The atmospheric and leakage flux of neutrons produced in the atmosphere by cosmic ray interactions

EARTH AND PLANETARY SCIENCE LETTERS 4 (1968) 393-398. NORTH-HOLLANDPUBLISHING COMP., AMSTERDAM

THE ATMOSPHERIC AND LEAKAGE FLUX OF NEUTRONS PRODUCED IN THE ATMOSPHERE BY COSMIC RAY INTERACTIONS * G.BOELLA, C.DILWORTH, M.PANETTI and L.SCARSI ** Istituto di Scienze Fisiche dell'UniversitY, Milano, Italy Gruppo G.I.F.C.O. del CN.R. Received 4 June 1968

The curve of the integrated neutron flux in the "thermal" to 20 MeV energy range, as a function of the atmospheric depth from sea level up to 0.1 mbar, has been determined, assuming as energy spectrum that predicted by Newkirk. The instrument employed is a combination of a hydrogeneous moderator with a slow neutron detector, with anticoincidence rejection of background events. Four balloon flights have been carried out in March 1966; one rocket flight in July 1966. With this flight the curve has been extended to 0.1 mbar from 6 mbar, attained with the balloon flights. The neutron flux as a function of the atmospheric depth is given, at 5.3 GV geomagnetic rigidity, in quiet Sun conditions. The albedo neutron flux has been measured: (0.42 + 0.04) neutrons cm- 2 sec-1. The mean free path for absorption in the atmosphere is: L = (165 + 5) g cm -2. The maximum intensity has been found at (90 + 10) g cm- 2 residual pressure.

1. INTRODUCTION The aim of the present experiment was to establish, at intermediate latitudes, a curve of the integrated neutron flux in the "thermal to ~ 20 MeV" energy range, as a function of atmospheric depth, from sea-level to a fraction of a millibar residual atmospheric pressure. Such a curve would then permit the evaluation, from a measurement at a given atmospheric depth, of the neutron flux at all heights and in particular of the albedo flux. In this way the variation of the albedo flux in time and in function of solar activity could be monitored by means of balloon sounding without recourse to direct measurements in rocket launches. This complete sounding has been obtained by a combination of four * This research has been sponsored in part by the Air Force Cambridge Research Laboratories through the European Office of Aerospace Research, OAR, United States Air Force, under Contract AF - 61 (052)-808. ** Now at Istituto di Fisica, University of Palermo.

balloon flights [7] and of a rocket launch with expulsion and parachute descent of the neutron counter. The balloon experiment was carried out in March 1966 at Aire sur l'Adour (vertical geomagnetic rigidity cut-off = 5.3 GV [1] ); the rocket launch at Salto di Quirra (vertical geomagnetic rigidity cut-off = 6.9 GV [1]) in July 1966. The choice of a different location for the balloon and the rocket sounding was imposed by the necessity of using the launching bases available at the time of the experiment. In principle a change in the neutron energy spectrum could occur in the upper region of the atmosphere (~< 100 g/cm2), as a consequence of the difference in the geomagnetic rigidity cut-off; this would render not directly comparable the counting rates obtained at the two locations with a counter with an energy dependent sensitivity curve. On the other hand, the absence of a detectable variation of the counting rate ratio observed in the ( 1 0 - 1 0 0 ) g/cm 2 range suggests that, in the limit of the experimental errors, the same neutron energy distribution can be assumed for Aire sur l'Adour and Salto di Quirra. Under this assumption,

394

G.BOELLA, C.DILWORTH,M.PANETTIand L.SCARSI

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2. THE EXPERIMENTAL METHOD 2.1. The neutron counter The neutron counter employed in this experiment [7] is similar in principle to the type used by our group in previous investigations on Cosmic Ray neutrons [3,4,5] and is based on the coupling of a hydrogeneous moderator with a slow neutron detector; the useful sensitive range extends from thermal energy up to about 20 MeV. The slowed down neutrons are revealed by a boron-plastic scintillator [6], chosen because of its small bulk, high efficiency for neutron detection and low efficiency for gamma rays and minimum ionizing particles. The main difference between this counter and the previous ones lies in the system of suppression of the charged-particle background. Whereas previously the neutron contribution was obtained by subtraction of the counting rate of two symmetrical channels, differing only in the isotopic composition of the boron, in this case the charged particles contribu-

tion to the counting rate is directly rejected by means of the anticoincidence of a polystyrene scintillator used in phoswich combination with the boron plastic. Apart from the obvious technical advantages of using a single counter instead of two which have to be maintained identical, this method of suppression reduces the statistical uncertainty inherent in the subtraction method, which is particularly felt in the higher reaches of the atmosphere. Moreover the response of the polystyrene scintillator gives directly the variation of the intensity of the total charged component. A schematic diagram of the counter is given in fig. 1. Two independent counting channels are used here to increase both the counting rate and overall reliability of the system. Each scintillator phoswich is composed of a disc of boron-plastic scintillator, 100 mm in diameter and 200/am thick, coupled to a polystyrene scintillator disc of the same diameter and 8 mm thick. The neutron moderator is represented by the common polyethylene cylinder, with the same diameter as the scintillators and height 30 mm, together with the lucite light guides and the polystyrene scintillators. Details on the circuits and the detector construction technique are given in ref. [7].

ATMOSPHERICAND LEAKAGEFLUX OF NEUTRONS 2.2. Calibration o f the counter Three identical units of the neutron detector have been built; they are identified as Counters I, II and III. All three of them have been used for the balloon flights; counter I has been afterwards employed successfully in the rocket experiment. The calibration has been carried out with three neutron sources of different neutron energy: with them the angular response of the instrument has been determined. The average sensitivity to an isotropic flux has been obtained from integration of the response curve over 47r. Details on this calibration are given in ref. [7]. 2.3. Local neutron production in the detector The detector counting rate due to neutrons locally produced by interactions of the nuclear active cosmic ray component in the material constituting the detector itself has been evaluated as a function of atmospheric depth, in the condition of solar minimum and 5 GV vertical geomagnetic rigidity cut-off. Details of the calculation are given in ref. [7], together with the curve showing the contribution from local production to the detector counting rate as a function of atmospheric depth.

3. EXPERIMENTAL RESULTS 3.1. The rocket experiment The sounding of the upper atmosphere, up to 0.1 mb, has been made using one of the counters of the balloon experiment (Counter I) as a payload for a Belier rocket~ The essential requirements of the experiments were: a) measurement free from the background of local production neutrons from the rocket body; b) exposure time at the various atmospheric depths sufficiently long to provide a statistically significant counting rate curve. These requirements were met by programming the rocket flight with a sequence calling for the expulsion of the counter from the rocket nose cone and the deploying of a 45 m 2 parachute, 14 seconds after reaching apogee at an altitude "~ 68 km * * The release system has been developed by the Sud Aviation Co,, under an ESRO contract.

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Fig. 4a. Ratio of the neutron counting rates obtained in the balloon flights at Aire sur l'Adour to that observed in the rocket experiment, as a function of atmospheric pressure. 4b. Ratio of the charged particles counting rates in the balloon flights at Aire sur L'Adour to that observed in the rocket experiment, as a function of atmospheric pressure. 4c. Temperature by thermistor as a function of atmospheric pressure in the rocket flight. nose cone, the temperature variations on this counter certainly did not exceed those registered by the thermistor in the centre. Fig. 3 gives the neutron and charged particle counting rates for channel A, as a function o f atmospheric pressure, obtained with the rocket launch. Fig. 4a and 4b shows the ratio of the neutron and charged particle rates obtained in the balloon flights at Aire sur l'Adour [7] to that observed in the rocket experiment for the ( 6 - 1 0 0 ) mb atmospheric pressure range; both ratios remain constant through the interval, the average values being (1.42 ± 0.05) for the neutrons and (1.24 ± 0.01) for the charged particles. In the 50 minutes employed by the rocket package to cover the (0.1 - 1 0 0 ) mb interval, the total temperature variation indicated by the thermistor was 6°C (fig. 4c), corresponding to a relative change of less than 5% in the counter channel A sensitivity.

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397

3.2. The neutron flux To transform the neutron counting rates to absolute flux values at 5.3 GV geomagnetic cut-off rigidity, the neutron energy spectrum proposed by Newkirk [2] has been used; this energy spectrum has been shown to be in good agreement with the experimental results, at 5 GV [3]. Fig. 5 gives the curve of the integrated n e u t r o n flux corrected for local production, between thermal energy and 20 MeV, as a function o f atmospheric depth, in the interval ( 0 - 6 0 0 ) mb; fig. 5 gives also the details for the interval ( 0 - 2 0 ) mb. In table 1 are listed the neutron flux values for selected atmospheric depths. Table 1 Neutron flux as a function of atmospheric depth. Atmospheric depth (mb)

Neutron flux (neutrons cm- 2 sec- 1)

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0.31 2.14 3.40 4.10 4.45 4.10 3.30 2.65 1.70 1.22 (0.42 + 0.04)

Aire sur l'Adour - vertical geomagnetic rigidity cut-off = 5.3 GV - March-July 1966. The mean free path for absorption, obtained from the flux values between 200 mb and 600 mb residual pressure, isL = (165 + 5) g cm - 2 . The m a x i m u m

Table 2 Summary of albedo neutron flux values obtained from balloon flights by the rocket data. Geomagnetic vertical cut-off

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march 1966

(0.42 + 0.04) cm- 2 sec-1

398

G.BOELLA, C.DILWORTH, M.PANETTI and L.SCARSI

intensity corresponds to (90 -+ 10) g cm - 2 residual atmosphere. In a previous experiment [3] the albedo neutron flux integrated in the "thermal energy - 2 0 MeV" range, at the geomagnetic vertical rigidity cut-off 4.3 GV, was estimated by extrapolation of the counting rate curve o f a series of balloon flights carried out both in Milano (5.1 GV) and in New Mexico (4.3 GV), the curve being normalized to the New Mexico data at maximum counting rate. These data can now be used, on the basis of the complete curve determined in the combined rocket-balloon experiment, to obtain the variation of the albedo flux between the cut-off rigidities 4.3 GV and 5.1 GV in 1963, and the time variation o f the flux at 5 GV between 1963 and 1966. The data are given in table 2.

ACKNOWLEDGEMENTS We are most grateful to Prof. G.Occhialini for guidance and criticism in the course of these experiments. We wish to thank the ESRO, Sud Aviation Co.

and Centro di Ricerche Aerospaziali (Rome) teams for their assistance in the preparation of the rocket payload, the CNES staff at Aire sur l'Adour for the balloon flights and the officers and men of the Salto di Quirra base for their generous and friendly help on the occasion of the rocket launch.

REFERENCES [ 1] M.A.Shea, D.F.Smart and K,G.McCracken, ]. Geophys. Res. 70 (1965) 4117. [2] L.L.Newkirk, J. Geophys. Res. 68 (1963) 1825. [3] G.Boella, G.Degli Antoni, C.Dilworth, M.Panetti, L.Scarsi and D.S.Intriligator, J. Geophys. Res. 70 (1965) 1019. [4] G.BoeUa,G.Degli Antoni, C.Dilworth, G.Giannelli, E. Rocea, L.Searsi and D.Shapiro, Nuovo Cimento 29 (1963) 103. [5] G.Boella, G.Degli Antoni, C.Dilworth, G.Pizzi, L.Scarsi and M.Tagliabue, Nuovo Cimento 37 (1965) 1232. [6] K.H.Sun, P.R.Malmberg and F.A.Pecjak, Nucleonics 14 (1956) 46. [7 ] R.Ballerini, G.BoeUa, A.Igiuni, P.Inzani and L.Scarsi, submitted for publication to Nuovo Cimento.