T H E F U T U R E OF B A L L O O N S I N

C O S M I C - R A Y R E S E A R C H

B. PETERS Danish Space Research Institute, Lyngby, Denmark

Abstract. In spite of the availability of satellites, the possiblity of increasing basic knowledge of cosmic radiation with the help of balloons has not diminished. New detection techniques have made it pos- sible to enter new areas of study which had been closed earlier.

The use of stratosphere balloons in the study of primary cosmic radiation has been re- sponsible for most of the progress made during the past 25 yr.

In spite of the availability of satellites, the possibility of increasing basic knowledge of cosmic radiation with the help of balloons has not diminished. New detection techniques have made it possible to enter new areas of study which had been closed earlier.

The main areas which have been opened up are (1) Extension of the study of the charge spectrum to include elements with abun-

dances 103-106 times lower than those studied until now. This includes all elements between iron and uranium (and possibly beyond).

(2) Study of the isotopic composition of the elements already identified in the primary radiation.

There are, of course, other important areas of cosmic ray study where balloons are inadequate and satellites are necessary, such as

(3) Investigation of the energy range above the geomagnetic sensitive region (ener- gy spectrum, chemical composition, positron-electron ratio, search for anti-nuclei).

(4) Investigations in the very low energy region, where even a small amount of residual atmosphere cannot be tolerated (chemical and isotopic composition, positron and electron spectra, modulation effects).

(5) Interplanetary intensity gradients. (6) Monitoring the emission of solar 'cosmic' rays. Nevertheless, the problems listed under (1) and (2)justify an intensification of effort

in balloon instrumentation for reasons which are quite general and can be formulated as follows:

Galactic cosmic ray nuclei represent the only sample of matte, r available to us which does not belong to the solar system. Its uniqueness in this respect is not likely to change in the foreseeable future.

Leaving aside information derived from cosmic rays, present knowledge of the com- position of matter in the universe is based entirely on electromagnetic radiation emit- ted or absorbed by atoms and molecules.

This source of information, although enormously rich and diversified, is, neverthe- less, deficient in at least two major respects:

Space Science Reviews 13 (1972) 313-318. All Rights Reserved Copyright �9 1972 by D. Reidel Publishing Company, Dordrecht-Holland

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(a) It can, (within present technology), provide no information on isotopic abun- dance ratios for any but the lightest elements. Even for light elements (below oxygen) differences in isotopic composition between various sources are detectable only when they are quite drastic.

This is a serious shortcoming because, from the point of view ofnucleo-genesis, stellar and galactic evolution, and cosmology, the mass number of a nucleus is at least as important as its proton number, if not more so.

(b) Virtually all matter whose chemical composition can be analysed through its electromagnetic radiation represents a mixture with widely different histories, with many sources of origin, distant both in space and time. Since the epoch when it was synthesized, the material has very likely been subjected to a large variety of processes giving rise to transformations and differentiation. This, of course, is a source of great difficulty for the interpretation of abundances in terms of well defined physical processes.

Cosmic rays are presumably tied more directly to processes of nucleo-synthesis, perhaps to very definite phases of nucleo-synthesis in well defined objects. Their ana- lysis provides therefore an as yet hardly explored source of basic data for the inter- pretation of stellar, galactic, and cosmological phenomena.

Although presumably the galactic cosmic-ray nuclei arrive fairly directly from re- gions where great energies are released and where complex nuclei are synthesized and destroyed, some modifications must have occurred in composition during their pas- sage through the interstellar gas. The physical processes involved in these changes can now all be studied in accelerator laboratories, so that interpretation of observed details in chemical and isotopic composition, in particular possible variations of abundances with energy, can be connected reliably with various models of cosmic particle acceleration and of diffusion of high energy particles in the galaxy. This is of great value for developing realistic models.

Since the energy Of cosmic radiation in our galaxy represents one of the most im- portant fractions of the total galactic energy, the origin and history of cosmic rays are inseparably connected with most other basic problems in astrophysics.

At present there is practically no reliable information on the isotopic composition of cosmic radiation and only very little on any but the most abundant elements (num- bering less than twenty).

1. The Study of the Abundances of Rare and Heavy Elements (Z > 26) with the Help of Balloons

One finds oneself here in approximately the same situation which confronted cosmic- ray physicists a little over 20 years ago in the study of elements lighter than iron. Then one was plagued by low flux, strong atmospheric absorption at balloon heights, lack of reference particles for absolute charge calibration and a precise knowledge of the various sources of error which affect the measurements. Now as then, one is confined to visual detectors, emulsions and plastics, which after appropriate chemical processing reveal tracks whose appearance and thickness is strongly charge dependent.

THE FUTURE OF BALLOONS IN COSMIC-RAY RESEARCH 315

Potentially the method is capable of resolution, adequate to distinguish neighbour- ing elements, and in some cases, at very low energy, even distinguish between particles of different mass and equal charge. At this moment one is far from having realized this performance, but progress is being made. With this rapidly evolving technology a comparatively quickly arranged balloon experiment is economically far superior to a satellite experiment, especially since recovery of the detector is necessary and therefore the longer exposure time of the satellite can be exploited only partially.

The electronic instruments which have been developed so far for measuring the charge of heavy ions have insufficient resolution to distinguish neighbouring ele- ments; their embarkation on satellites can as yet give only indications of the charge spectrum beyond the iron peak, rather than definitive results.

2. Isotopic Analysis of Primary Cosmic Rays with Balloons

(a) Low energy region (< 1 GeV/nucleon) - There exist at present two techniques, beginning to approach the perfection necessary to distinguish the mass of particles of a given atomic number.

One is the well known so-called E/dE/dx method used until now only to distinguish particles with different atomic numbers. The method consists in measured ionization (which is a function of charge Z and velocity fl) vs. range in the detector (which is a function of charge, velocity and mass). This method can be extended to include particles which do not actually come to rest in the detector, but nearly so, such that the increase in energy loss towards the end of the range can be used to obtain the range by extrapolation. For the determination of velocity, ionization may be replaced by other measurable quantities, such as time of flight or (~erenkov radia- tion.

In the best of such measurements the plots E vs dE/dx reveal, apart from separation of particles according to charge, indications of fine structure, due to the presence of different isotopes in the same charge component.

In the second method, applicable in the low energy region, the energy measurement is replaced by a measurement of momentum (deflections of particle trajectories in a known magnetic field). The difficulties of producing adequate magnetic fields above the atmosphere are considerable but soluble according to the latest communications from Berkeley. The precision in the velocity measurements necessary to derive from the measured deflection in the magnet, the particle mass, is adequate at present only in the energy region which overlaps and perhaps slightly exceeds thalL where the much simpler E/dE/dx method can be used.

For both these methods the stratosphere balloon is the natural vehicle. Both of them can be used for low Z; their resolution deteriorates rapidly with increasing charge (and increasing energy).

(b) At higher energies (2 < e < 50 GeV/nucleon) isotope analysis can be carried out with the help of the geomagnetic field by a difference method, which virtually elimin- ates the need to know the shape and strength of that field. One aspect of this method

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was first discussed by Cowsik et al. (1970). A more thorough discussion can be found in Lund et al. (1971).

It is applicable for use in balloons as well as in satellites, each vehicle having its advantages and disadvantages.

We outline below the method, using as illustration a particular experiment which will be carried out on NASA's High Energy Astronomical Satellite HEAO-B jointly by Centre d'Etudes Nucl6aire de Saclay and the Danish Space Research Institute in Lyngby.

One measures the following parameters for each galactic nucleus incident on the instrument:

(a) The charge Z (b) The momentum/nucleon, p (which is a function of velocity only) (c) The place of observation and (d) The direction of incidence.

Z and p are obtained from pulse heights produced by the particles in two (~erenkov counters of different refractive index.

The place of observation is given by the space craft coordinates. The direction of incidence is obtained by measuring the coordinates of each track by

means of layers of flash tubes, placed between the counters, and combining these meas- urements with the known attitude of the space craft axes at the time of observation.

These data can be used to obtain the isotopic composition of the various charge components in the manner described below:

We recall first that, (1) The rigidity spectra of all galactic cosmic-ray components, measured so far,

have been found to be equal within error of measurements, for magnetic rigidities > 3 GV/c. (For the purpose of discussing the principle of the experiment we shall

assume that they are strictly equal). (2) At a given magnetic rigidity the flux ratio of different cosmic-ray components is

an invariant, i.e. does not change if the particles move through a static magnetic field (e.g. the geomagnetic field).

From (1) and (2) it follows that, except for constant factors, spectra of all primary components arriving at a given place in the magnetosphere from a given direction are identical when plotted against any function of rigidity, ~ =pM/Z, where M is the atomic weight and Z the atomic number of the primary component.

Since this relation is valid for the rigidity spectra measured at any geographic loca- tion in any particular arrival direction, it remains valid if one adds spectra observed in different directions and different geographic locations, provided the time of observa- tion and the detection efficiency are the same for each primary component.

The problem of determining the abundance of various isotopes in a particular ele- ment of atomic number Z, reduces then to comparing its momentum/nucleon spec- trum with the spectrum of a reference element Z0, obtained under identical experi- mental conditions.

The practical problem of how best to superimpose spectra obtained at different geographical positions and from different directions is of some importance for satel- lite experiments (which cover data from regions of widely varying magnetic cut-off

THE FUTURE OF BALLOONS IN COSMIC-RAY RESEARCH 317

rigidities). In balloon experiments, however, this question is less important, and one may e.g. add the spectra over the entire opening angle of the instrument, even though particles from different parts of the sky correspond to different rigidity cut-offs as- sociated with different penumbra regions.

The differential spectra, showing number of observed particles in different loga- rithmic momentum/nucleon intervals when normalized to equal tolLal area, have the following property:

The higher the mass to charge ratio of a particular component, the more its spec- trum is displaced towards the low momentum range. For different charge groups the relative displacement depends only on the following parameters:

(a) The mass to charge ratio of the stable isotopes of the particular element. (These are known).

(b) The relative abundances of these various isotopes. (These are the quantities to be determined in the experiment).

(c) The parameters relating the true momentum/nucleon of a particle with the probability distribution of the signals received from the corresponding measuring in- strument. (These parameters can be obtained by calibration.)

The relative displacements do not depend on the nature of the magnetic field which the particles have traversed before being registered.

We cannot here go into the details of how the isotopic abundances (b) are related to the measured relative displacements of the spectra. This will be the subject of a forthcoming publication.

Here we only assert, that the presently obtainable accuracy in momentum/nucleon, @/p, is adequate for obtaining abundance ratios in the indicated energy range with precisions corresponding to N0.I neutron masses in the elements mean mass. Fur- thermore, that under certain conditions which are realizable in praxis, 6pip will be in- versely proportional to atomic number, up to Z ~ 26, so that the obtainable mass re- solution is approximately independent of atomic number for elements in the range 4~<Z~<26.

It can furthermore be shown that in order to obtain this precision also from the point of view of statistics, the number of particles in the momentum/nucleon spectra to be analyzed must exceed (for the case of only two stable isotopes)

( l+r~ z ( l + r ) z

N>\6r-r J log(Mi/Mzf 1 + l o g z '

where r is the abundance ratio of the light to the heavier isotope, 6r is the desired precision in the value of r and ? is the exponent of the power law which is the closest fit to the interplanetary rigidity spectrum. The parameter z (for z > 1) is given by:

(1 + r) log (0M/OS) 2

log(M1/M2)

Here Qu and Qs are the cut-off rigidities corresponding to the main (all white) and St6rmer (all black) cones and Ms, M2 the masses of the two isotopes.

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The number of particles required for a given statistical precision depends therefore on log 2 ( M a / M 2 ) ~ (A /~ /2Z) 2 where A/~ is the difference in neutron number of neigh- bouring isotopes. I t is lower, if the heavier isotope is the abundant one (r < 1) than if the lighter one dominates (r > 1). Finally it depends only logarithmically on the width of the penumbra region.

Assuming a penumbra width of 40% (i.e. ~ t / ~ s = 1.4), ~ = 1.5 and r - ~ 1, one finds

TLi ~-~ 8

ZVe = 35

I f the desired statistical precision in r is 6 r = 10%, one finds as an order of magnitude

f o r r = 1 f o r r ~ 1

NLI > 104 > 103

NFe >~ 105 ~> 104

Such statistical precision is obtainable only in fairly large balloon instruments ( ~ l m 2 sr) or with satellites, but even more modest instruments will give important information on isotopic composition since ~ r is approximately inversely proportional to the linear dimensions of the instrument.

A balloon experiment is of course confined to particular momentum cut-off regions. An important advantage is however, that the latitude can be chosen such that the cut- off momenta lie close to the (~erenkov counter thresholds, i.e. the region where their momentum resolution is highest.

These then are new fields of research in galactic cosmic radiation where stratosphere balloons will play a decisive role for many years to come.

R e f e r e n c e s

Cowsik, R., Lund, N., Pal, Y., and Peters, B.: 1970, Phys. Letters 31B, No. 8, pp. 553-556. Lund, N., Lundgaard Rasmussen, I., and Peters, B.: 1971, 'A Method for Determining the Mean

Atomic Mass of the Elements in the Primary Cosmic Radiation Throughout the Latitude Sensi- tive Part of the Spectrum', Cosmic Ray Conference Papers, 12th International Conference on Cosmic Rays, 16-25 August 1971, Hobart, Tasmania. p. 130, Vol. 1.