1510 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 3, JUNE 1996

Comparison of Activation Effects in y-ray Detector Materials

P.R. Truscott, H.E. Evans, C.S. Dyer, C.L. Peerless, J.C. Flatman, M. Cosby, and P. Knight Space Department, Defence Research Agency, Farnborough, Hampshire, GU14 6TD, England

C.E. Moss Astrophysics and Radiation Measurements, NIS-2, Los Alamos National Laboratory,

Los Alamos, NM 87545

Abstract

Activation induced by cosmic and trapped radiation in y- ray detector materials represents a significant source of background for space-based detector systems. Selection of detector materials should therefore include consideration of this background source. Results are presented from measurements of induced radioactivity in different scintillators activated either as a result of irradiation by mono-energetic protons at acceler- ator facilities, or flight on board the Space Shuttle. Radiation transport computer codes are used to help compare the effects observed from the scintillators, by identifying and quantifying the influence on the background spectra from more than one hundred of the radionuclides produced by spallation. For the space experiment data, the simulation results also permit determination of the contributions to detector activation from the different sources of radiation in the Shuttle cabin.

I. INTRODUCTION

Space-based '/-ray instruments are susceptible to background induced by primary energetic nucleons and nuclei, and their secondary radiations, experienced in the spacecraft environment [I] . The primary sources of radiation comprise trapped protons, solar flare particles, and very energetic cosmic-ray nuclei. These may undergo nuclear interactions with the spacecraft platform and payloads to generate second- ary protons, neutrons and mesons which, if sufficiently ener- getic, may themselves produce further spallation. Indeed, the multiplicity of cosmic-ray interactions often results in greater secondary particle fluxes in comparison with the primary flux [2,31.

The prompt effects of both the primary and secondary radiations can be largely negated through the use of veto- shields. In addition an active collimator (ie a thick veto-shield) suppresses most of the delayed y-ray flux from decaying radionuclides which are produced in the rest of the spacecraft by the spallation interactions. However, radionuclide decay of spallation and neutron capture products generated within or local to the central detector element constitute an important source of background for low- to medium-energy detector systems.

Previous investigations of induced radioactive background have individually looked at well-established detector materials such as sodium iodide, caesium iodide, and germanium, and more recently at bismuth germanate. These have included analyses of operational y-ray instruments (Apollo, HEAO-3, the OSSE instrument on the Compton Gamma Ray Observ- atory), as well as experiments design to quantify induced activation in scintillators (Shuttle Activation Monitor and ground irradiation experiments) [2,3,4].

Comparisons have also been made with predictions of detector activation and response. Previous work by the authors used Monte Carlo radiation transport techniques to calculate both the shielding effects of the spacecraft structure, and spallation rates and nuclide species generated in detectors as a result of the modified radiation (see $111). The good agreement of these predictions with experimental results has given confi- dence in the accuracy of the shielding calculations and our understanding of induced activation effects in detector materials.

This paper extends previous studies by presenting prelimi- nary results from investigations of activation background in more novel detector materials, such as barium fluoride and GSO. Comparisons are also made (using both Shuttle experi- ments and model results) of the characteristics and suscepti- bility of NaI, CsI and BGO scintillators to activation effects.

11. SPACE AND GROUND IRRADIATION EXPERIMENTS

A. The CREAM Space Shuttle Experiment

The Cosmic Radiation Effects and Activation Monitor (CREAM) experiment is a Space Shuttle middeck payload designed to collect data on a number of aspects of the radiation environment experienced in the Shuttle cabin area: (i) linear energy transfer spectra in PIN silicon diodes as a function of both time (and hence orbital location) and shielding afforded by the spacecraft structure - these data are relevant to the study of single-event upset (SEU) phenomena in spacecraft electronics; (ii) mission accumulated dose and neutron fluences for five differently shielded locations in the middeck area; (iii) induced activation in '/- and X-ray detector materials.

0018-9499/96$05.00 0 1996 British Crown Copyright

Published with the permission of Her Britannic Majesty's Stationery Office

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Mission

STS-48+

Since 1991 this experiment has had six successful flights (see Table 1), on the first two missions flying with another radiation experiment (Shuttle Activation Monitor or SAM) which collected complementary data on the y-ray environment in the Shuttle cabin using NaI and BGO detectors [2,5].

Date Incl. Altitude Duration [nmil [days1

Sep 1991 57" 307 5

STS-53 STS-56

I( STS-44' I Nov 1991 I 28.5" I 196 I 8 11

9 Dec 1992 57" 210/176 Apr 1993 57" 160

11 STS-68+ I Oct 1994 I 39" I 120 I 1 1 11

In the earlier flights of the CREAM experiment, a 7.6cm diameter x 7.6cm long cylindrical sodium iodide crystal was flown passively in a middeck locker. Pre- and post-flight measurements of internal energy-loss spectra as well as the escaping y-rays revealed the level of activation induced by spaceflight. More recently, CREAM has provided the oppor- tunity to examine activation effects in more exotic detector materials (at least for space applications). On Shuttle mission STS-63 the following crystals were flown in place of the sodium iodide: 5.1cm x 5.lcm cylindrical barium fluoride and gadolinium oxyorthosilicate detectors; 1.9cm x 2.2cm lutetium oxyorthosilicate; and two samples of cadmium zinc telluride totalling 57.48. As with the analysis of the NaI detector, activation analysis of these materials was performed pre- and post-flight through measurements of the internal energy- deposition in the scintillators (through a photomultiplier tube coupled to each crystal) and the escaping y-rays using germanium detectors.

B. Proton-Beam Irradiation Experiments

Several irradiations of a barium fluoride crystal have been conducted at the Paul Scherrer Institute in Switzerland, permitting activation background to be studied in a better defined radiation environment and at higher particle fluence levels. During irradiation a 2.5cm x 2.5cm cylindrical detector was bombarded by protons from a 3OOMeV beam aligned co- axially with the crystal and with a FWHM of 4.4cm. Three different fluence levels (2.12x107, 5.00~10' and 2.2~10'" p/cm2) were used to allow measurements of the internal spectra from 10 minutes to over a month after irradiation.

111. SIMULATION OF ACTIVATION PROCESS AND DETECTOR RESPONSE

Monte Carlo radiation transport computer models have been applied to predict activation levels in the space-flown and proton-beam irradiated scintillators. Spallation by 300 MeV protons and their secondaries has been modelled using the Light Heavy-Ion version of the High Energy Transport Code (LHIMETC) [6]. This program can perform particle transport simulation for protons, neutrons, charged pions, muons and light nuclei with energies >15 MeV, in complex 3-D geometries. In addition, all secondary, tertiary, etc. particles (>15 MeV) can be followed by the simulation to interaction, escape, or until the particle energy falls below the 15 MeV cutoff.

For the space-flown detectors, LHIMETC was used with Shuttle shielding models [2] to make an initial prediction for the residual primary proton and secondary proton and neutron flux levels at the detector surface. These particle spectra were then used in subsequent simulations to determine activation rates in the crystal. The primary trapped-proton environment was determined using the UNIRAD prediction suite in con- junction with the AP8 model of the Van Allen Belts [7,8], while the CREME model of Adams et al. [9] was used to estimate the cosmic-ray environment for each Shuttle orbit.

The decay of the direct spallation products and their daughter nuclides between the period of activation and time of observation was calculated using a generalisation of Bateman's solution [lo]. The resulting decay rates were then used in conjunction with libraries of response functions, each response function being a prediction for the average pulse-height spectrum from the decay of one radionuclide within a given scintillation detector. The results presented here utilise libraries containing between 146 to 300 response functions per detector. These have been generated using the Monte Carlo photon transport code BANKER, with decay scheme data from the Evaluated Nuclear Structure Data File (ENSDF) [11,12].

Iv. PROTON-BEAM IRRADIATION RESULTS

Fig. 1 shows the induced background spectrum recorded through a PMT mounted to the proton-irradiated barium fluoride detector. The count rates from the three irradiations have been normalised to the number of incident protons, and therefore show the gradual decay in activity from 10.8 minutes to 32.5 days post-irradiation. The peaks in these spectra are the result of electron-capture, isomeric-transition, or a-particle decays, while the continua are primarily due to p' or p- (the latter to a lesser extent since the spallation products are more often proton-rich than neutron-rich).

Included in Fig. 1 are predictions for the induced energy- loss spectrum, generated using the techniques described in $111. At higher energies, the predicted count rates (principally from P-decays) is too high: at 4 MeV by as much as an order of magnitude. However at lower energies (<2 MeV), there is remarkably good agreement between the predicted and

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Energy [MeV Fig 1 Spectra from 2.5cm x 2.5cm BaF, crystal after irradiation by

300 MeV protons compared with predictions (dashed hnes)

measured count rates. There is particularly good correspon- dence of peaks, and the predictions permit identification of the radionuclides producing these features - all appear to be direct or indirect products of barium spallation.

v. COMPARISON OF SPACE SHUTTLE EXPERIMENT RESULTS

Figs. 2 and 3 show the internal energy-loss spectra measured in the CREAM and SAM sodium iodide detectors following flights on Shuttle missions STS-56 and STS-48 respectively. The lower altitude of the Orbiter during STS-56 (160 nmi) means that the spacecraft was subjected to a lower mission-averaged trapped proton flux (2.7 p-cm 's-' for energies >15MeV) compared with the higher altitude mission STS-48 (31 p-cm%' >15MeV). Therefore, since the primary cosmic- ray proton flux was approximately the same, the post-flight count-rates from the SAM N d detector is slightly (30%) higher, despite the spectrum being taken 1.9 days (as opposed to 1.2 days) after touchdown, and the shorter duration mission.

The relative importance of the different radiation sources is confirmed from predictions of the contributions to the internal spectra from cosmic-ray and trapped proton primaries and secondaries (included in Figs. 2 and 3). Fig. 3 shows the greatest contributor to the post-flight counts is the flux of residual primary trapped protons which penetrate the Orbiter, and secondary protons and neutrons generated by trapped proton nuclear interactions in the spacecraft structure. In contrast, for the lower altitude STS-56 flight, secondary particles (principally neutrons) from cosmic-rays are predicted to be the major cause of NaI activation.

For the results from both missions, the predicted total count rate (upper dashed line) is in very good agreement with the measured spectra, again allowing identification the radionuclides producing the features observed.

In Fig. 4 a spectrum is shown taken from the SAM BGO detector 1.9 days after the end of mission STS-48 [2]. Unlike the sodium iodide spectra, BGO displays a number of high- energy peaks rather than continua. This is due both to the greater importance of electron-capture (as opposed to p') decays of the high-Z bismuth spallation products, and the higher fraction of isomeric-transition decays of meta-stable nuclides produced from interactions with bismuth and

1 +"'I + others I 1 , , 1 , ~ I I I I I i / ~ ~

10-'0:0 - 0 2 0 . 4 0 . 6 0.8 1 . 0 Energy [MeV]

1 0 2 ' ' ' ' ' r - 1 " " ' " ' ' 1 " " ' ' I " I " " 'j ' ' '

I KEY 1 =. Measured Total ~redicted (UDDer dashed) 1 j

-L

- .- .- . . . . . . . Secolidaries from cosmic-rays

Residua primary cosmic-rays Residual primary trapped protons

1 0 ' I 2 3 4 Energy [MeV1

Fig. 2 CREAM STS-56 NaI spectra at 1.2 days post-flight (upper graph: O-2MeV, lower graph: 0-6 MeV)

Total predicted (upper dashed) Secondaries from cosmic-rays Residual primary cosmic-rays Residual primary trapped protons Secondaries from trapped protons 1 0 1 - I

$ Y 2. $ 1 0 0

s m w

Y

lV1

.. , 10 1 2 3 4

Energy [MeV]

Fig. 3 SAM STS-48 NaI spectrum at 1.9 days post-flight

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a, r ?

0 0 1 2 3 4

Energy [MeV] - All residual primaries + al secondaries - - - Secondary protons and neutrons from CA protons + alphas & trapped protons) ii residual primaries + all secondaries

CR & trapped protons) k esidual trapped protons

trapped protons Secondary protons & neutrons from mmic-ray protons

- - - - Residual cosmic-ray protons

a ....... ....

. . . -.

Fig. 4 SAM STS-48 BGO spectrum 1.9 days post-flight

102

101

0 . 2 0.4 0.6 0.8 1 .0 Energy [MeV]

Fig. 5 CREAM STS-63 BaF, spectrum 23 hrs post-flight

lo i . 0 4 0 . 5 1.0 1.5 2.0

Energy [MeV]

Fig. 6 CREAM STS-63 GSO spectrum 17 hrs post-flight

germanium. As with the sodium iodide results, predictions of internal spectra from the BGO detector are in extremely good agreement with experimental count rates.

Figs. 5 and 6 show internal spectra from the barium fluoride and GSO crystals respectively following flight on Shuttle mission STS-63. Due to the smaller mass of the crystal flown, and the natural radium content of the material, the count rates from the 5.1cm x 5.1cm barium fluoride detector have poorer statistics compared with the NaI and BGO crystals from previous flights. However, activation is apparent at low energies (<OS MeV) and is in good agreement with the predicted count rates due to nuclear interactions of primaries and secondaries from cosmic-rays and trapped protons (dashed line). As with the mono-energetic proton irradiation results, spallation products of barium feature strongly. The 5.lcm x 5 . lcm GSO detector also showed post-flight activation with comparable count rates to the STS-63 BaF, over 0-1 MeV (Fig. 6). There is good agreement between trends of the pre- dicted and measured count rates, albeit the prediction is generally lower. Peaks in the experimental spectrum between 0.1-0.3 MeV also appear to be duplicated in the model results (the peak at approximately 0.4 MeV arises not from activation, but is due to a slight calibration/resolution mismatch of the pre- and post-flight spectra). Identification of the features in the GSO spectra awaits a more detailed analysis.

Post-flight analysis of the CdZnTe samples flown on STS- 63 identified no signs of activation. This however was most probably the result of the small quantity of material used (4.5% of the sodium iodide mass) rather than a lower susceptibility of the material to activation.

VI. COMPARISON OF DETECTOR MATERIALS USING COMPUTER SIMULATIONS

Simulation models by themselves may also be applied to compare activation effects in different detector materials. In Fig. 7, internal energy-loss spectra are presented for NaI, CsI and BGO detectors which have y-ray detection efficiencies comparable to one of the OSSE detectors on the Compton Observatory. The estimate for the induced background is based upon constant bombardment by primary cosmic-ray protons

Energy [MeV]

Fig. 7 Comparison of near-equilibrium spectra from NaI (dashed), CsI (dotted) and BGO (solid) detectors under constant cosmic-ray

proton irradiation

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20.00%

10.00%

t

I

I

>15MeV of flux 1 p-cm2s over the period of 50 days (when the count-rates reach a state of near-equilibrium). For the purposes of this comparison, the cosmic-ray spectrum is taken for the STS-48 orbit (57" inclination, 307nmi altitude). The results show that between 0.7-2 MeV, BGO (solid line) shows comparable or slightly worse count rates than the alkali halides. However, between 0.1-0.7 MeV the bismuth germanate spectrum is generally lower than NaI or CsI, whilst above 2MeV BGO appears to demonstrate a significantly reduced background (by up to a factor of eight).

In Fig. 8 the same example of cosmic-ray proton activation has been used to determine the percentage of "single-site'' decays in the NaI, CsI and BGO detectors. Single-site event rejection has been proposed as a technique for reducing activa- tion background in a number of future y-ray instruments [13]. The technique operates on the assumption that many of the radionuclides generated in the detector material undergo p'- decay with the emission of either no or only low-energy photons. Such an event produces only localised (single-site) ionization, whereas incident photons undergoing Compton- scatter or pair-production interactions frequently produce multi- site events. Fig. 8 shows the percentage of decays which occur without the emission of photons with energies >E, and demon- strates that background reduction using tkis technique would be less viable in BGO than for NaI or CsI.

VII. CONCLUSIONS

The CREAM expel-iment is providing a valuable opportun- ity to quantify space-flight induced activation effects in a variety of detector materials, and has provided the first results for barium fluoride and GSO. Data from CREAM and other experiments is permitting essential validation of models for spacecraft shielding, detector activation and response. In addition the good agreement makes radiation transport simula- tion techniques an essential tool for identifying and quantifying the effects of the spallation nuclides to the spectra, especially for space-flown detectors where contributions from different sources of activation may be determined.

The computer models have been used to compare activation background effects in detector materials, and may be used to aid materials selection and compare background reduction techniques in the design of future instruments.

VIII. ACKNOWLEDGEMENTS

Flights of the CREAM and SAM experiments on Shuttle have been sponsored by the US BMDO and organised by the USAF Space Test Programme. The authors are indebted to Penny Haskins and Jack McKisson of Radiation Technologies, Inc., for the use of the SAM experiment data. Activation analysis of the CREAM and SAM detectors for missions STS- 63, STS-56 and -48 was conducted at the Environmental Engineering Department of the University of Florida and at the Nuclear Chemistry Division of Lawrence Livermore National Laboratory. The authors would like to acknowledge the enthusiastic assistance provided by the members of both laboratories, particularly Professor Emmett Bolch, Ryan Richards, Thabet Tolaymat, George Harder and Dave Camp. Richard Conwell of Aurora Technologies Corp. is thanked for providing samples of CdZnTe at short notice to fly on the Shuttle. The proton beam irradiations were conducted in collaboration with ESTEC using facilities at the Paul Schemer Institute in Switzerland, and thanks are extended to Len Adams and Bob Nickson of ESTEC, and Wojtek Haydas of PSI.

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PI

[31

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PI

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