Gamma rays in the MeV region at balloon altitude1

K. OKUDAIRA AND Y. HIRASIMA Department of Physics, Rikkyo University, Tokyo, Japan

Received June 21, 1967

Gamma rays in the MeV region were observed from balloons at h = 26" N on September 29, 1966. A scintillation counter constructed with two NaI ?A in. x 2 in. qj crystals separated by 1 cm x 2 in. @ lead was used to measure the directional distribution of the gamma-ray flux. This counter was flown at an atmospheric depth of 14.2 g cm-2. As the response of each crystal of this counter depends on the direction of incidence of the gamma rays, an anisotropic distribution of gamma rays gives rise to a difference between the counting rates of two crystals. It was ascertained from the observation that albedo gamma rays from the lower atmosphere are predominant a t this high altitude. The deviation from the calculated values of the dif- ference in counting rate assuming only atnlospheric gamma rays nlay be due to an extrater- restrial origin of part of the gamma-ray flux. For the measurement of the gamma-ray spectrum, a phoswich counter ( 1 in. x 1 in. qj NaI crystal surrounded by %-in.-thick plastic scintillator) was flown to 10 g cm-2. Though the main part of the gamma-ray flux is probably due to atmospheric gamma rays, an upper limit for the isotropic cosmic gamma-ray flux is deduced to be ( 1.25 F 0.05) X 10-2 counts c111-%-l sr-1 MeV-1 in the energy rangc 1.2-3.1 MeV.

I. DIRECTIONAL DISTRIBUTION O F GAAslMA-RAY FLUX

A scintillation counter for the measurement of the directional distribution of gamma rays was constructed with two NaI(T1) crystals of % in. x 2 in. & size and lead of 1 cm x 2 in. I#J size, called a "two-crystal counter". As shown ill Fig. ~ ( L I . ) , the lead was inserted

P b , -.. . , , PIa~tic Scintlllotor

1/4" Thlck

I / ~ ' ' x 2" 4 Plastlc Scintillator

I rrn X q" .A N o 1 I ' ' X I ' ' @

(0) TWO-Crystal Counter (b) Phoswich Counter

FIG. 1. TWO types of scintillation counters. ( a ) Two-crystal counter for the measurenlent of the gamma-ray directional distribution. ( b ) Phoswich counter for the nleasurenlent of the energy spectrum of gamma rays.

between two NaI(T1) crystals. The response of each crystal of this counter d e ~ e n d s on the

I

incident direction of gamma rays. A curve showing the directional response for photo- peak detection of lSCs gamma rays (662 keV)

1Presentecl at the Tenth International Conference on Cosmic Rays, helcl in Calgary, June 19-30, 1967, OG-28A.

INCIDENT DIRECTION 8

FIG. 2. The directional response for hotopeak detection of 137Cs gamma rays (66% keV7 by one crystal of the two-crystal counter.

by one crystal is shown in Fig. 8. The energy calibration was performed using 13?Cs gamma rays. Also, the discrimination level of energy loss in each crystal was determined by com- paring the counting rate of 13iCs gamma rays with the pulse-height spectrum shape of these

Canadian Journal o f Physics. Volume 46. S-LO-k (1068)

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0KUD:IIRA ASD HIRASIMA: G .AMMA RAYS IN MeV REGION 5495

137Cs gamma rays. The discrimination levels of energy loss in the crystals are 440 keV and 400 keV for NaI(T1) A and NaI (Tl ) B respec- tively.

The balloon was flown to ceiling altitude (atmospheric depth 14.2 g ~ m - ~ ) at geomag- netic latitude X = 26" N on September 29, 1966, and after level flight for 110 min de- scended gradually to 49 g ~ m - ~ at the time of separation of a gondola from the balloon. The independent counts of two crystals and coincidence counts were transmitted by a 298.1-Mc/s FM-FM telemeter. Coincidence events were eliminated from the counts of the single crystals as being due to charged par- ticles. The ratios of counts obtained from the two crystals are shown in Fig. 3. The counter axis was vertical, with NaI(T1) A in the upper position, for 150 min after the start of level flight. Next, the orientation of the counter axis was changed by 90" to horizontal. The count ratio diminished gradually owing to the difference between the heat capacity of the NaI(T1) crystal and the heat capacity of a circuit inserted to compensate for any tem- perature effect. At the time of the change of orientation, a jump of the count ratio was observed. This jump indicates the difference in count ratios of two crystals in vertical and horizontal axis orientations. When the counter axis is horizontal, the incident gamma-ray fluxes from sides A and B are equal, and the

Count Ratio N a l A

Orlentotlon Change A B

I I

0 . 4 l o l o ,ao .a0 :lo I

Mia Time

FIG. 3. Observed count ratio of the two-crystal counter. The jum at the change of orientation indi- cates the count iifFerence of the two crystals with vertical orientation of the counter axis.

Atmospheric Depth gcm-'

Ph3'j\ rr

R counting Rate o f ( Na Upper) I A

R' counting R a t e of No I B ( Lower)

0 5 10 15 20 9 cm -'

Atmospheric Depth

FIG. 4. Calculated ratio of flux from upper side of the atmosphere to that from lower side, and calculated count ratio of the two crystals. Also the comparison between the calculated count d8erence of the two crystals and that observed.

difference between counting rates depends on the difference between pulse-height discrimi- nation levels. Therefore the count ratio in the vertical orientation just before the orientation change was normalized by dividing it by the count ratio in the horizontal orientation just after the orientation change.

Atmospheric gamma rays which enter the counter at an atmospheric depth <20 g ~ m - ~ were calculated, assuming that the gamma-ray production rate per unit air mass density is uniform and the gamma rays are attenuated exponentially by air. The calculated ratio of the flux from the upper side to that from the lower side is shown in Fig. 4. Also, the count ratio and the count difference calculated using this flux ratio and the observed directional re- sponse of the counter shown in Fig. 2, and the observed count differences during the flight, are shown in Fig. 4.

As the counts of the crystal in the upper position were less than those in the lower

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S496 CANADIAN JOURNAL O F PHYSICS. VOL. 46. 1968

position, it was ascertained that albedo gamma rays from the atmosphere below are pre- dominant at these high altitudes where the atmospheric depth is comparable with or less than the attenuation mean free path of gamma rays. Consequently, it might be pos- sible to shield a counter effectively from atmospheric gamma rays by putting shielding matter only beneath the counter. The observed difference of the count rate from that calcu- lated assuming only atmospheric gamma rays to be present may be due to an extraterres- trial origin of some of the gamma rays.

11. ENERGY SPECTRUM OF GAMMA RAYS 10%

Another scintillation counter, a spectrom- eter for the measurement of the energy spectrum of gamma rays, was constructed with a "phoswich counter", a 1 in. x 1 in. $I NaI (TI ) crystal surrounded by %-in.-thick plastic scintillator, as shown in Fig. l ( b ) . The surrounding plastic scintillator was used for elimination of charged particles. As a charged particle penetrates the plastic scintil- lator it produces a faster flash than the NaI (Tl ) flash; pulses accompanied by fast pulses were eliminated. A test of the effectiveness of elimination of charged particles was per- formed using p mesons at sea level. Th,e energy spectrum of gamma rays in the region 1.2-3.3 MeV was measured. For the energy calibra- tion, O°Co gamma rays (1.33 MeV) and 24Na (1.37 MeV, 2.75 MeV) were used. NaI(T1) flashes caused by gamma rays were analyzed by a balloon-borne 16-channel pulse-height analyzer. The counts in each channel were stored for 7.5 s in a four-stage binary storage, and then were read out for 0.5 s. The counts were transmitted by telemetry.

The balloon flight was carried out at a ceiling altitude of atmospheric depth 10 g cm-' at h = 26" N, following the flight of the preceding two-crystal counter on September 29, 1966.

The altitude dependence of the gamma-ray counting rate for energies greater than 1.2 MeV is shown in Fig. 5. The transition maxi- mum of 2.0 counts cm-%-l at 100 g ~ m - ~ was observed. The mean absorption length is 195 g cm-2 for pressures from 270 to 830 g cm-'.

Atmospheric Depth

FIG. 5. Altitude dependence of the countin, 0 rate of gamma rays with energies greater than 1.2 MeV found by the phoswich counter.

The energy spectrum of gamma rays in the MeV region has been observed on a number of occasions (Metzger et nl. 1964; Peterson 1965; Rocchia et al. 1966; Frost et al. 1966; Peterson et al. 1966). The present data on the gamma-ray spectrum in the 1.2-3.3 MeV energy range are plotted in Fig. 6, together with data obtained by Metzger et al. and Peterson on gamma rays in space. Our values at the top of the atmosphere were extrapo- lated from the observed values at 10 g cm-', using the calculated altitude dependence. Our data were obtained at a low geomagnetic latitude ( h = 26" N) . In our past work it had been found that on account of the latitude effect, the atmospheric gamma-ray flux at low geomagnetic latitude (26" N) is 1/2.5 to 1/3 of the flux at high latitudes (40-50" N ) . Though most of the observed gamma-ray flux is probably due to atmospheric gamma rays, we can consider it to be an upper limit for the isotropic cosmic gamma-ray flux of extra- terrestrial origin. This upper limit of intensity

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OKUDAIRA AND HIRASIMA: GAMMA RAYS IN MeV REGION S497

in the 1.2-3.1 MeV energy range is (1.25 ' O F

2 z U I ~

N E

Y : In + !=

3 o U

12

2 10

0 Ours ( 0 g 1 0.05) x 10-2 counts cm-"-l sr-l MeV-l. ?---I Ours ( 0 g cm" )

%. 0 Metzger e t al. ACKNOWLEDGMENTS

(Ranger III ) The authors wish to thank Professor S. - Peterson Nakagawa for his constant interest and en- . --- . a, ( O S O I ) couragement, and are to Professor J.

Nishimura and the balloon group of the Insti- - tute of Space and Aeronautical Science, Uni- + versity of Tokyo, for the balloon flights. The

- assistance of Mr. T. Yamagami is appreciated. - O. e - b REFERENCES 4 , FROST, K. J., ROTHE, E. D., and PETERSON, L. E.

@ 8 0 1966. J. Geophys. Res. 71, 4079. - @+ METZGER, A. E., ANDERSON, E. C., VAN DILLA,

M. A., and ARNOLD, J. R. 1964. Nature, 204, - 766. - PETERSON, L. E. 1965. Presentation to 6th COSPAR

International Space Symposium, Buenos Aires; -1 ' " L ~ " I I I I 1 , t I 1 Space Res. 6 ( 1966).

1 0 I lo MeV PETERSON, L. E., SCHWARTZ, D. A., PELLING, R. M., Photon Energy and MCKENZIE, D. 1966. J. Geophys. Res. 71,

FIG. 6. Energy spectrum of gamma rays at the 5778. top of the atmosphere. The data were extrapolated R o c c ~ ~ , R., LABEY-, J., DUCROS, G., and BOCLET, from the observed values at 10 g cm-2. D. 1966. Proc. Intern. Conf. Cosmic Rays, Lon-

don, 1, 423.

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