Analysis of Activation Effects in Space-Borne & Proton-Beam Irradiated y-Ray Detectors

P.R. Truscott, H.E. Evans, C.S. Dyer, M. Cosby Space Ditpartment, Defence Evaluation & Research Agency, Farnborough, Hampshire, GU14 OLX, England

C.E. Moss Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Abstract - The selection of detector materials for space-borne discrete energy decay (electron capture, isomeric transition, y-ray system has plreviously been based on the response of the or a-decay) separated by a continuum principally due to p'- scintillator or semiconductor to incident source or background decay. y-rays, and the practicalities associated with the use of these materials in space. 'Today the availability of a greater variety of The availability of an increasing variety of detector detector materials allows potential optidsation for induced materials introduces the possibility of including background radioactive backgroipnd as well, which contributes an important characteristics of scintillators and semiconductors as part of source of backgrounld for instruments in the space environment. the trade-off process in the design of an instrument. This Studies have been conducted on induced activation effects in a paper reports on results collected from Space Shuttle and variety of y-ray detector materials Of interest for space proton beam irradiation experiments to quantify induced applications, including BaF2 and GSO. Results are Presented radioactive background levels in some novel detector

experiments. Detailhd radiation transport computer simulations radiation transport calculations, used to predict the processes are used to compai'e and identify the effects observed in the of shielding, activation, radioactive decay and detector experimental spectra and, for the space experiments, to quantify the contributions l o detector activation from the different response. sources of radiation bxperienced in the Shuttle cabin.

from Space and Protonmbeam (300 MeV and GeV) materials. These data are compared with output from

11. EXPERIMENT DESCRPTION I. INTRODUCTION

Space-based y-ray observatories have to operate in a severe natural radiation erlvironment, yet still be able to distinguish the faintest of y-ray signals emanating from within, and beyond, our own galaxy. The evolution of astrophysical y- ray detector desig,, Over three decades has resulted in a steady improvemimt in detector sensitivity. instruments, such as those proposed for the International Gamma Ray Laboratory (INTEGRAL), are expected to extend the limits of' sensitivity up to an order of magnitude beyond those currently being achieved by detectors on the Compton G ~ ~ ~ ~ - R ~ ~ Observatory and GRANAT. T~ achieve this, significant effort is being placed on employing techniques which will veto background events created by the natural radiation environment, or failing this, permit their effects to be accurately quantified. Results from previous

radioactivity induped in the instrument materials by spallation represent a major source of background [l]. Activation local to the central detector element, and within the active collimator, give rise to y-ray lines characteristic of the radionuclides produced. The response to radioactivity induced in the central detector itself is a series of peaks from

A. The Cosmic Radiation Effects and Activation Monitor (CREAM) experiment

The CREAM experiment comprises active and Passive detectors to the single event effects (SEE) environment, and mission accumulated dose and neutron

Future fluence in the Space Shuttle middeck area. Earlier flights also included a 7.6cm x 7.6cm sodium iodide crystal, flown passively in a Shuttle locker to investigate activation effects resulting from space flight [2,3]. For more recent Shuttle missions (STS-68, -63, and -81), samples including BaF2, GSO, LSO and CdZnTe have been flown to compare their activation properties. For mission STS-63 in February 1995, Orbiter Discovery was placed in a 51.6" inclined, 3501397km near-circular orbit. The detector materials remained in a locker in the SpaceHab module, until just prior to landing

locker was one of five passive detector packages, which contained nickel and gold neutron activation foils and TLD- 700 thermoluminescent dosimeters (TLD). Upon landing, the scintillators were rapidly retrieved to low-background counting facilities and internal energy-loss spectra (from natural background and induced radioactivity) were recorded through photomultiplier tubes coupled to the scintillators.

low- to medium-energy detector systems have shown that when were retrieved to the middeck. Included in the

0 British Crown Copyright 1997/DERA Published with the permssion of the Controller of Her Britannic Majesty's Stationery Office

0-7803-4335-2/98,/$10.00 0 1998 British Crown Copyrights. 2 1

Activation in the BaF2, GSO and CdZnTe crystals were also monitored through escaping y-rays measured using germanium detectors located close to the space-returned samples.

Trapped protons I 2.1

B. Proton beam experiments Proton irradiations of a 5.lcm x 5.lcm GSO and 2.5cm x

2.5cm BaFz have been performed using the Saturne accelerator facility, at Saclay in France (1 GeV) and the Proton Irradiation Facility (PIF) at the Paul Scherrer Institute (PSI) in Switzerland (300 MeV). Following irradiation, measurements were made of internal energy deposition spectra from 19 minutes to 21 hours post-irradiation for GSO and 11 minutes to 33 days for BaF2.

0.11 0.32

111. SIMULATION PROCESS The calculation process for determining detector activation

levels, radionuclide decay rates and detector response involves the use of a number of space environment and radiation transport models, summarised by Fig. 1.

Shielding G e o y e h y

(trapped protons) (cosmic-rays)

Fig. 1: Calculation scheme for detector activation and response

A. Primary particle environment Predictions for the primary cosmic-ray proton and a-

particle environment have been made using the CREME code of Adams et al. [4]. Averaged over the complete STS-63 mission, the cosmic-ray flux >15 MeVInucleon has been estimated to be 0.53 protons-cm-%' and 0.065 a-cm'2s-'. The trapped proton environment has been determined using the AP8 Van Allen proton belt model at solar minimum [5], in conjunction with a geomagnetic field model extrapolated to the epoch of the AP8 data. The mission-averaged trapped proton flux >15 MeV was estimated to be 14 ~ m - ~ s - ' .

B. Shielding calculations Simulations have been performed for trapped protons,

cosmic-ray protons and a-particles incident upon 5 12- element, Shuttle spherical shield models for the different CREAM experiment locations. These radiation transport

calculations utilised the Defence Evaluation and Research Agency's Integrated Radiation Transport Suite (IRTS) which permits 3-D Monte Carlo simulation of [6]:

high-energy (>15 MeV) nucleon, meson and light ion transport;

low-energy (thermal - 15 MeV) neutron transport; 0 nuclear and electromagnetic interactions for all primary and secondary particles.

Table I shows the residual primary and secondary particle fluxes determined for the CREAM SpaceHab locker. This location affords relatively little shielding, with minimum and median shielding levels of 5.8 g/cm2 and 12.9 g/cm2 respectively. Despite this, the net secondary particle flux from cosmic-ray protons is expected to exceed the primary flux outside the spacecraft. For cosmic-ray a ' s , the four nucleons making up each particle increases the multiplicity of the secondaries to almost a factor of three of the a-particle flux outside the spacecraft. The results in Table I also show that shielding against trapped protons is beneficial.

The measured fast neutron flux, determined from post- flight activation induced in the nickel metal foils from

Ni(n,p)'*Co reactions, is in extremely good agreement with the predicted neutron flux. Furthermore, the dose rate at the locker, which arises principally from trapped protons, is predicted to be 29 mrad(Si)/day compared with the measured dose rate of 22k1 mrad(Si)/day.

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TABLE I. PREDICTED AND MEASURED PARTICLE FLUXES >15 MEV FOR STS-63

CREAM LOCKER (FLUX LEVELS IN CM-'S-')

Source I Residual I Secondary I Secondary I neutrons

CR protons 0.41 0.22 CR &particles 0.03 1 0.060 0.12

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TABLE 11. TABLE 111. PREDICTED SPALLATION AND NEUTRON-CAPTURE YIELDS FOR 1 GEV,

COSMIC-RAY AND TRA?PED PROTONS, AND SECONDARY NEUTRONS FROM COSMIC-RAY PROTONS ON 5 . l C M X 5.1CM GSO CRYSTAL

PREDICTED SPALLATION YIELDS FOR COSMIC-RAY AND TRAPPED PROTONS, AND SECONDARY NEUTRONS FROM COSMIC-RAY PROTONS ON 5 . ICM X 5. lCM

B A S , AND 300 MEV PROTONS ON 2.5 X 2.5CM BAFz CRYSTAL

'56Gd Stable 1.8

lszGd l.lE+14y 1.1

produced with significant yields arise from interactions with the oxygen (and po:isibly even silicon) in GSO but are stable, e.g. 15N, I4N, I2C, and I3C (fractional yields between 6 ~ 1 0 - ~ to 3 ~ 1 0 . ~ for lGeV protons). Similarly, for BaF2 significant yields of stable products arise from interactions with fluorine, e.g. 14N, l60, l 8 0 , and 1 7 0 , with fractional yields from 6 ~ 1 0 . ~ to 3 ~ 1 0 . ~ for 300 MeV protons. By far the majority of radionuclides are produced by interactions with the heaviest element, gadolinium for GSO and barium for BaF2. Detector response should therefore be strongly influenced by these spallation and neutron-capture products, and their progeny.

D. Simulation of radionuclide decay and detector response The Monte Carlo photon transport code BANKER has

been used to determine the average pulse-height spectrum for the decay of each radionuclide in each detector concerned. Typically, response functions are based on 50,000 to 100,000 simulated decays using nuclear decay scheme information from the Evaluated Nuclear Structure Data File (ENSDF) [7] and atomic de-exci tation data from the MEDNEW database [8]. For the results presented in SIV, each library (for a given detector size, material and radionuclide distribution)

contained approximately 300 response functions. Where appropriate, allowance has been made for the non-linear scintillation efficiency of the material for ionisation by a- particles. The response functions were convolved with nuclide decay rates of the direct spallation products and progeny, determined from the nuclide yield predictions.

I v . EXPERIMENT RESULTS AND COMPARISON WITH PREDICTIONS

Figs. 2 and 3 compare the predicted and measured internal spectra, collected from the space-flown (STS-63) and ground irradiated GSO detectors respectively. The contribution to the total predicted spectrum from the primary and secondary radiations has been quantified in Fig. 2. This figure shows the residual trapped proton flux is responsible for -50% of the observed activation 17 hours after flight, with secondaries (principally neutrons) from cosmic-ray and trapped protons inducing most of the remainder. The net predicted spectrum in Fig. 2 is in relatively good agreement with that measured - the poor statistics in the latter are due to the short duration of

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Detector

rotons GSO 6.St 9.7 5.9

0.39'* 6.2 4.0 2.5

Normalisation factors [cm']

1 GeV I Cosmic-ray I Cosmic-ray I Trapped

I I I I

For mono-energetic irradiations, proton beam was coaxial to crystal. For space radiation sources, an isotropically incident particle flux is assumed.

Note this is for a 2.5cm x 2.Scm crystal - others are 5.lcm x S.lcm.

the Shuttle flight. Peaks, identified based on the prediction, arise from the decay of Tb, Gd, and Eu radionuclides originating from proton interactions with gadolinium. Tb nuclides from (p,xn) reactions feature particularly strongly. For trapped protons, these nuclides are produced in similar

I02

10 1

10-1

Total prediction Residual trapped protons Residual cosmic rays Secondanes from cosmic rays Secondanes from trapped protons

, . -.- ._.__,. .; 'I

0 5 1 .o I .5 2.0 Energy (MeV)

Fig. 2: Measured intemal spectra from CREAM 5.lcm x 5.lcm GSO detector 17 hours after STS-63 touchdown, compared with predictions.

Energy (MeV)

Fig. 3: Predicted (dashed line) and measured intemal spectra from S.lcm x 5.lcm GSO detector irradiated by lGeV protons.

Energy [MeV]

Fig. 4: Predicted (dashed line) and measured internal spectra from 2.5cm x 2.5cm BaFz detector irradiated by 300 MeV protons.

quantities to their Gd isobars, but are not favoured for higher- energy proton spallation (1 GeV or cosmic-rays).

The background spectra measured from the proton-beam irradiated GSO are also in good agreement with those predicted (Fig. 3), and identities for some of the peaks have been provided based on the model output. For early spectra, the continua above approximately 1 MeV are a factor of 2-3 higher, suggesting that the contribution from short half-life p- emitters has been over-estimated.

Fig. 4 presents the predicted and observed spectra from the proton-irradiated BaF2 crystal. At high energies, the p- continua again appear to have been overestimated, but very good agreement between the simulation and measured data is seen below -1 MeV, permitting peak identification. As expected from Table 111, many of the features in the spectra arise from barium spallation products.

V. CQNCLUSIONS Both the CREAM and proton-beam experiments are

permitting essential validation of simulation codes and techniques, which may ultimately be applied to the materials selection process to optimise background in future instruments.

ACKNOWLEDGMENT Flights of CREAM on Shuttle have been sponsored by the

US Ballistic Missile Defense Organization and organised by the USAF Space Test Program. Short-term post-flight activation analysis was performed using facilities at the University of Florida, Gainesville, and the assistance of Emmett Bolch, Thabet Tolaymat and George Harder is gratefully acknowledged. Proton beam facilities at the Paul Scherrer Institute were provided through collaboration with Len Adams and Bob Nickson of ESTEC, and with the enthusiastic assistance of Wojtek Haydas of PSI. Irradiations

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at the Laboratoire National Saturne were performed in collaboration with the Max Planck Institute, Mainz, and the assistance of Johannes Briickner and the staff of MPI and LNS is gratefully acknowledged.

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[3] P.R. Truscott and C.S. Dyer, “LEO radiation environment measurements made by the CREAM experiment on Space Shuttle mission STS-56”. Defence Evaluation and Research Agency Technical Report DRA/CIS(CSC3)/TI!/94/1, June 1994.

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[5 ] D.M. Sawyer and J.I. Vette, NSSDCNDC-A-R&S, 76-06, NASA- GSFC report TMX-72605, 1976.

[6] N. Hammond, EDS .Defence Ltd Report C309270/S-09/1, 1996.

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