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Produced by the NASA Center for Aerospace Information (CASI)
(NASA-CR- 1 5j.9) A MERCURIC DETECTOR SYSTEM N84-16095

FOR X-RAY ASTR?UNOMY. 2. RESULTS FROM

PLIGHT TESTS OF A BALLUUN bUhNE INSTRUMENT

(Massachusetts Inst. of Tech.) 30 p U.nclas

4C A03/MF A01 C:SCL 03A G3/89 11383

A MERCURIC DETECTOR SYSTEM FOR X-RAY ASTROKOMX:

II. RESULTS FEOM FLIGHT TESTS vF A BALLOON BORNE INSTRUMENT

J.V. Vallerga, R.K. Vanderspek, and G.R. Ricker

Center for Space Research

Massachusetts Institute of Technology

Cambridge, MA 02139

z

Y .

CSR-HEA--82-46

ABSTRACT

t To establish the expected sensitivity of a new hard X-ray telescope

design, described in Ricker, Vallerga and Wood (Paper 1), an experiment was

conducted to measure the background counting rate at balloon altitudes (40 km

of mercuric iodide, a room temperature solid state X-ray detector. The

prototype detector consisted of two thin mercuric iodide (HgI ) detectors

surrounded by a large bismuth germanate (Bi 4Ge301 ) scintilla? or operated in 's

anticoincidence. The bismuth germanate shield vRoed most of the background

counting rate induced by atmospheric gamma-ra rs, neutrons and cosmic rays.

A balloon-borne gondola containing a prototype detector assembly was

designed, constructed and flown twice in the spring of 1982 from Palestine,

Texas. The second flight of this instrumen4 established a differential

background counting rate of 4.2 t 0.7 x 10 --' cma keV over the

energy range of 40 to 80 keV. This measuremient was within 50% of the

predicted value. The measured rate is 'u5 times lower than previously achieved

in shielded NaI/CsI or Ge systems operating in the same energy range. The

prediction was based on a Monte Carlo simulation of the detector assembly in

the radiation; environment at float altitude. Details of the simulation can be

found in Paper .

r

This-work is supported in part by NASA grant- NSG-7339 and NAGW-14.

Y

.fit _
2

! 1.0 INTRODUCTION

In order to test and confirm detector models used in the previous paper

(paper I), a small balloon-borne gondola containing a prototype detector for a

hard X-ray diffraction telescope was designed, constructed and flown at

balloon altitudes. Its purpose was to measure the background counting rate at

the eventual telescope operating environment (>40 km altitude, balloon or

satellite). The radiation environment (X- and gamma rays, high energy charged

particles, neutrons) at high altitude is quite different from that at sea

level. Atmospheric shielding and ground radiation have much greater effects

v near sea level. In this paper, the measurements at high altitude are compared

^ with predictions made by a detector Monte Carlo simulation (paper I). Once

the simulation is checked, the design parameters (geometry of detectors,

IN shielding components and discriminator settings) can be varied in the

simulation to optimize (lower) the expected background counting rate.

The amall gondola used in these experiments can be conceptually divided

into two subsystems: the prototype detector consisting of a bismuth germanate

shielded mercuric iodide detector (paper I, fig. 2) described in Section 2;

and the detector support systems consisting of electron;.--s, power systems, and

data recording, a:.l described in Section 3. Section 4 gives the results of

the background measurements at sea level and at balloon altitudes.

2.0 Prototype Detector System

i The overall design goal for the Hard X-ray Diffraction Telescope (HXDT)

described in Paper I is to minimize the number of background events in the

energy range of 40-80 keV while keeping the response to concentrated signal

X-rays as high as possible. The proposed concentrator requires a detector

with an area approximately equal to the size of its focal "spot" (-1 cm for a

lithium fluoride concentrator optimized for 40-80 xeV). The detector must

w
3

also have a field of view larger than the solid angle subtended by the

concentrator to "see" all diffracted signal X-rays (a-.12 steradian for the

HXDT concentrator). The active anticoincidence shield should surround the

entire detector except for the entrance aperture to be most effective at

vetoing non-aperture radiation. Passive mass viewed by the detector (e.g.,

structural material such as aluminum) should be minimized because secondary

radiation generated in this mass might not be vetoed.

The prototype detector system consists of two HgI 2 detectors mounted back

to back and operated in anticoincidence. The two detectors are placed inside

a bismuth germanate (Bi4 Ge3 012 or "BGO") scintillating shield along with two

hybrid charge sensitive preamps. The BGO shield is also operated in

anticoincidence with the main (top) detector. The axially symmfir,ric "well"

containing the detectors is gas-tight (.5mm Al entrance window) to retain a

pressure of one atmosphere at balloon altitudes. Such a gas-tight enclosure

eliminates the problem of high voltage corona breakdown encountered wt low

atmospheric pressures and avoids the need for potting the detector assembly.

2.1 Primary Mercuric Iodide Detector

The "main" HgI2 detector that will eventually be used to detect X-rays

from a celestial X-ray source is the top detector in the back to back geometry

(Fig. 1). The HgI 2 detector used in the test flights has an effective area of

0.8 cm (defined by the size of the palladium electrode) and is 390 microns

thick. The bottom detector has the same area and is -i(,00 microns thick. The

bottom detector's main purpose is to provide active shielding from radiation

generated in the passive material underneath (printed circuit boards and

preamps) as well as better protection from aperture charg , .d particle flux.

The HgI 2 detectors are separated by a thin (1311m) mylar insulator and mounted

on a Rexol.ite substrate. The HgI 2 crystal: are grown and then fabricated into
4

tae "back to back" units by EG&G, Inc., Santa Barbara.

The charge sensitive preamps are mounted on paper based printed circui

boards and are held in place underneath tke bottom detector by a low mass

stainless steel structure. A glass epoxy Lased printed circuit board was

avoided because of the high concentration of potassium 40 in natural glass

(ref. 1, pg. 775). The preamps were based on a low power design used for th

i SAS-C X-ray Satellite` and were modified and hybridized by EG&G, Las Vegas.

They were placed, close to the detectors to minimize input capacitance and

noise pickup. These compact, self-contained preamps were chosen because of

their proven reliability End availability. In future designs we will consid

placing only the input field effect transistor (FET) in the well to reduce

the passive mass. Small copper wires bring preamp power, high voltage, and

ground connections dower into the well to the detector assembly and the top a

bottom detector signals out of the well. The well is lined with electricall

grounded aluminum (.25 mm thick) to provide electromagnetic shielding.

RC filters for the high voltage and preamp power are mounted outside the wel

and the field of view of the detectors but inside the pressure container.

2.2 Bismuth Germanate Shield

Bismuth germanate was chosen as the shield material because of its very

high photoelectric cross section and density. 3 In terms of its stopping pow

it can almost be considered "active lead". The crystal is 7.5 cm in diamete

and 12.5 cm long with a machined 7.4 cm deep x 2.5 cm diameter axially

symmetric well in which the HgI 2 detectors are placed. The outside surface 

the crystal is coated with reflective paint and encased in aluminum to prote

the cr.-stal and prevent light leaks. A Ha.mmamatsu R1307 photomultiplier is

mounted on the unpainted bottom of the BGO crystal opposite the well (paper 

fig. 2) and surrounded by a mu metal shield to protect it from magnetic fiel

i

Y
5

induced gain changes.

At the time when the BGO was ordered from Harshaw Chemical Co., the 7.

cm diameter was the lar vest size available. The 2.5 cm inner diameter of th

well was determined by the preamp size and ease of construction. In the

energy range where BGO has the smallest attenuation coefficient (°1 to 3 MeV

non-aperture gamma rays that intersect the HgI 2 must traverse at least 2.5 c

of BGO (-1 attenuation length). At lower and higher energies the attenuati

is much greater. The top HgI 2 detector is placed 2.6 cm deep into the well

measured from the top of the BGO. The BGO then acts as the collimator- with 

flat topped response and an effective circular field of view of -.12

steradians.

With such a large field of view the diffuse atmospheric and cosmic X-r

background would dominate the background count rate_ For the diffracting

concentrator concept to work, X-rays that can reach the HgI 2 detector direc

through the aperture must be stopped so that only diffracted X-rays from a

celestial X-ray source are detected. The "direct" (undiffracted) field of

view must be blocked using a graded shield of lead, tungsten, and tin. The

lower atomic nu=mber materials are includr!d to stop the characteristic K X-ra

of the higher. For the test flight of the prototype detector this passive

shield was placed directly over the entrance window though in actual use wit

a concentrator it would be further from the detector so as not to block the

diffracted X-rays. This is shown schematically in Fig. 2. The transmission

of 1 mm of Pb + 1 mm of W + 2.5 = of Sn is shown in Fig. 3.

3.0 Balloon Gondola and Associuted Electronics

The detector ?p rototype (BGO shield and HgI 2 de-cectors) is contained in 

small (30 cm x 30 cm x 45 cm) aluminum framed package along with its support

electronics. Power for the electronics and heaters is supplied by lithium

It" ri
6

batteries while the high voltage for the HgI 2 crystals comes from a charged

0.2 microfarad capacitor. The capacitor stays within 300 of its original

voltage for -30 hours because of the small leakage currents of the detector

cables, and the capacitors themselve-s. The capacitator is fully charged ,ju

before launch. This simple detector biasing method provides lower noise

levels than does a lo-.r-to-high voltage converuer. Low voltage experiment

power is turned on at approximately 6 km altitude by parallel pressure

switches operating a relay. This conserves the batteries and protects agai

electrical shorting at launch and landing.

Once power is turned on in flight, data collection begins. The main p

of the electronics Lakes random, asynchronous voltage pulses from the detec

preamp, converts these pulses into synchronous, 8 bit digital, words and rou

these words to an appropriate ma gnetic tape track. The remainder of the

electronics monitors the condition of the gondola (temperatures, voltages,

currents, etc.) and records this housekeeping data on a separate magnetic t

track. The gondola is self contained during flight (i.e., no telemetry, no

commands, no interface). However, it is essential that the cassette tape b

recovered!

3.1 Event Logic Electronics

Pulse height spectra are recorded for both top and bottom HgI 2 detecto

Output pulses from the BGO photomultiplier are fed into an Amptek A-101

logarithmic amplifier/discriminator which produces a digital pulse if the

charge pulse amplitude is above a set threshold. For the BGO shield, only 

integral count rate is recorded; no pulse height information is retained.

Small voltage pulses from both HgI 2 detector preamps are brought to pu

shaping voltage amplifiers (time constant -154s). The last stage of these

amplifiers is a "peak stretcher" which retains the maximum voltage attained 

^TM
7

the pulse and holds it at the input of an analog to digital converter until

reset. If the pulse height triggers the lower level discriminator, the pulse

height will be converted 22 useconds later to an 8 bit digital word. For the

top HgI2 detector, the event logic determines if an event occurred in the BGO

crystal or the bottom detector in the time interval from 13us before the top

event to 9 PS after. If no " ,v eto" event occurs during this interval, then th

8 bit digital word is shifted to the "valid" buffer for tape track #1. If a

"veto" event occurs in either tho BGO or the bottom crystal then the 8 bit

word is shifted into the "veto" buffer for tape track #2. A11 events

occurring in the bottom crystal are converted and shifted into the "bottom"

buffer for tape track #3- Once the shift into the buffer takes place, the

peak stretcher is reset and ready for a new pulse. Total time from event to

reset is approximately 50 nicroseconds: The event igic diagram is shovrn in

Fig. 4.

Because of the very low count rates in the HgI 2 detectors at float (-1

ct/sec) the dead times due to the finite conversion time are negligible. The

BGO count rate of 1000 cts/sec plus the veto flag duration of 22 microsecond

gives a 2% probability that a valid event will be ,i ,etoed by accident.

4.0 RESULTS OF BACKGROUND MEASUREMENTS AT SEA LEVEL

AND AT BALLOON ALTITUDES

4.1 Sea Level 3ackground

The radiation environment on the ground consists mostly of high energy

gamma rays due to the iecay of naturally occurring radioactive isotopes such

as potassium 40 and uranium and thorium daughter products. Primary cosmic

rays and neutrons are shielded by the atmosphere but secondary moons do reach

ground level. The gamma ray intensity is a strong function of the type of
8

building where the measurements take place. Wood frame buildings are usually

less active than concrete buildings due to the amount of radionuclides in san

and cement.

Tong term background spectra were accumulated with the prototype detecto

gondola in many locations and many configurations. Fig. 5 shows the

normalized count rates for spectra obtained in: Building 37, 5th floor, MIT;

a ground floor preparation room at the National Scientific Balloon Facility i

Palestine, Texas; and the iron cave in the basement of Building 2 at MIT.

Integrated count rates are compared in Table 1 along with the BGO shield coun

rate. The iron cave results provide an upper limit to the intrinsic

background of the prototype detector (background due to radioactive material

used in the construction of the detector ,%Ssembly itself) though extrinsic,

muons are more likely to be the cause of this background. The gamma, ray

contribution can be scaled with the bismuth K line observed in the veto

spectrum. Me veto ratio (the number of vetoed counts divided by the total

counts) increased in the iron cave as the bismuth line decreased. This would

be expected if the iron cave background is dominated by charged muons.

The effectiveness of the various shielding systems is shown in Fig. 6

where the HgI 2 valid spectra are shown with the BGO shield off and the bottom

HgI2 crystal off. The BGO shield does provide most vetoes, especially vetoing

the K ling of bismuth as expected. The bottom HgI2 does decrease the

integrated count rate by another factor of -2 in the range of 40-80 keV.

4.2 Energy Calibration

The 8 bit pulse height analyzer (0-255) was calibrated using gamma rays

from radioactive sources such as 145 PM

( 37 keV), 241 Am ( 59.5 keV) and 57Co

(122 keV). Unlike photomultipliers, solid state detectors are not strongly

affected by small changes in the bias voltage. The peak position of the 241 

P A
9

60 keV line feature moved less than one channel as the detector bias was

decreased from 300 volts to 200 volts, a drop expected in a 30 hour balloon

flight. An in-flight calibration source was not included on the gondola

because of this gain stability plus the fact that the bismuth K X-ray

calibration is already "built in" due to fluorescence of the BGO shield.

The voltage amplifier gain was adjusted to place the 59.5 keV line

approximately near channel 60 (in actuality channel 57, see Fig. 7). The

dynamic range then was from 15 keV to 265 keV, with all larger events ending

up in channel 255. The voltage amplifier was modified before the second

flight to give an approximately logarithmic gain above 120 keV though it

remained, linear below an energy of 120 keV. The complete dynamic range

extended from 10 keV to 670 keV.

The detector used for flight has a resoluti_n of 5.5 keV at 60 keV so t

quantization error of one channel/keV was negligible. This detector wa3

chosen for its stability and size and was operated at only 300 volts to

decrease leakage current "bursts" that mimic background counts. Some energy

resolution was sacrificed since the dominant background contributors of

interest produce a continuum of counts vs. ener&y.

4.3 Balloon Flights

1 The small prototype detector gondola was flown twice from Palestine,

Texas. The first time, flight 1282P, was a "piggyback" flight in which the

small gondola was strapped on the side of a large (1400 kgs.) payload flown b

Martin Israel's group at Washington University, St. Louis. The first flight

was launched at 12:25 UT, May 8, 1982. It reached float altitude at 14:48 UT

ry

at an altitude of 38 km (3.8 gm/cm` vertical atmospheric column density). Th

flight lasted 35.5 hours and was successfully recovered in Mississippi.

Analysis of the housekeeping data showed that the batteries began to fail
10

after 13 hours, probably due to a defective lithium cell since they were ex

ed to last 35 hol ir s .

Another problem observed in th- first flight was multiple triggering 

energetic events due to extreme saturation of the voltage amplifier, The

extra events had a characteristic signature of preferentially occurring wit

channel number of 4N-1. This effect was duplicated in the laboratory by

i

applying large pulses to the input of the Voltage amplifier as would have

occurred during flight if a very energetic particle interacted in the

detector. This was a small effect in the veto and bottom detector spectra 

it dominated the valid spectrum, increasing the count rate by a factor of 5

Two modifications were made to the electronics. Diodes were placed at the

input and on the feedback loops of the voltage amplifier so that large puls

from the preamps would only slightly saturate the last stage. In addition 

time constant was changed in the event logic so that the analog to digital

converter would not accept another pulse for 7504sec after it converted a

pulse. Despite this lock-out period, the additional dead time was negligib

(.07% for a count rate of -1 ct/sec). The change in the voltage amplifier

required a new energy calibration (see above).

The second flight, 1296P, was a solo flight for the small gondola usin

small (62x103 m3 ) balloon. The launch was at 12:00 UT on June 21, 1982 and 

float altitude of 40.2 km (2.8 gm/cm2 vertical atmospheric column density) 

reached at 14:01 UT. The successful 7 hour flight was terminated at 21:00 

and reached ground 43 minutes later -60 km Northwest of Pecos, Texas. The

payload struck a 130,000 volt power line, resulting in the parachute being

ripped apart and the payload free-falling the last 15 meters. Although its

Styrofoam thermal cover burned on the outside, the payload received only mi

° mechanical damage and the data tape was recovered intact. The free fall al

`M

't.vra_.Q, ^,,.°r^•^-^r

_ - ^rxsar^.r
11

demonstrated the ruggedness of the BOO shield which operated flawlessly upon

its return to the laboratory.

4.4 Flight Results and_Comparison to Predictions

The background spectra of flight 1282P for both H gI2 detectors, top and

bottom, are shown in Fig. 8. Onay the vetoed spectrum of the top detector i

presented because the valid spectrum was contaminated with spurious events d

to multiple triggering of the event logic by energetic particles. The spectr

from flight 1292P are presented in Figs. 9 and 10. The electronic

modifications made between flights were successful in suppressing the multip

triggering and the valid spectrum (Fig 10) is free of the single channel peak

All these spectra were integrated over time starting at float and endin

at flight termination (or power failure for flight 1282P). Arrival at float

altitude coincided with a leveling off of the count rate versus time, as show

in Fig. 11. The two maxima in the count rate vs. time plot are due to the

Pfotzer maximum, the altitude (-17 km.) at which the flux of cosmic ray

secondaries reaches its peak. The payload passed through this region slowly

on the way up and quickly on the way down.

The vetoed spectra for both flights are very similar in that they are

featureless except for the bismuth and possibly mercury K X-ray line features

The K beta X-ray of bismuth was resolved in the second. flight. The overa.Ll

count rate at float decreased by about 25 to 35% from the first flight to the

second. This occurred in both detectors, top and bottom. Changes this larg

can"ot be explained by the differences in altitude between the two flights.

To i::terpret this difference as being due to the altitude dependence of the

backgra+ind would require that the float altitudes of the two flights be

different by "6 km,when, in actuality, they only differed in altitude by 2 km

The only other differences between the two flights was the change in the even
12

logic electroni : s and .a absence of the adj acent 1400 kg. payload in the

second flight. Multiple triggering only accounted for a small part of the

higher count rate in the first flight based on the number of events in the

single channel peaks ( see Section 4.3). A massive payload above the detecto

as in flight 1282P, however, would produce an increase in the local radiation

flux due to interactions with the primary cosmic rays. If the massive payloa

is rQsponsible for the higher count rate in the first flight it would be

equivalent to an atmospheric slab of 9 gm/cm2 vertical column density.

As expected, the total number of valid background counts accumulated

during the second flight was small ( 31 counts, 40 to 80 keV) and the spectrum

is featureless. There are a few extra counts ( 6 count3 total) close to 59

keV. For a flight of this duration, 2.6 counts would be expected at this

energy from inelastic neutron scattering from the 1^7 I nucleus.

Unfortunately, the tungsten K-alpha X-ray energy is also 59 keV so this barel

significant feature might also be fluorescence of the passive graded shield o

a combination of both. Based on their timing signature (many simultaneous lo

energy events), the excess number of counts at very low energies (< 20 keV)

were found to be due to leakage current "bursts".

Table 2 contains the float background count rate predictions of Paper I

as well as the measured values. The valid count rate agrees surprisingly

well. The gamma ray prediction of the Monte Carlo simulation can be directl

tested against the strengths of the K alpha and beta line features of mercury

and bismuth in the vetoed spectrum. During flight 1292P, the measured value

was 25% smaller than predicted, which might indicate that the gamma ray

contribution was overestimated. This is also indicated in the energies above

160 keV. If the gamma ray prediction is scaled down to match the measured

results for the line features and the higher energies then the discrepancy at

^6
13

lower energies Would be enhanced. This iiscrepancy probably indicates that

the neutron contributions at lower energies were underestimated, which ",xi

easily be the case, considering the uncertainties in the neutron fluxes and

cross sections.

5.0 CONCLUSIONS

Previous to the research work described in these papers, it had been

conjectured that HgI2 might have great potential for use in hard X-ray

astronomy. The combination of high energy resolution at room temperature p

a high photon quantum efficiency seemed ideal for many balloon and satellite

detector applications. Yet, experience in field operations with HgI2 was

minimal and background count rate information obtained with an optimized

system at high Utitudes did not exist Development of HgI 2 specifically f

use astronomical X-ray detector over the last 3 years has changed thi

,AV_1-.,,tion (refs. 5 .,6,7,$). Procedures for handling and packaging detectors

have been developed that produce stable and dependable HgI 2 detectors for u

in the field. HgI 2 detector assemblies have been constructed and flown at

balloon altitudes and the detectors operated flawlessly on all occasions.

A major limitation to the application of HgI 2 in X-ray astronomy is t

inability to fabricate individual detectors larger than --1 cm 2 . A scheme t

overcome this limitation involves the use of X-ray concentrators to increas

the effective area of a shielded, low-background HgI 2 detector, (the HXDT; 

Paper I). The concentrator consists of a paraboloid shell lined with lithi

fluoride crystals that Bragg diffract on-axis incoming X-rays to the focu^ 

the paraboloid, where a small area detector is located. To establish the

expected sensitivity of this concentrator/detector design, the background

counting rate of the shielded HgI 2 detector at balloon altitudes had to be

determined
14

A balloon-borne gondola containing a prototype detector assembly was

designed and constructed to measure this background rate. This gondola was

flown twice in the spring of 1982 from Palestine, Texas. The second flight of

this prototype instrument established a background counting rate for a bismuth

germanate- Shielded mercuric iodide detector of 4.2 ±0.7 x 1005 counts -q / sec cm'

keV over the energy range of 40 -80 keV. This measurement was within 50% of

` the predicted value. The measured rate is -5 times lower than previously

6

found with shielded NaI/CsI or Ge systems operating in the same energy range.

The prediction was based on a Monte Carlo simulation of the detcc : tor assembly

'.n the radiation environment at float altitude (-40km). The Monte Carlo

simulation can now be used to investigate the detector design parameters

(geometry, discriminator levels, etc.) to attempt to lower the background rat

even further.

Based on a background rate of 4 . 2 x 10 cts/sec cm -5 ` keV, calculations

indicate that with 4 concentrators and 4 detectors systems a celestial X-ray

source with 1/7.:10 the intensity of the Crab Nebula could be detected in 1.5

hours. 'rhis excellent sensitivity is achieved by simultaneously reducing the

background by shielding a HgI 2 detector with BGO, and by increasing the

I

effective area using hard X -ray concentrators.

The prescription for reducing the background rate by a factor of -10 over

previous balloon-borne hard X - ray detectors can be summarized very simply:

` use very high quantum efficiency detectors shielded by dense, high Z active

scintillators. Because they are well suited to this experimental approach,

HgI2 and BGO are both likely to become major detector materials in X-ray

astronomy. Furthermore, their ease of handling and improves'•. performance at

moderately reduced temperatures 7 enhance their desirability for balloon and

I satellite use.

Y 
15

ACKNOWLEDGEMENTS

The authors wish to thank: Robert Kubara and the launch crew at the

National Scientific Balloon Facility for two highly successful balloon

flights; Wayne Schnepple, Carol Ortale and John Warren at EG & G, Santa

Barbara for assistance in crystal selection and testing; Andrez,j Dabrowski at

the Medical Imaging Group, U.S.C. for numerous helpful discussions. Mar-7

Israel kindly provided a "hitchhike" flight opportunity (flight 1282P). At th

Center for Space Research at MIT, we wish to thank John Doty, Bob Goeke, Dan

h Griscon, John Kruper and Pete Tappan for assistance at every stage of gondola

design, construction, and testing. This work is supported in part by NASA

grants NSG-7339 and NAGW-14.
REFERENCES

1. Knoll, G.F. 1979, Radiation Detection and Measurement (Wiley & Sons:

New York) .

2. Goeke, R. 1975, IEEE Trans. on Nucl. Sci., NS-22, 600.

3• Farukhi, M.R. 1978, Technical Paper No. TP16 REVA 0679, Harshaw

Chemical Co., Solon, Ohio.

4. Wood, D.R. 1982, "Effects of Atmospheric Gamma Rays on the

Background of a Bismuth Germanate-Shielded Mercuric Iodide

Detector: A Monte Carlo Analysis", Senior thesis, MIT.

5. Vallerga, J.V., Ricker, G.R., Schnepple, W. ezd Ortale, C., 1982,

I.E.E.E. Trans. on Nucl. Sci., NS-29, No. 1. p.151

6. Oga.wara, Y., Mitsuda, K., Vallerga, J.V., Cominsky, L., Kruper, J.,

Grunsfeld, J., Ricker, G.R., 1982, Nature, 295, p• 675

7. Ricker, G.R., Vallerga, J.V., Dabrowski, A., Entine, G., 1982, Rev.

of Sci. Inst., 53, , No. 5, P. 700

8. Vallerga, J.V., 1 01 82, "Studies of New Hard X-ray Detection Methods

and Observations of Cosmic X-ray Sources", Ph.D. Thesis, M.I.T.

+F

t

r^
Table 1

Measured Background Count Rate of BGO-Shielded

Top HgI 2 Detector for Various Surface Locations

Valid Count Rate2 (40-80keV) Veto Bismuthtt BGO

Location (cts/sec cm keV) Ratio K X-ray Count Rate

Balloon Lab 1.8x10 -5 91% 14x10-4 1231 cts/sec

MIT Bldg. 37

Balloon Base 1.0x10 -5 89% 5x10-4 791 cts/sec

Palestine, Tx.

Iron Cave .2x10-5 94% 1x10-4 527 cts/sec

MIT Bldg. 2

t Ratio of number of vetoed counts to total counts (veto + valid)

over the energy range 40-160 keV

tt Counts/sec cm observed in the vetoed spectrum
Table 2

Comparison of Measured and Predicted Count Rates at Float

in the Top HgI2 Detector for Flight 1292P

Valid Count Rate (10- 5 cts/sec cm2 keV)

Enemy (keV) Predicted Measured

4o-8o 2.9 4.2 +.7

80 -120 1.6 2.6 + .6

120-160 1.0 1.9 + . 5

160-200 1.8 1.5 •5

200-24o 1.7 1.0 + .4

Bismuth and Mercury K X-ray line featurett.

Predictedt Measured

3.6 x 10-2 cts/sec cm2 2.7 x 10-2 ets/sec cm2

t Sum of predicted background due to gamma rays and neutrons

(See paper I). The gamma ray contribution was determined by a Monte

Carlo simulation while the neutron contribution was calculated

analytically.

tt Present only in vetoed spectrum

0
Figure captions for Paper II:

Figure 1 Detailed view of mercuric iodide detector assembly showing the

components that fit in the axially symetric well inside the

bismuth germanate shield.

Figure 2 Schematic of the diffraction concentrator showing the passive

graded shields (lead, tungsten, and tin) stopping background

aperture X-rays that would normally impinge on the mercuric

iodide detector. Only on-axis X-rays that are diffracted by

the concentrator reach the detector.

Figure 3 Transmission of photons vs. energy through a graded shield of

lmm of lead, 1 mm of tungsten and 2.5 mm of tin. The lower Z

metals act to stop the fluoresced K X-rays of the higher Z

metals.

Figure 5 Sea level valid background spectra in top mercuric iodide

detector for various experiment sites.

Figure 6 Valid background spectra in top mercuric iodide detector

showing the effectiveness of the anticoincidence shields.

Figure 7 Actual mercuric iodide flight detector calibration spectrum

using a 241 Am radioactive source. The energy resolution at

300V (flight bias) was 5.5 keV FWHM at 60 keV.

Figure 8 Vetoed and bottom spectra from flight 1282P. The veto rate of

the top detector was .15 counts/sec over the energy range 40

to 80 keV. As can be seen, the energy resolution of the bottom

detector (used as an anticoincidence shield) was substantially

broader than the top detector.

Figure 9 Vetoed and bottom spectra from flight 1292P. Note the

decrease in count rate from the previous flight as well as

the resolved bismuth K-beta X-ray at 87 keV in the vetoed

spectrum.

Figure 10 Valid spectrum from flight 1292P. The measured background

rate was 4.2x10 s counts/sec cm 2 keV (40-80 keV). There is a

possible feature at 59 keV (see text). The increase in count

rate below the 40-80 keV range of interest was due to detector

leakage current "bursts" (see text).

Figure 11 Count rate vs. time of the bottom detector during flight 1292P

showing both ascending and descending passages through the

Pfotzer maximum and the steady count rate at float altitude.
Top Hg 12

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Hg 1 2

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ORIGINAL PAGE 9

OF POOR QUALI'T'Y(

H9I2 Flight Detector Calibration Spectrum

1500 . Bia® Voltage 300V

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9
20

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ENERGY (keV)

OF POOR QUALITY

FLIGHT 1282P MAY 8, 1982

20

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FLIGHT 1292P JUNE 219 1982

FIGURE 9

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ENERGY (keV)
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25 FLIGHT 1292P JUNE 21 9 1982

20

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r

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