independent crosssection measurements of isotope producing processes. To reproduce the isotope production using Monte Carlo simulations, we have divided the sample in thin slices (2.5 mm) along the beam direction and calculated the number of produced isotopes for each individual slice. The total number of produced isotopes can then be determined by adding contributions from all slices (i): N 0 sim ¼ X i N sim;i N beam;i ¼ X n dx i s i ðeþz i ðeþ=n beam;i. i Therefore, we need to determine: (1) The effective fluence ðn beam;i Þ and beam energy in each slice. To do this, we simulated the development of the beam inside the crystal and determined the average beam energy (E) in each slice. (2) The acceptance of the germanium detector for each measured energy. The acceptance ðz i ðeþþ is defined as the fraction of the gamma-rays, created in slice i and that deposits all of its energy in the germanium 2 A thick sample means here that there is a significant drop of the average beam energy inside the crystal. ð2þ

6 244 ARTICLE IN PRESS E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research A 578 (7) cross-section [mb] σ inelas,tot (Br+p) σ inelas,tot (La+p) cross-section [mb] Br(p,X) 76 Br Br(p,X) 77 Br Br(p,X) 79 Br Fig. 6. Total inelastic scattering for protons on Br and La (a). In (b) the cross-sections for the processes nat Brðp; XÞ 76 Br, nat Brðp; XÞ 77 Br and nat Brðp; XÞ 79 Kr are plotted. Table 3 Measured and calculated number of gamma-rays from isotope decay (normalized to the number of beam particles) Isotope Line (kev) Intensity (%) N 0 sim N 0 meas N 0 sim N 0 meas 76 Br : : : : Br : : : : : : : : Kr : : : : detector. The acceptance is, of course, different for each slice and gamma-ray energy. The determination of the acceptance also included the self-absorption of the LaBr crystal. (3) The isotope production cross-section ðs i ðeþþ is a function of proton energy. Because the commonly used Geant software does not include proton induced isotope production for materials like La, Br and Ce, we made use of a special and dedicated software package called Talys [6]. In Fig. 5(a) we plot the beam inside the crystal. As can be seen from the plot, seven beam energies as discussed in Section 2, enter the sample and gradually lose energy in the crystal. Note that Bragg peaks occur at a depth of 1.1, 1.7 and 2.5 cm for protons with an initial energy of 60, 80 and 100 MeV, respectively. In Fig. 6(a) and (b) we plot, respectively, the total inelastic scattering cross-section and the cross-section for isotope production as given by the Talys package. Note that the plotted cross-sections are for naturally occurring Br, which consists of 79 Br (50.7%) and 81 Br (49.3%). Although, the total inelastic crosssection for bromide is lower than for lanthanum (or cerium), it dominates the activation, because for each La atom there are about three Br atoms in the crystal. We have carried out the above-described procedure for three isotopes which induce strong and well-separated lines. A comparison of simulated and measured number of isotopes is given in Table 3. If we estimate a total error of 25% on the ratio of the two numbers, then we can conclude that the simulation and the measurements are in good agreement, apart from the determination of the 76 Br isotope production. 6. Conclusions We have investigated the intrinsic activity and the activation of LaBr 3 :Ce after proton irradiation. Using a well-calibrated germanium gamma-ray spectrometer several long-lived isotopes were identified. During the measurements which took place a few days after irradiation, it was found that bromide is the source for the highest activation; proton induced activation of 79 Br and 81 Br results in the production of 76 Br, 77 Br and 79 Kr isotopes with lifetimes of 16, 57 and 35 h, respectively. The comparison of the measured isotope production of bromine and krypton with simulations showed an agreement within a factor 2. The production of 76 Br is overestimated in the simulations, albeit still reasonable close to the measured value. The activation is a potential problem for application of LaBr 3 :Ce in space missions. Isotope decay causes

7 E.J. Buis et al. / Nuclear Instruments and Methods in Physics Research A 578 (7) gamma-ray lines that might interfere with observables in nuclear astrophysics. However, Fig. 6(b) shows that isotope production cross-sections drop significantly for proton energies above 50 to 60 MeV. A few g/cm 2 of aluminum shielding could achieve a low energy cut-off of solar proton events. On the other hand, the study has shown that any material surrounding the sensitive material (such as packaging material) increases the amount of activation. For planetary gamma-ray spectroscopy, the activation is less problematic: the activation above 2 3 MeV is featureless and orders of magnitude lower. For the crystal that was irradiated with the highest dose ð10 12 p=cm 2 Þ the integrated activation above 2 MeV dropped from 420 Bq=cm 3 at 23 h after irradiation to 47 emissions/s/cm 3 at 83 h after irradiation. For typical solar protons events the total protons fluence is several orders of magnitude lower and hence the activation accordingly. Acknowledgments We thank G. Vacanti from cosine Science & Computing for fruitful discussions and A. Koning from NRG Petten for providing the Talys software package. References [1] A. Owens, A.J.J. Bos, S. Brandenburg, E.-J. Buis, C. Dathy, P. Dorenbos, C.W.E. van Eijk, S. Kraft, R.W. Ostendorf, V. Ouspenski, F. Quarati, Nucl. Instr. and Meth. A 572 (7) 785. [2] Geant4 Collaboration, S. Agostinelli, et al., Nucl. Instr. and Meth. A 506 (3) 250. [3] J. Wilson, et al., Shielding strategies for Human Exploration, NASA Conference Publication, vol. 3360, 1997, pp [4] R. Brun, F. Rademakers, Nucl. Instr. and Meth. A 389 (1997) 81. [5] B.D. Milbrath, R.C. Runkle, T.W. Hossbach, W.R. Kaye, E.A. Lepel, B.S. McDonald, L.E. Smith, Nucl. Instr. and Meth. A 547 (5) 504. [6] A.J. Koning, S. Hilaire, M.C. Duijvestijn, TALYS: Comprehensive nuclear reaction modeling, 5, p