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Composition of radiation trapped in the geomagnetic field at altitudes up to 1000 kilometers
COMPOSITION OF RADIATION TRAPPED IN THE GEOMAGNETIC FIELD AT ALTITUDES UP TO 1000 KILOMETERS FRANCIS E. HOLLY Air Force Special Weapons Center and RICHARD G. JOHNSON
Lockheed Missiles and Space Research Laboratory Abstract--Several methods are available for making measurements above the Earth's atmosphere. One of these is a "parasite" package which rides "piggy-back" on ICBM developmental flights. This paper discusses one such system in use by the Air Force Special Weapons Center for making measurements in the region of the lower Van Allen radiation belt. One such experiment and results presently available are discussed. Data from one phase indicate that the radiation penetrating 30 mg/cm2 is predominantly electrons. I. I N T R O D U C T I O N The Air Force Special Weapons Center has, for some time, been interested in measurements about the Earth's atmosphere. It is the purpose of this treatise to discuss one vehicle used in such measurements and one experiment, of several, made to determine the particulate composition of the lower Van Allen radiation belt. II. VEHICLE There are several types of systems which are suitable as instrument carriers in such a program, among these are: satellites, amphibiotic sounding rockets and the parasitic instrument package which uses some other rocket or missile as a carrier. The latter has been used successfully in several instances, O-3) using ICBM developmental flights as carriers. The disadvantages of such a system are obvious, one cannot select the launch time or the trajectory and the failure probability is greater than with a developed sounding rocket system. Among the advantages is the "free ride" to altitude. Within limits all necessary weight is available; however, with the development of the more exotic sounding systems such as Javelin/Journeyman and Scout 411
this is not an advantage singularly enjoyed by I C B M "parasite" vehicles. The AFSWC pod, a self-contained system, was designed for use on the Thor-Able system but was actually used exclusively on Atlas systems, weighs approximately 50 lb and is an approximately rectangular shape (Fig. 1) with outside dimensions of 6 × 12 × 24 in. Mounting is accomplished with a four point suspension to brackets already provided on the missile. Two points are used for attachment, and two points serve as supports against longitudinal forces and contain ejection springs, each of which are compressed to approximately 50 lb force at the time of installation. A one point attachment is desirable; however, in order to leave the central portion free from obstructions (as desired for placement of experiments) a two-point attachment must be utilized. Attachment is accomplished by means of two release bolts (Fig. 2). These bolts are presently piston-actuated, each piston being driven by a pair of squibs (one is sufficient) operating from a pre-set timer. The timers are started at lift-off by a pull-out lanyard system. The timers which are at the rear of the pod are connected in parallel. The timers and lanyard system are
412
FRANCIS E. HOLLY and R I C H A R D G. J O H N S O N
Fig. I. AFSWC instrumentation package.
Fig. 2. Ejection and release bolt mechanism.
shown in Figs. 1 and 3. Explosive bolts manufactured by Hercules Powder Company are under consideration for future programs. Telemetry for the present system consists of a time multiplexed eight channel capability on a single F M carrier. The power output of the power amplifier is 10 W which is fed to a quarter wave antenna attached to the top of the package.
Fig. 3. AFSWC instrumentation package, bottom view.
III. HISTORICAL BACKGROUND Several measurements have been made in the region of the radiation belts first discovered by Van Allen which support the hypothesis by Stormer and others that charged particles may be trapped in the geomagnetic field. Several theories have been advanced about the origin of these belts but, as yet, relatively little is known about the charge, mass and energy spectrum of the trapped particles. The measurements
RADIATION TRAPPED 1N THE GEOMAGNETIC FIELD described herein and elsewhere 11-4) seem to support the theory that the inner zone consists dominantly of electrons from beta-decay of "splash-albedo" fast and slow neutrons (endpoint energy 782 keV) and protons with energies up to several hundred MeV. Our measurements and those of Van Allen CS) indicate that the electron flux is at least three orders of magnitude greater than the proton flux. IV. EXPERIMENTAL TECHNIQUE Three instrumented packages (Figs. 1 and 3) were prepared each containing eight Geiger
413
In each of the packages at least one pair of counters were included which were identical in every respect (including orientation) except the entrance aperture of one was in a transverse magnetic field. These were counters A (2,3), B (1), C (3) and E (2). This magnetic field turns the effective collimator axis for electrons in the energy region of interest by more than 20°; the effective collimator axis for protons of energy great enough to penetrate the absorber is turned a negligible amount. In addition the overall electron sensitivity of these counters in an isotropic flux is lower because of reduced trans-
L ~i...
r
~.~,~.~ln
Fig. 4. AFSWC instrumentation package, close-up of Geiger.
counters shielded and collimated in various ways (Fig. 4). Both side and end window counters were used, the end window variety being collimated to half angles of nearly 6 ° by cylindrical entrance apertures. Table 1 shows the various amounts of shielding which were used and the associated effective electron and proton energy thresholds. Numbers in parentheses indicate which counters were contained in each of the three packages.
mission through the collimator, while the proton sensitivity remains the same. The velocity vectors of trapped radiation at low altitudes have been shown to be confined predominantly to a plane. C6) This, plus the fact that the instrument package rotates or tumbles in flight permits the use of data from such identical pairs of counters to distinguish between electron and protons (or other heavy particles) as follows: If the z-axis is taken along
FRANCIS E. HOLLY and RICHARD G. JOHNSON
414
the transverse magnetic field, then the xy-plane contains the collimator axes, separated by more than 20 ° for electrons but coincident for protons. Two simple cases may be considered. First, if the axis of rotation lies in the plane of the flux and the z-axis is parallel to the axis of rotation, each counter will have a peak counting rate
Table I. Counter
Shielding (mg/cmtz)
A (1,2,3)
1'7 Mica 30 Aluminum 10 Aluminum 30 Steel 36 Aluminum 150 Aluminum 400 Aluminum 600 Aluminum 2000 Brass 3000 Brass
B (1)
C D E F G H / J
(3) (1,2,3) (2) (1,2,3) (l) (2,3) (1) (2,3)
that the remaining particles are protons, a crude proton spectrum (for lower energies) may be determined. V. RESULTS TO D A T E On February 7, 1959 and July 21, 1959, packages 1 and 2 were carried aloft and released.
Counter Data
twice each revolution as it looks into the plane of the flux. The electron peak in the counter containing the magnet will occur approximately 20 ° out of phase with its twin counter but the proton peak occurs at the same time in both counters for electrons the peak rate in the counter containing the magnetic field will be smaller than in its twin because of reduced transmissions, while for protons the peak rates will be equal. Second, if the axis of rotation lies in the plane of the flux and the z-axis is perpendicular to the axis of rotation (or a line parallel to it), each counter will again have a peak rate twice each revolution (when xy-plane coincides with the plane of the flux), but the peak rates will occur at the same time in both counters for both electrons and protons; however, the ratio of peak rates in the two counters will still depend on whether the radiation is electrons or protons. Utilizing the data from these pairs of counters as well as the range spectra provided by the other counters, one can determine a fairly good energy spectrum for trapped electrons. Assuming
Electron Threshold i Proton Threshold (keV) : (MeV) 30 160 120 190 200 460 1000 1500 4000 6000
0-5 3.8 2.8 4.6 4.8 10.0 16.5 23 41 52
At this writing data from only the first have been analysed. Fig. 5 shows the trajectory of the instrument package after release as well as the altitude variation of the peak counting rate in several of the counters, the curves being normalized to unity. Because of the altitude variation of the iso-count contours at several latitudes the trajectory achieved maximum penetration into the radiation after reaching apogee. As shown in Table 1, this package contained counters of types A, B, D, F, G and L Two counters of type D had a relatively large active area and thus were saturated at the points of interest; however, they were helpful in determining package orientation. The remaining six counters provided the data to be discussed. Fluxes as high as 2.2 × 105 particles/cm~/sec ± 10 per cent and as low as 75 -k- 6 particles/ cm2/sec were encountered in counters A and I respectively. Type B counters were used in this package as the matched pair, and during maximum penetration of the radiation data were obtained, at various times, which closely approximated the
R A D I A T I O N TRAPPED IN THE G E O M A G N E T I C FIELD
415
I0~
SSC
90C
85C
SO(
75C
70C
|9
18
17
16 15 14 DESREES NORTH LATITUDE
13
12
II
Fig. 5. Package trajectory and radiation profile.
idealized conditions described in the previous section. A quantitative analysis yielded an '
I
'
l
~S50~BER SPECTRUM 9BOKM, I5°N LAT.
io6
radiation penetrated 150 mg/cm 2 of aluminum; therefore, no estimate of proton contribution to the radiation can be made from this data, except in the 30-150 mg/crn ~ interval. However, a qualitative look at relative counting rates provided by our data and that of White (~) and Rosen (3) leads one to suspect that the radiation which penetrates 400 mg/cm 2 of aluminum is predominantly protons. This is not in conflict with the Earth neutron albedo concept.
105
I04
b
\ o 1°3 _J
-_
\ g ,o2
-
upper limit of 3 per cent ( ÷ 5/-- 3 per cent) for the proton contribution to the radiation penetrating the 30 mg/cm ~ absorber. Calibrations were made with TL 2°4 and W 185 sources, the combined spectra of which closely approximated the observed radiation. Fig. 6 shows the integral absorber spectrum of the observed radiation. Less than 1 per cent of the
-
-. "'a~
Acknowledgments--The authors wish to thank L. Allen ioI - -
I tO-2
I0 3
~o
, ]
I
I
I0-1 I00 2 ABSORBER THICKNES$(GM-Cht )
,~o
i01
4~o
ELECTRON ENERGY( KEV )
17 23 P~OTON ENERGY(MEV)
41 52
75
Fig. 6. Neasured absorber spectrum (integral).
and J. A. Welch, Jr., of the Air Force Special Weapons Center, and R. D. Moffat, M. Walt and A. Meyerott of Lockheed Missiles and Space Research Laboratory for their assistance in instrument design and data interpretation. The authors also wish to express their appreciation to the Mathematics Branch of the Physics Division at Air Force Special Weapons Center for help in data reduction and machine calculations.
416
FRANCIS E. HOLLY and R I C H A R D G. JOHNSON REFERENCES
1. F. E. HOLLY and R. G. JOHNSON, Composition of Radiation Trapped the Geomagnetic Field at Altitudes up to 1000 Kilometers, USAF-AFSWC-TN 59-15. 2. S. R. WHITE and S. C. FREDEN,Protons in the Earth's Magnetic Field, UCRL-5581-T; also Phys. Rev. Letters 3, No. 9 (1959). 3. L. ROSEN et al., Los Alamos Scientific Lab., Flux and
Energy of Charged Particles at 300 and 600 Miles Altitude, private communication. 4. J. A. VAN ALLEN et al., Physics Dept., State Univ. of Iowa, J. Geophs. Res. 64, 217 (1959). 5. J. A. VAN ALLEN, private communication. 6. L. ALLEN et al., Angular Distribution of Van Allen Radiation with Respect to Geomagnetic Field, Postdeadline paper presented at Amer. Phys. Soc. Meet., Washington (April 1959).
Lockheed Missiles and Space Research Laboratory Abstract--Several methods are available for making measurements above the Earth's atmosphere. One of these is a "parasite" package which rides "piggy-back" on ICBM developmental flights. This paper discusses one such system in use by the Air Force Special Weapons Center for making measurements in the region of the lower Van Allen radiation belt. One such experiment and results presently available are discussed. Data from one phase indicate that the radiation penetrating 30 mg/cm2 is predominantly electrons. I. I N T R O D U C T I O N The Air Force Special Weapons Center has, for some time, been interested in measurements about the Earth's atmosphere. It is the purpose of this treatise to discuss one vehicle used in such measurements and one experiment, of several, made to determine the particulate composition of the lower Van Allen radiation belt. II. VEHICLE There are several types of systems which are suitable as instrument carriers in such a program, among these are: satellites, amphibiotic sounding rockets and the parasitic instrument package which uses some other rocket or missile as a carrier. The latter has been used successfully in several instances, O-3) using ICBM developmental flights as carriers. The disadvantages of such a system are obvious, one cannot select the launch time or the trajectory and the failure probability is greater than with a developed sounding rocket system. Among the advantages is the "free ride" to altitude. Within limits all necessary weight is available; however, with the development of the more exotic sounding systems such as Javelin/Journeyman and Scout 411
this is not an advantage singularly enjoyed by I C B M "parasite" vehicles. The AFSWC pod, a self-contained system, was designed for use on the Thor-Able system but was actually used exclusively on Atlas systems, weighs approximately 50 lb and is an approximately rectangular shape (Fig. 1) with outside dimensions of 6 × 12 × 24 in. Mounting is accomplished with a four point suspension to brackets already provided on the missile. Two points are used for attachment, and two points serve as supports against longitudinal forces and contain ejection springs, each of which are compressed to approximately 50 lb force at the time of installation. A one point attachment is desirable; however, in order to leave the central portion free from obstructions (as desired for placement of experiments) a two-point attachment must be utilized. Attachment is accomplished by means of two release bolts (Fig. 2). These bolts are presently piston-actuated, each piston being driven by a pair of squibs (one is sufficient) operating from a pre-set timer. The timers are started at lift-off by a pull-out lanyard system. The timers which are at the rear of the pod are connected in parallel. The timers and lanyard system are
412
FRANCIS E. HOLLY and R I C H A R D G. J O H N S O N
Fig. I. AFSWC instrumentation package.
Fig. 2. Ejection and release bolt mechanism.
shown in Figs. 1 and 3. Explosive bolts manufactured by Hercules Powder Company are under consideration for future programs. Telemetry for the present system consists of a time multiplexed eight channel capability on a single F M carrier. The power output of the power amplifier is 10 W which is fed to a quarter wave antenna attached to the top of the package.
Fig. 3. AFSWC instrumentation package, bottom view.
III. HISTORICAL BACKGROUND Several measurements have been made in the region of the radiation belts first discovered by Van Allen which support the hypothesis by Stormer and others that charged particles may be trapped in the geomagnetic field. Several theories have been advanced about the origin of these belts but, as yet, relatively little is known about the charge, mass and energy spectrum of the trapped particles. The measurements
RADIATION TRAPPED 1N THE GEOMAGNETIC FIELD described herein and elsewhere 11-4) seem to support the theory that the inner zone consists dominantly of electrons from beta-decay of "splash-albedo" fast and slow neutrons (endpoint energy 782 keV) and protons with energies up to several hundred MeV. Our measurements and those of Van Allen CS) indicate that the electron flux is at least three orders of magnitude greater than the proton flux. IV. EXPERIMENTAL TECHNIQUE Three instrumented packages (Figs. 1 and 3) were prepared each containing eight Geiger
413
In each of the packages at least one pair of counters were included which were identical in every respect (including orientation) except the entrance aperture of one was in a transverse magnetic field. These were counters A (2,3), B (1), C (3) and E (2). This magnetic field turns the effective collimator axis for electrons in the energy region of interest by more than 20°; the effective collimator axis for protons of energy great enough to penetrate the absorber is turned a negligible amount. In addition the overall electron sensitivity of these counters in an isotropic flux is lower because of reduced trans-
L ~i...
r
~.~,~.~ln
Fig. 4. AFSWC instrumentation package, close-up of Geiger.
counters shielded and collimated in various ways (Fig. 4). Both side and end window counters were used, the end window variety being collimated to half angles of nearly 6 ° by cylindrical entrance apertures. Table 1 shows the various amounts of shielding which were used and the associated effective electron and proton energy thresholds. Numbers in parentheses indicate which counters were contained in each of the three packages.
mission through the collimator, while the proton sensitivity remains the same. The velocity vectors of trapped radiation at low altitudes have been shown to be confined predominantly to a plane. C6) This, plus the fact that the instrument package rotates or tumbles in flight permits the use of data from such identical pairs of counters to distinguish between electron and protons (or other heavy particles) as follows: If the z-axis is taken along
FRANCIS E. HOLLY and RICHARD G. JOHNSON
414
the transverse magnetic field, then the xy-plane contains the collimator axes, separated by more than 20 ° for electrons but coincident for protons. Two simple cases may be considered. First, if the axis of rotation lies in the plane of the flux and the z-axis is parallel to the axis of rotation, each counter will have a peak counting rate
Table I. Counter
Shielding (mg/cmtz)
A (1,2,3)
1'7 Mica 30 Aluminum 10 Aluminum 30 Steel 36 Aluminum 150 Aluminum 400 Aluminum 600 Aluminum 2000 Brass 3000 Brass
B (1)
C D E F G H / J
(3) (1,2,3) (2) (1,2,3) (l) (2,3) (1) (2,3)
that the remaining particles are protons, a crude proton spectrum (for lower energies) may be determined. V. RESULTS TO D A T E On February 7, 1959 and July 21, 1959, packages 1 and 2 were carried aloft and released.
Counter Data
twice each revolution as it looks into the plane of the flux. The electron peak in the counter containing the magnet will occur approximately 20 ° out of phase with its twin counter but the proton peak occurs at the same time in both counters for electrons the peak rate in the counter containing the magnetic field will be smaller than in its twin because of reduced transmissions, while for protons the peak rates will be equal. Second, if the axis of rotation lies in the plane of the flux and the z-axis is perpendicular to the axis of rotation (or a line parallel to it), each counter will again have a peak rate twice each revolution (when xy-plane coincides with the plane of the flux), but the peak rates will occur at the same time in both counters for both electrons and protons; however, the ratio of peak rates in the two counters will still depend on whether the radiation is electrons or protons. Utilizing the data from these pairs of counters as well as the range spectra provided by the other counters, one can determine a fairly good energy spectrum for trapped electrons. Assuming
Electron Threshold i Proton Threshold (keV) : (MeV) 30 160 120 190 200 460 1000 1500 4000 6000
0-5 3.8 2.8 4.6 4.8 10.0 16.5 23 41 52
At this writing data from only the first have been analysed. Fig. 5 shows the trajectory of the instrument package after release as well as the altitude variation of the peak counting rate in several of the counters, the curves being normalized to unity. Because of the altitude variation of the iso-count contours at several latitudes the trajectory achieved maximum penetration into the radiation after reaching apogee. As shown in Table 1, this package contained counters of types A, B, D, F, G and L Two counters of type D had a relatively large active area and thus were saturated at the points of interest; however, they were helpful in determining package orientation. The remaining six counters provided the data to be discussed. Fluxes as high as 2.2 × 105 particles/cm~/sec ± 10 per cent and as low as 75 -k- 6 particles/ cm2/sec were encountered in counters A and I respectively. Type B counters were used in this package as the matched pair, and during maximum penetration of the radiation data were obtained, at various times, which closely approximated the
R A D I A T I O N TRAPPED IN THE G E O M A G N E T I C FIELD
415
I0~
SSC
90C
85C
SO(
75C
70C
|9
18
17
16 15 14 DESREES NORTH LATITUDE
13
12
II
Fig. 5. Package trajectory and radiation profile.
idealized conditions described in the previous section. A quantitative analysis yielded an '
I
'
l
~S50~BER SPECTRUM 9BOKM, I5°N LAT.
io6
radiation penetrated 150 mg/cm 2 of aluminum; therefore, no estimate of proton contribution to the radiation can be made from this data, except in the 30-150 mg/crn ~ interval. However, a qualitative look at relative counting rates provided by our data and that of White (~) and Rosen (3) leads one to suspect that the radiation which penetrates 400 mg/cm 2 of aluminum is predominantly protons. This is not in conflict with the Earth neutron albedo concept.
105
I04
b
\ o 1°3 _J
-_
\ g ,o2
-
upper limit of 3 per cent ( ÷ 5/-- 3 per cent) for the proton contribution to the radiation penetrating the 30 mg/cm ~ absorber. Calibrations were made with TL 2°4 and W 185 sources, the combined spectra of which closely approximated the observed radiation. Fig. 6 shows the integral absorber spectrum of the observed radiation. Less than 1 per cent of the
-
-. "'a~
Acknowledgments--The authors wish to thank L. Allen ioI - -
I tO-2
I0 3
~o
, ]
I
I
I0-1 I00 2 ABSORBER THICKNES$(GM-Cht )
,~o
i01
4~o
ELECTRON ENERGY( KEV )
17 23 P~OTON ENERGY(MEV)
41 52
75
Fig. 6. Neasured absorber spectrum (integral).
and J. A. Welch, Jr., of the Air Force Special Weapons Center, and R. D. Moffat, M. Walt and A. Meyerott of Lockheed Missiles and Space Research Laboratory for their assistance in instrument design and data interpretation. The authors also wish to express their appreciation to the Mathematics Branch of the Physics Division at Air Force Special Weapons Center for help in data reduction and machine calculations.
416
FRANCIS E. HOLLY and R I C H A R D G. JOHNSON REFERENCES
1. F. E. HOLLY and R. G. JOHNSON, Composition of Radiation Trapped the Geomagnetic Field at Altitudes up to 1000 Kilometers, USAF-AFSWC-TN 59-15. 2. S. R. WHITE and S. C. FREDEN,Protons in the Earth's Magnetic Field, UCRL-5581-T; also Phys. Rev. Letters 3, No. 9 (1959). 3. L. ROSEN et al., Los Alamos Scientific Lab., Flux and
Energy of Charged Particles at 300 and 600 Miles Altitude, private communication. 4. J. A. VAN ALLEN et al., Physics Dept., State Univ. of Iowa, J. Geophs. Res. 64, 217 (1959). 5. J. A. VAN ALLEN, private communication. 6. L. ALLEN et al., Angular Distribution of Van Allen Radiation with Respect to Geomagnetic Field, Postdeadline paper presented at Amer. Phys. Soc. Meet., Washington (April 1959).