The Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission spacecraft was primarily designed and calibrated for nuclear gamma ray line measurements, but also has a high energy mode which allows the detection of gamma rays at energies above 10 MeV and solar neutrons above 20 MeV. The GRS response has been extrapolated until now for high energy gamma rays from an early design study employing Monte Carlo calculations. The response to 50 to 600 MeV solar neutrons was estimated from a simple model which did not consider secondary charged particles escaping into the veto shields. In view of numerous detections by the GRS of solar flares emitting high energy gamma rays, including at least two emitting directly detectable neutrons, the calibration of the high energy mode in the flight model has been recalculated by the use of more sophisticated Monte Carlo computer codes. New results presented show that the GRS response to gamma rays above 20 MeV and to neutrons above 100 MeV is significantly lower than the earlier estimates

Chupp, E. L.

Cooper, J. F.

Forrest, D. J.

Kinzer, R. L.

Reppin, C.

Share, G. H.

English

NASA Technical Reports Server

474
SH 9.1-6
MONTE CARLO CALIBRATION OF THE SMM GAMMA RAY
SPECTROMETER FOR HIGH ENERGY GAMMA RAYS AND NEUTRONS
J. F. Cooper and C. Reppin
Max Planck Institute for Extraterrestrial Physics
D-80tt6 Garching, F.R.G.
D. 3. Forrest and E. L. Chupp
University of New Hampshire, Durham, N.H. 03824, U.S.A.
G. H. Share and R. L. Kinzer
Naval Research Laboratory, Washington, D.C. 20375, U.S.A.
1. Introduction. The Gamma Ray Spectrometer 11/ on the Solar Maximum
Mission spacecraft was primarily designed and calibrated for nuclear gamma
ray line measurements, but also has a high energy mode which allows the
detection of gamma rays at energies above 10 MeV and solar neutrons above
20 MeV. The GRS response has been extrapolated until now for high energy
gamma rays from an early design study /2/ employing Monte Carlo
calculations. The response to 50-600 MeV solar neutrons was estimated from a
simple model which did not consider secondary charged particles escaping into
the veto shields /3/. In view of numerous detections by the GRS of solar
flares emitting high energy gamma rays /_/, including at least two emitting
directly detectable neutrons /3,5,6/, the calibration of the high energy mode
in the flight model has been recalculated by the use of more sophisticated
Monte Carlo computer codes described in Section 2. New results presented in
Section 3 show that the GRS response to gamma rays above 20 MeV and to
neutrons above 100 MeV is significantly lower than the earlier estimates.
2. Monte Carlo Simulation of the GRS. The simulation of GRS response in
the high energy mode has required several stages of software development: (1)
representation of the principal materials and structures, both active and
passive, by combinatorial geometry routines, (2) propagation of primary
photons, and their secondary interaction products, through the GRS with the
Electron-Gamma Shower Code (EGS) /7/, (3) propagation of neutrons and their
secondary products with the High Energy Transport Code (HETC) /8/ and with
EGS, (4) calculation of charged particle scintillation efficiencies /9/, and (5)
the final selection of events satisfying pulse height logic criteria.
The combinatorial geometry routines give an accurate representation of the
GRS flight model /1/, consisting of seven (7.6-cm x 7.6-cm) Nal detectors in
an upper array and a large (24.2-cm x 7.6-cm) Csl detector, located 18 cm
below the upper array. These detectors are surrounded by an array of plastic
and CsI veto shields to reject charged particle events.
In the calculations the flight model is represented by fifty active "
scintillator or passive structure elements. These volume elements are
defined in terms of intersections or unions of solid cylinders, cylindrical shells,
hemispherical shells, and planar boundaries, in order to represent the upper
NaI and lower Csl detector crystals, photomultiplier tube assemblies between
the upper and lower detectors, the plastic and CsI shields, and the principal
supporting structures, including the metal housings for the active detectors
and shields. A significant improvement over the previous study [2[ is in the
treatment of empty interstitial spaces between the material volumes. Tracking
of Monte Carlo particles through all of these volumes and boundaries is
accomplished with common geometry routines in EGS and HETC.
https://ntrs.nasa.gov/search.jsp?R=19850026808 2020-03-20T16:50:15+00:00Z
475 SH 9.1-6
The compatibility of the tracking routines is particularly useful for the
simulation of neutron interactions, which produce secondary products requiring
two-stage tracking, first by HETC for nucleons and pions, and then by EG5
for secondary gamma rays from pion decay or the de-excitation of nuclei
excited by collisions. Interactions of nucleons and pions with target nuclei, and
the residual excitation energy emitted as gamma rays, are simulated with the
two-stage intranuclear-cascade and nuclear evaporation routines in HETC. A
low energy cutoff of 15 MeV is used for the propagation of nucleons and
pions in HETC, although the intranuclear cascade model, decomposing
interactions with nuclei into individual collisions with the constituent nucleons,
is not valid for collision energies below 100 MeV. EG5 uses cross sections
derived from analytical formulae or experimental data to simulate the
interactions of gamma rays above 100 KeV and electrons above _00 KeV.
Cumulative errors due to Monte Carlo statistics and the accuracy of the
interaction cross sections used in both HETC and EGS are about 10-20
percent.
The present calculations use only a rough approximation procedure for
energy deposition in the detectors by secondary neutrons produced at energies
below 15 MeV. The neutron response calculations have been run for two cases=
(1) the low energy neutrons produced below 1_ MeV deposit their full energy
locally via nuclear recoil or inelastic interactions, or (2) these neutrons escape
from the GRS without any further interaction. The final neutron responses are
the average of the results from these two cases, significantly different for
• _0-I00 MeV neutrons but much less so for higher energies.
The pulse height logic criteria for gamma ray or neutron events in the
high energy mode are the following: (l) 10-100 MeV electron-equivalent energy
deposited in either the upper Nal detector assembly or the lower Csl
detector, or in both, (2) no deposits greater than about 3 MeV in the annular
CsI side shields, and (3) no deposits greater than about 500 KeV in either the
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GpJ'II'IA RPtY ENERGY :£N rEv NEUTRON ENERGY :IN GEV
Figure 1: (A) 10-140 MeV gamma ray response. (B) 50-1000 MeV neutron
response. Nominal high energy matrix response with (solid curves) and without
(dashed curves) shield veto. Gamma ray response from /2/ with corrections for
the GRS flight model (boxes). Neutron response without shield veto and 100-
MeV cutoff from present work (diamonds) and /31 (circles).
476 SH 9.1-6
front or back plastic shields. Energy deposits in the upper and lower detector
arrays are pulse-height analysed separately with four channels below 100 MeV,
while deposits above 100 MeV in either array are not recorded due to preset
electronics limitations. The four channels correspond to electron-equivalent
energy deposits of 10-25, 25-40, _0-65, and 65-100 MeV. The GRS event logic
also records a two-dimensional high energy matrix which includes "mixed"
events triggering both the upper and lower detectors, as the result of
secondary charged particle and photon showers, and depositing total energies
up to 1_0 MeV. The total energy deposited in the matrix gives at least a
minimum measure of the incident particle's energy, while the fraction of
mixed events allows some discrimination between gamma rays and neutrons.
3. High Energy Gamma Ray and Neutron Response. Calculated results for the
total GRS responses to gamma rays and neutrons as functions of incidence
energy are shown respectively in Figures I(A) and I(B). The incident photons
or neutrons are perpendicularly incident onto the front surface of the GRS, as
would be the case for radiation of solar origin. The GRS response to high
energy gamma rays from off-axis sources has also been calculated and used
for in-orbit calibration and cosmic burst studies, the response being nearly
constant up to 30 degrees off-axis at all energies and falling off by up to a
factor of three at 90 degrees and 20 MeV.
The high energy matrix response for solar .gamma rays, shown by the solid
curve in Figure I(A), is heavily attenuated above 15 MeV by the vetoing of
events which produce shower electrons escaping through the active shields,
where these electrons trigger the side and bottom shields at about the same
rates. The new responses are ten percent below the earlier ones /2/ at 20
MeV, but the differences increase to thirty percent at higher energies. These
differences arise from the use of different instrument models and shield veto
criteria which produce increasingly divergent results above 20 MeV.
The strong forward showering of secondary photons and electrons preduces a
characteristic response in the GR5 to solar gamma rays above 20 MeV, where
these events contribute mixed events to the high energy matrix. At increasing
energies the mixed events increase in significance but are also associated with
high veto rates and low total response. The calculated ratios of mixed and
total matrix events are 0.01, 0.2, 0.6, and 0.4 at 30, 70_ lt_0_ and 200 MeV
respectively, decreasing above 140 MeV due to matrix energy limits.
In Figure I(B) the solid curve shows the large cumulative effect on the
neutron response from shield vetoes and the 100-MeV cutoff_ without which
the present and earlier /3/ calculations converge above 500 MeV. The relative
significance of shield vetoing alone is illustrated by the increasing difference
between the solid and dashed curves at higher energy. The large error bars
for the response to neutrons at 50-100 MeV arise from the low energy neutron
approximation discussed in Section 2. The neutron response is characterized by +
a relatively negligible fraction (less than 0.03) of mixed events, arising from
the physical separation of the upper and lower GRS detectors, and from the
relatively isotropic scattering of neutron interaction products.
4. Discussion. The new Monte Carlo calibration gives a significant
improvement of the earlier results /2/ for higher energy gamma rays
producing mixed events, the difference between the two calculations at 20
MeV being relatively negligible. Since average gamma ray energies in the high
energy GRS flares /4/ were less than 25 MeV in the impulsive phase_ the
integral fluences for those flares do not require revision. Analyses of flares
emitting higher energy gamma rays from pion decay /lO/ are highly dependent
477 SH 9.1-6
on the measured Iraction of mixed matrix events and require the new
calibration.
Aside from residual uncertainties regarding the newly calculated GRS
response to solar neutrons below a few hundred MeV, the important effects of
shield vetoing, the 100-MeV high energy mode cutoff, and details of the
secondary interaction product distributions, have otherwise been accurately
modeled for the first time. The earlier estimates /3J of GRS response to
neutrons, derived from simple cross section arguments without consideration
of these effects, greatly exceed the new ones at neutron energies above I00
MeV. At 20-100 MeY the earlier estimates should be used, since neutron cross
sections in the new HETC calculations have probably been overestimated at
those energies. The total matrix responses of the GRS with shield vetoing to
neutrons at 50, I00, 200, 300, 400, 500, and I000 MeV are then II, 40, 59,
50, 47, 44, and 24 cm2 respectively, and the corresponding ratios of the new
and old responses are 1.0, 1.0, 0.57, 0.36, 0.31, and 0.28 up to 500 MeV.
The in-flight GRS data provide some confirmation that the current
calibration of the high energy mode is reasonably accurate. In-flight
calibration data have confirmed the stability of energy deposit thresholds for
the shields, although thresholds at I0 MeV and above for the high energy
matrix are still uncertain by a few MeY. The gamma ray calibration has been
checked at 30-200 MeV by measurements of atmospheric gamma rays, as the
GRS points toward the earth's horizon during the night portion of its orbit,
which agree within 20-30 percent with comparable SAS-2 data /II/. For a
check of the neutron response, delayed neutrons from the 3une 21, 1980 solar
flare [3[ provide a relatively "clean" neutron source, uncontaminated by high
energy gamma rays emitted only in the initial impulsive phase. These neutrons
produced a very small fraction of mixed events in the high energy matrix
after background subtraction ]6J, thereby confirming the predicted response
from these calculations.
5. Acknowledgements. This work was supported partially by contracts
OIOKOIT"ZA/WSJWRK 0275:4 at the Max Planck Institute, NAS _-23761 at the
University of New Hampshire, and by NASA contract S.70926A at NRL.
References
1. Forrest, D. 3., et alL: 1980, Solar Physics; 65, 15.
2. Kinzer, R. L., et aL: 1977, Monte Carlo Simulation of the SMM Gamma
Ray Detector; unpublished report, Naval Research Laboratory, Washington,
D.C.
3. Chupp, E. L., et al.: 1982, Ap. 3. Lett._ _ L95.
4. Rieger, E., et al.: 1983, 18th Int. Cosmic Ray Conf. Papers; Bangalore,
338.
- _. Chupp, E. L., et ak: 1983, 18th Int. Cosmic Ray Conf. Papers_ Bangalore,
334.
6. Chupp, E. L., et al.: 1985, this conference, Paper SH 1.4-I.
7. Ford, R. L., and W. R. Nelson: 1978, The EGS Code System; Stanford
Linear Accelerator Center Rep. 210; Stanford University, Palo Alto, Ca.
. * "\ .
8. Chandler, K. C., and T. W. Armstrong: 1972, Operating Instructions for
the High Energy Nucleon-Meson Transport Code HETC; Oak Ridge
National Laboratory Rep. 4744_ Oak Ridge, Tenn.
9. Birks, 3. B.: 1964, The Theory and Practice of Scintillation Counting,
Pergammon Press_ Oxford.
I0. Forrest, D. 3., et al.: 1985, this conference, Paper SH 1.4-7.
II. Thompson, D. T., et "aL: 1981, 3. Geophys. Res._ 86, 126_.


Monte Carlo calibration of the SMM gamma ray spectrometer for high energy gamma rays and neutrons

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