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A measurement of the absolute energy spectra of galactic cosmic rays during the 1976–77 solar minimum
NW/. Tracks Radiat. Meas., Vol. 20, No. 3, pp. 415-421, 1992 ht. J. Radiat. Appl. Instrum., Part D
Pergamon Press Ltd
Printed in Great Britain
A MEASUREMENT OF THE ABSOLUTE ENERGY SPECTRA OF GALACTIC COSMIC RAYS DURING THE 1976-77 SOLAR MINIMUM J. H. DERRICKSON,* T. A. PARNELL,* R. W. AUSTIN,*W. J. SELIG* and J. C. GREooRYt *Space Science Laboratory, NASA Marshall Space Flight Center, Huntsville, AL35812, U.S.A. and TDepartment of Chemistry, University of Alabama in Huntsville, Huntsville, AL 35899, U.S.A. (Received
31 October
1991; in revised form 6 January
1992)
Abstract-An
instrument designed to measure elemental cosmic ray abundances from boron to nickel in the energy region 0.5-2.0 GeV nucl-’ was flown on a high altitude balloon from Sioux Falls, South Dakota, on 30 September through 1 October 1976 at an average atmospheric depth of -5 g cm-‘. Differential energy spectra of B, C, N, 0, Ne, Mg, Si and Fe, extrapolated to the top of the atmosphere, were measured. The float altitude exposure of 17 h ended near Alpena, Michigan. The flight trajectory maintained a north easterly heading out of Sioux Falls traversing the upper mid-west region between 84” and 97” west longitude while remaining between 43.5” and 45” north latitude. The maximum vertical cut-off for this flight path was 1.77 GV or 0.35 GeV nucl-’ (Shea and Smart (1975), Report No. AFCRL-TR-75-0185, Air Force Cambridge Research Laboratories).
abundances are compared to results from other experiments made during that era. The absolute intensities are compared with the University of Chicago telescope data taken on board the IMP-8 spacecraft during the period 1974-76 (Garcia-Munoz et al., 1977) and the University of Minnesota balloonborne Cosmic Ray Isotope Instrument System (CRISIS) detector launched from Aberdeen, South Dakota, on 20 May 1977 (Young et al., 1981).
1. INTRODUCTION
THE RECENT resurgence of interest in manned exploration of the Earth’s Moon and Mars has sparked a re-examination of the intensity of galactic cosmic rays. The U.S. Presidential Commission (Stafford et al., 1991) formed to study the “exploration initiative” strategy, cited two principal health hazards that needed further study: ionizing radiation (galactic cosmic rays and solar flare particles) and weightlessness, and the possible synergism between them. In examining the hazard due to galactic cosmic rays, there is a need to define more accurately the absolute intensities of particles at the lower end of the cosmic ray energy range throughout the solar cycle. This measurement of the galactic cosmic ray fluxes made in the fall of 1976 was contributed to a workshop held at the 22nd International Cosmic Ray Conference in Dublin, Ireland, in August 1991. An objective of the workshop, entitled “Cosmic Radiation: Constraints on Space Exploration”, was to provide a reliable data base of intensities of galactic cosmic radiation, particularly those during solar minimum periods. The present situation concerning prediction of radiation dose for interplanetary flight was described at the workshop by Adams et al. (1991). This paper describes a balloon-borne measurement of the intensity of heavy cosmic ray primaries made at the end of September during the 1976 solar minimum period. The smoothed sunspot number was 18 and the smoothed 2800 MHz radio flux was about 75. We describe the instrument, data analysis procedures, and results on absolute intensities of the more abundant species C-Fe. The measured relative
2. THE INSTRUMENT The instrument, shown in Fig. 1, contained eight gas-filled (Ar/CO,) multi-wire proportional counters (Parnell et al., 1973; Rizzo et al., 1974; Austin and Selig, 1974) divided into 4 x and 4 y planes with 104 anode wires per plane, the wires being spaced 0.5 cm apart. There were two discriminators per wire. The lower level discriminator was adjusted to detect 90% of muons during ground calibration and the upper discriminator was set at an ionization level 30 times higher, triggering between normally incident boron and carbon. The dual level discriminators allowed a separation of the cosmic ray primary tracks from the accompanying delta rays, removing most of the delta ray influence when determining the heavy primary track parameters. All high and low discriminators that were fired were telemetered in an “event” message of variable length. This hodoscope allowed identification of fragmentation by-products and shower particles, permitting efficient elimination of background events. The angular resolution of particle tracks was N 1” for carbon and -2” for iron. 415
J. H. DERRICKSON
416
et al.
these Cerenkov detectors was 14% which was corrected for in the data analysis. --_-__________________-___~~~__ A plastic scintillator (Ne-110) was included as an r-1 ionization-dependent fast trigger. The response of this scintillator viewed edge-on through adiabatic light guides was not very uniform and it was used only for triggering and consistency checks to reject background. The instrument was set to trigger on minimum ionizing lithium which resulted in a flight count rate of 3.4 events s-‘, which included showers and nuclear interaction events. A telemetry rate of 20 kbits s-l, and an average event length dependent on the number of set hodoscope wires, resulted in an average dead time of 22%. The geometric factor, 0 determined by the effective area of the gated detectors and the spacing between them, was 0.1505 m* sr. The I__. __________________________________ - - - - _ _ _ _ A net exposure for the data reported here was Flight 139%. Sioux Falls, SD.. Oct. 1976 6735 m* sr s. --._
--1
r:i
CT
-
CP
-
Teflon Cerenkov Counter Pilot 425 Cerenkov Counter
SC PCHl8.2
-
Ne 110 Plastic Scintillatar
-
ArC02 Filled Multi-wire Proporlonal
IClBP
-
X&H,
PMT
-
Photomultipller Tube
ALG
-
Adiabatic Light Guide
FIG. 1. The balloon-borne during the 1976-77
3. DATA ANALYSIS Counters
FIlled Ion Chambers
cosmic ray instrument solar minimum period.
flown
PROCEDURE
The data analysis procedure used to obtain the corrected elemental abundances at the top of the atmosphere from the raw data has been described in 0.5
0.55 0.6
0.8
1.0 1.5 2.0 GeVlnuc
The instrument included two 8.4 cm thick parallelplane pulse ion-chambers (Gregory, 1974; Gregory et al., 1981) filled with Xe-CH, for a larger signal, an extended primary charge range, and better charge resolution than with argon gas mixtures. The electric field over the active region varied by less than 1%. Two Cerenkov counters using sheets of Teflon and Pilot 425, were used to measure the energy over the ranges 0.45 to - 1.4 GeV nucl-’ and 0.32 to - 1.0 respectively. The analog-to-digital GeV nucl-‘, converters for this flight were dual range devices (IO: 1) in order to meet the overall dynamic range requirements of the experiment. Table 1 shows the average number of photo-electrons for singly charged relativistic particles (fi - 1) that was collected by a set of four RCA 4525 photomultiplier tubes (12.7 cm cathode diameter) viewing the Teflon Cerenkov radiator in a diffuse white box lined with millipore paper and BaSO, paint. The yield for the Pilot 425 Cerenkov radiator, also viewed by four 12.7 cm photomultipliers, is shown in the same table. The variation in response as a function of position for
Square Root of C, 0.5 0.55 0.6
0.0 .l.O ,I.5 '2.0 GeVlnuc
Table 1.Average number of photo-electrons produced in the Cerenkov counters
Muons Protons
at altitude
Teflon (neR= 1.36)*
Pilot 425 (neR= 1.52)*
6.1 5.4
21 22
*neRis the effective index of refraction which includes a small scintillation component from the BaSO, paint that coats the walls of the diffuse white box.
Square Root of C, FIG. 2. The square root of the average ion-chamber signal plotted against both the square root of the Teflon and the Pilot 425 Cerenkov signal. The grid overlay depicts the initial charge and energy assignments.
ABSOLUTE
ENERGY
417
SPECTRA OF GCR
26
27
FIG. 3. The charge histogram for the elements boron (Z = 5) through iron (Z = 26) in the energy interval 0.9-1.4 GeV nucl-’ as determined from the cross-plots in Fig. 2.
detail (Derrickson, 1983). The hodoscope information, combined with pulse height consistency criteria between detector pairs with a similar velocity and charge response, was used to reject air showers,
nuclear fragmentation background, and highly inclined events. Many off-line checks were performed to insure that these rejection criteria were not significantly biasing the measurement. Because there were fewer than 2% dead anode wires and each x, y view of the hodoscope contained four planes, the efficiency for track recovery was believed to be almost 100%. During the initial evaluation of the track fitting routines, it was found that “good events” were rejected ~3% of the time while “bad events” were accepted < 1% of the time. After the path length and detector mapping corrections were applied to the data, the charge and energy of each cosmic ray event was determined by comparing the square root of the average of the ion chamber outputs to both the square root of the Teflon and Pilot 425 detector responses as depicted in Fig. 2. A consistency check was performed between the Teflon and Pilot 425 results. At this stage of the data analysis, essentially all background events were rejected and the acceptable cosmic ray events were identified with respect to their charge and energy. A summary of the number of events retained and rejected in each procedure is included in Derrickson (1983). The next step was to correct the data for the finite charge resolution of the experiment. An example of a charge histogram derived from the cross-plots of Fig. 2 for the energy channel 0.9-1.4 GeV nucl-’ is shown in Fig. 3. The charge resolution was determined by fitting Gaussian distributions to the measured elemental charge distributions. The average
charge resolution (a,) determined by this method over the energy range 0.5-2.0 GeV nucl-’ is given in Table
2. The derived charge resolution is in reasonable agreement with predicted values based upon delta-ray energy straggling in the ion chambers and residual errors in the trajectory measurement (Derrickson, 1983). Electronic (amplifier) noise was negligible over the charge range reported here. Table 3 shows the mass and material composition of the instrument, from which it may be seen that the 50 mg cm-* Xe counters are overlaid with about 5 g cm-* of material. The fluctuation of the energy deposition in the ion chamber is attributed mainly to the energy straggling of the knock-on electrons that accompany the nuclei. The slight asymmetry in the charge distribution can be attributed to the honeycomb covers located on and above the ion chambers which introduce a mass fluctuation in the detector system, consequently producing a difference in the number of the energetic knock-on electrons for each event. After completing the charge deconvolution procedure, the data were corrected for the finite energy resolution (QJ of the Cerenkov detectors. An estimate was made of the probability that a cosmic ray with the true energy E would be assigned an energy value that places it in a neighboring energy
Table 2. Charge resolution for the ion chambers
Oxygen Silicon Iron*
(a.)
0.25 0.30 0.40
*Estimate of oz limited by statistics.
J. H. DERRICKSON
418
Table 3. The mass profile of the 1976 instrument. Refer to Fig. 1 for explanation of structural elements Structural element Gondola lid Cr
SC
PCHl ICI 2x
IC2 2x PCH2 Cr
Material
Mass thickness (g cm-?
Height (cm)
Al Al Teflon Al Ne 110 Al Stainless steel Al Al Stainless steel Xe Stainless steel Xe Stainless steel Al Al Stainless steel Al Al Pilot 425
0.643 0.137 2.730 0.274 0.655 0.137 0.071 0.137 0.137 0.030 0.025 0.118 0.025 0.030 0.137 0.137 0.071 0.137 0.137 1.511
99.04 60.61 49.74 22.42 -
-
bin. It was assumed that the energy dispersion followed a Gaussian distribution. The accuracy of the energy deconvolution correction is affected by the energy resolution (a3 compared to the size of the energy bins. The energy resolution depends on the number of photo-electrons collected as well as on the steepness of the Cerenkov response curve. The uncertainty in the energy measurement can be attributed mainly to the fluctuation in the number of photo-electrons collected (rrpe).Coupled with this are the fluctuations due to path length error (a& and system electronic noise (uSN)which are described in Derrickson (1983). The total composite energy resolution is
aZ,=a*pe +af,+u* SN 800 -
.
et al.
Table 4. Energy resolution for the Teflon counter (cE in GeV mm-‘) Energy Element
0.55 GeV nucl-’
1.5 GeV nucl-’
Oxygen Silicon Iron
0.014 0.007 0.003
0.21 0.12 0.08
To estimate upe requires a knowledge of the number of photo-electrons detected by the Cerenkov counter as a function of charge and energy. To determine the fi = 1 point for the Cerenkov detector, we examined the pulse height distribution of oxygen in the Teflon counter (Fig. 4). From the half width at half maximum at the fi = 1 edge, we deduced that 400 photoelectrons were observed for oxygen which agreed very well with the prediction of 390 photo-electrons that was based on the muon calibration data (Table 1). The oxygen data had sufficient statistical significance for this comparison and was not appreciably affected in the relativistic regime by the small abundance of fluorine. From the calibration with oxygen nuclei coupled with a simulation of the Cerenkov detector response, the photo-electron output of the Teflon counter was projected from B to Ni as a function of energy (N,(E)), and the energy variance (0,) associated with the values NPe+ ,/Npe was derived. Table 4 lists the composite energy resolution for some elements and for energies just above the threshold, and below the Cerenkov saturation level. The data were next corrected for nuclear fragmentation in both the instrument and the overlying atmosphere. The fragmentation program used for this purpose was adapted from a program written by Hagen (1975) and Ormes (per% commun.) and
* Cameron Abundances Assumed
B= 1 Edge
Galaxv IISML Exponential Pathlength (h, = 7.0g/cmn) 90% H + 10% He - 2 Interactions Allowed
l
l
600
N
Atmosohere
Slab Model - 70% N + 22% 0 1 Interaction Allowed
l
400
-
l
v Slab Model -Instrument Materials (H, C, 0, F, At, ...) No Interactions Allowed (Presumably All Interactions Have Seen Removed From the Measured Flux)
l
200
l
a,
FIG. 4. The distribution of pulse heights detected in the Teflon Cerenkov counter for the element oxygen.
FIG. 5. A schematic of the propagation model used to trace the elemental abundances through the atmosphere and through the instrument. This model accounts for the spallation in the instrument and the overlying atmosphere. The isotopes for each element traced through the galaxy and the atmosphere can be found in Derrickson (1983).
ABSOLUTE Atmosphere 8
ENERGY
Depth Profile for Fit-1395C The Event Profile Has Been Normalized to
0.05-
4.3
5.0
8.0
9.0
Instrument Depth Profile for Pi-139%
&
0.10
t
0.08
c
The Event Profile Has Been Normalizedto
SPECTRA OF GCR
419
incorporated the nuclear fragmentation crosssections of Silberberg and Tsao (1973a, b). The fragmentation model is illustrated in Fig. 5. The slab models for the atmosphere and the instrument were weighted with the corresponding mass profiles presented in Fig. 6. The mass profile of the instrument F,(X) was determined by the vertical mass thickness (7.58 g cnm2) and the angular distribution of the primaries derived from the track fitting program. The mass profile of the atmosphere F,(X) was found by combining, on an event-by-event basis, the float altitude time profile with the observed angular distribution of the cosmic ray events. The atmosphere and instrument depth profiles in Fig. 6 have 0.1 g cm-* resolution and are normalized to one event. Table 5 summarizes the corrections described above, which were made on the cosmic ray elements B-Fe in the energy interval 0.5-2.0 GeV nucl-I.
B ;
0.06
=3 zl ?2 p P
0.04
4. RESULTS AND COMPARISONS OTHER MEASUREMENTS
0.02 7.5 6.0
10.0 9.0 x c Depth (g/cmq
11.0
FIG. 6. The mass profile of the atmosphere F&Y) and the instrument F,(X) for the balloon flight 139%. These profiles are used in conjunction with the nuclear fragmentation corrections.
Applying all corrections summarized in Table 5 results in the differential energy spectra of the more abundant cosmic ray elements shown in Table 6. To obtain the fluxes at the top of the atmosphere requires one further correction listed in Table 7. The absolute differential energy spectra are presented in Fig. 7 after the adjustments shown in Table 7 are made for the energy loss in the atmosphere. A sharp rollover of the
Table 5. A compilation of the fractional changes in the number of events for each stage of the correction process for the combined energy interval 0.5-2.0 GeV nucl-’
Element B C N 0 Ne Mg Si Fe
Charge deconvolution correction
1.003 1.058 0.888 1.058 1.082 1.087 I .094 1.559*
Energy deconvolution correction 1.002 0.999 0.969
1.003 1.006 1.000 1.005 0.998*
Fragmentation (instrument) 1.307 1.332 1.369 1.393 1.450 1.496 1.537 1.794
Fragmentation (atmo.sDhere) 1.052 1.180 1.133 1.261 1.233 1.293 1.337 1.513
*These corrections are limited by poor statistics. Fe actually includes the charge group [Mn, Fe, Co]. Table 6. The differential elemental cosmic ray fluxes* in particles per (m2 sr s GeV nucl-‘) Energy channel (GeV nucl-I) Element
(2) 0.54-0.6
(3) 0.6-0.7
(4) 0.7Xl.9
(5) 0.9-l .4
:
6.33 1.87 f 0.08 0.16
5.14f0.11 1.49 f 0.06
0.97 3.37 *+ 0.03 0.06
0.80 2.52 + It 0.02 0.03
: Ne Mg Si Fe
5.69 1.92 f+ 0.86 f 1.09 + 0.68 f 0.03 f
4.52 1.58 _+ f 0.06 0.11 0.68 f 0.04 0.96 + 0.05 0.68 f o.ost 0.26 f 0.037
3.64 1.15*0.04 f 0.07 0.61 k 0.03 0.83 f 0.04 0.67 f 0.03 0.32 f 0.03
0.80 2.29 f 0.39 f 0.51 f 0.41 * 0.20 +
0.09 0.16 0.06 0.07 0.06t 0.02t
WITH
0.03 0.02 0.01 0.02 0.02 0.01
*These fluxes are obtained after all corrections in Table 5 have been applied to the data. Correction for slowing in the atmosphere has nor been applied in Table 6. Fluxes after this correction has been made are shown in Fig. 7. tFluxes suppressed by instrument trigger (see text).
420
J. H. DERRICKSON
Table 7. Correction Numbers show the atmosphere for each to
for energy loss in the atmosphere. bin-center energy at the top of the charge, after this correction is applied each energy channel Energy channel
At the top of the instrument Element B C N 0 Ne Mg Si Fe
(2)
(3)
(4)
(5)
0.570
0.650
0.800
1.150
0.600 0.610 0.617 0.623 0.636 0.649 0.662 0.726
0.679 0.688 0.695 0.701 0.713 0.726 0.738 0.801
0.827 0.836 0.842 0.848 0.860 0.872 0.884 0.943
1.175 1.183 1.189 1.195 1.206 1.217 I.228 1.284
Fe energy spectrum and to a lesser degree the Si energy spectrum at low energies seen in this figure is an instrumental artifact. The reason is the triggering arrangement of the instrument (refer to Fig. 1, noting that the Pilot 425 Cerenkov counter is located at the bottom of the detector stack). While traversing the apparatus the differential energy loss of the low energy, highly inclined Si to Fe events was great enough to cause the Pilot 425 response to fall below the discriminator level. In Table 8 we compare the relative abundance ratios normalized to oxygen with other measurements acquired during the same solar cycle. The agreement
h-
C
0
+k N
\
Mg
+-A?_ B
Si
Ne
+i
Fe
I-
0.5
1 .o
2.0 0.5
1.0
Kinetic Energy (GeV/nuc
2.0
)
FIG. 7. The absolute differential energy spectra of the cosmic ray elements B, C, N, 0, Ne, Mg, Si and Fe in the 0.5-2.0 GeV nuclk’ energy interval.
et al.
ABSOLUTE
ENERGY
Oxygen
421
OF GCR
oxygen spectrum measurement.
10.0 l
SPECTRA
would
be steeper
than the IMP-8
This work (1976) .
s
t IMP-8 (1974-76;
s
REFERENCES
P
Adams J. H., Badhwar
+
Q . :cn
.
. t tj 1.c
.
3 3 5! .g t n. x 3 ii
tt Iron l This work (1976)
$+ +t
+ CRISIS
(1977)
0.1 1.0 0.5 Energy (GeVlnuc)
2.0
FIG. 8. A comparison with independent observations of the absolute differential energy spectra of the elements oxygen and iron collected during 197477. Details of the IMP-8 measurement of oxygen are in Garcia-Munoz et al. (1977) and a description of the CRISIS measurements is in Young
et al. (1981). with other measured relative abundances appears reasonable. Possible systematic errors due to the uncertainty in the fragmentation cross-sections, the uncertainty in establishing the energy bin edges, and the uncertainty associated with the estimates of the finite charge and energy resolution have not been defined for our experiment or compared for the other experiments listed in Table 8. In Fig. 8 the absolute abundances for both oxygen and iron are compared with measurements made within 2 yr of our balloon flight (Garcia-Munoz et al., 1977; Young et al., 1981). The absolute flux of iron agrees with the measurement by Young et al. within the statistical errors. The statistical errors in our data and limited energy range prevent an extensive comparison of the iron spectra. Our balloon-borne measurement of the oxygen spectrum is somewhat steeper than the IMP-8 measurements. Our oxygen data were measured during the solar minimum. The IMP-8 data were taken during 1974-76 and suffered about the same degree of modulation as data taken in 1973 (Garcia-Munoz et al., 1977). We have not applied a solar modulation model to the data, but it is reasonable that our
G. D., Mewaldt R. A., Mitra B., O’Neill P. M., Ormes J. F., Stemwedel P. W. and Streitmatter R. E. (1991) Papers of the 22nd International Cosmic Ray Conference, 065.2.7, Dublin, Ireland. Austin R. W. and Selig W. J. (1974) Nucl. Instrum. Meth. 117, 429. Derrickson J. H. (1983) A measurement of the cosmic ray energy spectra of the elements boron to iron in the intermediate energy region 0.5-2.0 GeV/amu. Ph.D. thesis, University of Alabama in Huntsville. Dwyer R. D. and Meyer P. (1981) Proc. 17th Int. Cosmic Ray Con& Vol. 2, p. 54. Englemann J., Goret P., Juliusson E., Koch-Miramond L., Masse P., Petrou N., Rio Y., Soutoul A., Byrnak B., Jakobsen H., Lund N., Peters B., Rasmussen I. L., Rotenberg M. and Westergaard N. (1981) Proc. 17fh Int. Cosmic Ray Conj, Vol. 9, p. 97. Garcia-Munoz M., Mason G. M., Simpson J. A. and Wefel J. P. (1977) Proc. 15th Inr. Cosmic Ray Con& Plovdiv, Vol. 1, pp. 230-235. Gregory J. C. (1974) NASA Contract NAS8-24953, Final Report. Gregory J. C., Parnell T. A. and Watts J. W. (1981) Papers of the 17th Int. Cosmic Ray ConJ, Vol. 9, p. 299. Hagen F. A. (1975) Ph.D. thesis, University of Maryland. Julliot C., Koch L. and Petrou N. (1975) Proc. 14th Inf. Cosmic Ray Con/Y, Munich, Vol. 12, p. 4118. Lezniak J. A. and Webber W. R. (1978) Asfroph,vs. J. 233, 676. Lund N., Rasmussen I. L., Peters B., Rotenberg M. and Westergaard N. J. (1975a) Proc. 14th Int. Cosmic Ray Conj, Munich, Vol. I, p. 263. Lund N., Rasmussen I. L., Peters B. and Westergaard N. J. (1975b) Proc. 14th Int. Cosmic Ray ConJ, Munich, Vol. 1, p. 257. Maehl R. C., Ormes J. F., Fisher A. J. and Hagen F. A. (1977) Astrophys. Space Sci. 47, 163. Parnell T. A., Guenther G. A. and Pollvogt U. (1973) Papers of the 13th Int. Cosmic Ray ConJ, Vol. 4, p. 3029. Rizzo A., Parnell T. A. and Guenther G. (1974) IEEE Trans. Nucl. Sci. NA-19, 440. Shea M. A. and Smart D. F. (1975) Tables of asymptotic directions and vertical cutoff rigidities for a five degree by fifteen degree world grid as calculated using the international geomagnetic reference field for epoch 1975.0, Report No. AFCRL-TR-75-0185, Air Force Cambridge Research Laboratories. Silberberg R. and Tsao C. H. (1973a) Astrophys. .I. (Supplement Series No. 220(I)), 25, 315. Silberberg R. and Tsao C. H. (1973b) Asfrophys. J. (Supplement Series No. 220(11)), 25, 335. Stafford T. P. e/ al. (1991) Report of the Synthesis Group on America’s Space Exploration Initiative, Superintendent of Documents, GPO, Washington, DC 20402. Webber W. R., Damle S. V. and Kish J. (1972) Astrophys. Space Sri. 15, 245. Young J. S., Freier P. S., Waddington C. J., Brewster N. R. and Fickle R. K. (1981) Asfrophys. J. 246, 1014-1030.
Pergamon Press Ltd
Printed in Great Britain
A MEASUREMENT OF THE ABSOLUTE ENERGY SPECTRA OF GALACTIC COSMIC RAYS DURING THE 1976-77 SOLAR MINIMUM J. H. DERRICKSON,* T. A. PARNELL,* R. W. AUSTIN,*W. J. SELIG* and J. C. GREooRYt *Space Science Laboratory, NASA Marshall Space Flight Center, Huntsville, AL35812, U.S.A. and TDepartment of Chemistry, University of Alabama in Huntsville, Huntsville, AL 35899, U.S.A. (Received
31 October
1991; in revised form 6 January
1992)
Abstract-An
instrument designed to measure elemental cosmic ray abundances from boron to nickel in the energy region 0.5-2.0 GeV nucl-’ was flown on a high altitude balloon from Sioux Falls, South Dakota, on 30 September through 1 October 1976 at an average atmospheric depth of -5 g cm-‘. Differential energy spectra of B, C, N, 0, Ne, Mg, Si and Fe, extrapolated to the top of the atmosphere, were measured. The float altitude exposure of 17 h ended near Alpena, Michigan. The flight trajectory maintained a north easterly heading out of Sioux Falls traversing the upper mid-west region between 84” and 97” west longitude while remaining between 43.5” and 45” north latitude. The maximum vertical cut-off for this flight path was 1.77 GV or 0.35 GeV nucl-’ (Shea and Smart (1975), Report No. AFCRL-TR-75-0185, Air Force Cambridge Research Laboratories).
abundances are compared to results from other experiments made during that era. The absolute intensities are compared with the University of Chicago telescope data taken on board the IMP-8 spacecraft during the period 1974-76 (Garcia-Munoz et al., 1977) and the University of Minnesota balloonborne Cosmic Ray Isotope Instrument System (CRISIS) detector launched from Aberdeen, South Dakota, on 20 May 1977 (Young et al., 1981).
1. INTRODUCTION
THE RECENT resurgence of interest in manned exploration of the Earth’s Moon and Mars has sparked a re-examination of the intensity of galactic cosmic rays. The U.S. Presidential Commission (Stafford et al., 1991) formed to study the “exploration initiative” strategy, cited two principal health hazards that needed further study: ionizing radiation (galactic cosmic rays and solar flare particles) and weightlessness, and the possible synergism between them. In examining the hazard due to galactic cosmic rays, there is a need to define more accurately the absolute intensities of particles at the lower end of the cosmic ray energy range throughout the solar cycle. This measurement of the galactic cosmic ray fluxes made in the fall of 1976 was contributed to a workshop held at the 22nd International Cosmic Ray Conference in Dublin, Ireland, in August 1991. An objective of the workshop, entitled “Cosmic Radiation: Constraints on Space Exploration”, was to provide a reliable data base of intensities of galactic cosmic radiation, particularly those during solar minimum periods. The present situation concerning prediction of radiation dose for interplanetary flight was described at the workshop by Adams et al. (1991). This paper describes a balloon-borne measurement of the intensity of heavy cosmic ray primaries made at the end of September during the 1976 solar minimum period. The smoothed sunspot number was 18 and the smoothed 2800 MHz radio flux was about 75. We describe the instrument, data analysis procedures, and results on absolute intensities of the more abundant species C-Fe. The measured relative
2. THE INSTRUMENT The instrument, shown in Fig. 1, contained eight gas-filled (Ar/CO,) multi-wire proportional counters (Parnell et al., 1973; Rizzo et al., 1974; Austin and Selig, 1974) divided into 4 x and 4 y planes with 104 anode wires per plane, the wires being spaced 0.5 cm apart. There were two discriminators per wire. The lower level discriminator was adjusted to detect 90% of muons during ground calibration and the upper discriminator was set at an ionization level 30 times higher, triggering between normally incident boron and carbon. The dual level discriminators allowed a separation of the cosmic ray primary tracks from the accompanying delta rays, removing most of the delta ray influence when determining the heavy primary track parameters. All high and low discriminators that were fired were telemetered in an “event” message of variable length. This hodoscope allowed identification of fragmentation by-products and shower particles, permitting efficient elimination of background events. The angular resolution of particle tracks was N 1” for carbon and -2” for iron. 415
J. H. DERRICKSON
416
et al.
these Cerenkov detectors was 14% which was corrected for in the data analysis. --_-__________________-___~~~__ A plastic scintillator (Ne-110) was included as an r-1 ionization-dependent fast trigger. The response of this scintillator viewed edge-on through adiabatic light guides was not very uniform and it was used only for triggering and consistency checks to reject background. The instrument was set to trigger on minimum ionizing lithium which resulted in a flight count rate of 3.4 events s-‘, which included showers and nuclear interaction events. A telemetry rate of 20 kbits s-l, and an average event length dependent on the number of set hodoscope wires, resulted in an average dead time of 22%. The geometric factor, 0 determined by the effective area of the gated detectors and the spacing between them, was 0.1505 m* sr. The I__. __________________________________ - - - - _ _ _ _ A net exposure for the data reported here was Flight 139%. Sioux Falls, SD.. Oct. 1976 6735 m* sr s. --._
--1
r:i
CT
-
CP
-
Teflon Cerenkov Counter Pilot 425 Cerenkov Counter
SC PCHl8.2
-
Ne 110 Plastic Scintillatar
-
ArC02 Filled Multi-wire Proporlonal
IClBP
-
X&H,
PMT
-
Photomultipller Tube
ALG
-
Adiabatic Light Guide
FIG. 1. The balloon-borne during the 1976-77
3. DATA ANALYSIS Counters
FIlled Ion Chambers
cosmic ray instrument solar minimum period.
flown
PROCEDURE
The data analysis procedure used to obtain the corrected elemental abundances at the top of the atmosphere from the raw data has been described in 0.5
0.55 0.6
0.8
1.0 1.5 2.0 GeVlnuc
The instrument included two 8.4 cm thick parallelplane pulse ion-chambers (Gregory, 1974; Gregory et al., 1981) filled with Xe-CH, for a larger signal, an extended primary charge range, and better charge resolution than with argon gas mixtures. The electric field over the active region varied by less than 1%. Two Cerenkov counters using sheets of Teflon and Pilot 425, were used to measure the energy over the ranges 0.45 to - 1.4 GeV nucl-’ and 0.32 to - 1.0 respectively. The analog-to-digital GeV nucl-‘, converters for this flight were dual range devices (IO: 1) in order to meet the overall dynamic range requirements of the experiment. Table 1 shows the average number of photo-electrons for singly charged relativistic particles (fi - 1) that was collected by a set of four RCA 4525 photomultiplier tubes (12.7 cm cathode diameter) viewing the Teflon Cerenkov radiator in a diffuse white box lined with millipore paper and BaSO, paint. The yield for the Pilot 425 Cerenkov radiator, also viewed by four 12.7 cm photomultipliers, is shown in the same table. The variation in response as a function of position for
Square Root of C, 0.5 0.55 0.6
0.0 .l.O ,I.5 '2.0 GeVlnuc
Table 1.Average number of photo-electrons produced in the Cerenkov counters
Muons Protons
at altitude
Teflon (neR= 1.36)*
Pilot 425 (neR= 1.52)*
6.1 5.4
21 22
*neRis the effective index of refraction which includes a small scintillation component from the BaSO, paint that coats the walls of the diffuse white box.
Square Root of C, FIG. 2. The square root of the average ion-chamber signal plotted against both the square root of the Teflon and the Pilot 425 Cerenkov signal. The grid overlay depicts the initial charge and energy assignments.
ABSOLUTE
ENERGY
417
SPECTRA OF GCR
26
27
FIG. 3. The charge histogram for the elements boron (Z = 5) through iron (Z = 26) in the energy interval 0.9-1.4 GeV nucl-’ as determined from the cross-plots in Fig. 2.
detail (Derrickson, 1983). The hodoscope information, combined with pulse height consistency criteria between detector pairs with a similar velocity and charge response, was used to reject air showers,
nuclear fragmentation background, and highly inclined events. Many off-line checks were performed to insure that these rejection criteria were not significantly biasing the measurement. Because there were fewer than 2% dead anode wires and each x, y view of the hodoscope contained four planes, the efficiency for track recovery was believed to be almost 100%. During the initial evaluation of the track fitting routines, it was found that “good events” were rejected ~3% of the time while “bad events” were accepted < 1% of the time. After the path length and detector mapping corrections were applied to the data, the charge and energy of each cosmic ray event was determined by comparing the square root of the average of the ion chamber outputs to both the square root of the Teflon and Pilot 425 detector responses as depicted in Fig. 2. A consistency check was performed between the Teflon and Pilot 425 results. At this stage of the data analysis, essentially all background events were rejected and the acceptable cosmic ray events were identified with respect to their charge and energy. A summary of the number of events retained and rejected in each procedure is included in Derrickson (1983). The next step was to correct the data for the finite charge resolution of the experiment. An example of a charge histogram derived from the cross-plots of Fig. 2 for the energy channel 0.9-1.4 GeV nucl-’ is shown in Fig. 3. The charge resolution was determined by fitting Gaussian distributions to the measured elemental charge distributions. The average
charge resolution (a,) determined by this method over the energy range 0.5-2.0 GeV nucl-’ is given in Table
2. The derived charge resolution is in reasonable agreement with predicted values based upon delta-ray energy straggling in the ion chambers and residual errors in the trajectory measurement (Derrickson, 1983). Electronic (amplifier) noise was negligible over the charge range reported here. Table 3 shows the mass and material composition of the instrument, from which it may be seen that the 50 mg cm-* Xe counters are overlaid with about 5 g cm-* of material. The fluctuation of the energy deposition in the ion chamber is attributed mainly to the energy straggling of the knock-on electrons that accompany the nuclei. The slight asymmetry in the charge distribution can be attributed to the honeycomb covers located on and above the ion chambers which introduce a mass fluctuation in the detector system, consequently producing a difference in the number of the energetic knock-on electrons for each event. After completing the charge deconvolution procedure, the data were corrected for the finite energy resolution (QJ of the Cerenkov detectors. An estimate was made of the probability that a cosmic ray with the true energy E would be assigned an energy value that places it in a neighboring energy
Table 2. Charge resolution for the ion chambers
Oxygen Silicon Iron*
(a.)
0.25 0.30 0.40
*Estimate of oz limited by statistics.
J. H. DERRICKSON
418
Table 3. The mass profile of the 1976 instrument. Refer to Fig. 1 for explanation of structural elements Structural element Gondola lid Cr
SC
PCHl ICI 2x
IC2 2x PCH2 Cr
Material
Mass thickness (g cm-?
Height (cm)
Al Al Teflon Al Ne 110 Al Stainless steel Al Al Stainless steel Xe Stainless steel Xe Stainless steel Al Al Stainless steel Al Al Pilot 425
0.643 0.137 2.730 0.274 0.655 0.137 0.071 0.137 0.137 0.030 0.025 0.118 0.025 0.030 0.137 0.137 0.071 0.137 0.137 1.511
99.04 60.61 49.74 22.42 -
-
bin. It was assumed that the energy dispersion followed a Gaussian distribution. The accuracy of the energy deconvolution correction is affected by the energy resolution (a3 compared to the size of the energy bins. The energy resolution depends on the number of photo-electrons collected as well as on the steepness of the Cerenkov response curve. The uncertainty in the energy measurement can be attributed mainly to the fluctuation in the number of photo-electrons collected (rrpe).Coupled with this are the fluctuations due to path length error (a& and system electronic noise (uSN)which are described in Derrickson (1983). The total composite energy resolution is
aZ,=a*pe +af,+u* SN 800 -
.
et al.
Table 4. Energy resolution for the Teflon counter (cE in GeV mm-‘) Energy Element
0.55 GeV nucl-’
1.5 GeV nucl-’
Oxygen Silicon Iron
0.014 0.007 0.003
0.21 0.12 0.08
To estimate upe requires a knowledge of the number of photo-electrons detected by the Cerenkov counter as a function of charge and energy. To determine the fi = 1 point for the Cerenkov detector, we examined the pulse height distribution of oxygen in the Teflon counter (Fig. 4). From the half width at half maximum at the fi = 1 edge, we deduced that 400 photoelectrons were observed for oxygen which agreed very well with the prediction of 390 photo-electrons that was based on the muon calibration data (Table 1). The oxygen data had sufficient statistical significance for this comparison and was not appreciably affected in the relativistic regime by the small abundance of fluorine. From the calibration with oxygen nuclei coupled with a simulation of the Cerenkov detector response, the photo-electron output of the Teflon counter was projected from B to Ni as a function of energy (N,(E)), and the energy variance (0,) associated with the values NPe+ ,/Npe was derived. Table 4 lists the composite energy resolution for some elements and for energies just above the threshold, and below the Cerenkov saturation level. The data were next corrected for nuclear fragmentation in both the instrument and the overlying atmosphere. The fragmentation program used for this purpose was adapted from a program written by Hagen (1975) and Ormes (per% commun.) and
* Cameron Abundances Assumed
B= 1 Edge
Galaxv IISML Exponential Pathlength (h, = 7.0g/cmn) 90% H + 10% He - 2 Interactions Allowed
l
l
600
N
Atmosohere
Slab Model - 70% N + 22% 0 1 Interaction Allowed
l
400
-
l
v Slab Model -Instrument Materials (H, C, 0, F, At, ...) No Interactions Allowed (Presumably All Interactions Have Seen Removed From the Measured Flux)
l
200
l
a,
FIG. 4. The distribution of pulse heights detected in the Teflon Cerenkov counter for the element oxygen.
FIG. 5. A schematic of the propagation model used to trace the elemental abundances through the atmosphere and through the instrument. This model accounts for the spallation in the instrument and the overlying atmosphere. The isotopes for each element traced through the galaxy and the atmosphere can be found in Derrickson (1983).
ABSOLUTE Atmosphere 8
ENERGY
Depth Profile for Fit-1395C The Event Profile Has Been Normalized to
0.05-
4.3
5.0
8.0
9.0
Instrument Depth Profile for Pi-139%
&
0.10
t
0.08
c
The Event Profile Has Been Normalizedto
SPECTRA OF GCR
419
incorporated the nuclear fragmentation crosssections of Silberberg and Tsao (1973a, b). The fragmentation model is illustrated in Fig. 5. The slab models for the atmosphere and the instrument were weighted with the corresponding mass profiles presented in Fig. 6. The mass profile of the instrument F,(X) was determined by the vertical mass thickness (7.58 g cnm2) and the angular distribution of the primaries derived from the track fitting program. The mass profile of the atmosphere F,(X) was found by combining, on an event-by-event basis, the float altitude time profile with the observed angular distribution of the cosmic ray events. The atmosphere and instrument depth profiles in Fig. 6 have 0.1 g cm-* resolution and are normalized to one event. Table 5 summarizes the corrections described above, which were made on the cosmic ray elements B-Fe in the energy interval 0.5-2.0 GeV nucl-I.
B ;
0.06
=3 zl ?2 p P
0.04
4. RESULTS AND COMPARISONS OTHER MEASUREMENTS
0.02 7.5 6.0
10.0 9.0 x c Depth (g/cmq
11.0
FIG. 6. The mass profile of the atmosphere F&Y) and the instrument F,(X) for the balloon flight 139%. These profiles are used in conjunction with the nuclear fragmentation corrections.
Applying all corrections summarized in Table 5 results in the differential energy spectra of the more abundant cosmic ray elements shown in Table 6. To obtain the fluxes at the top of the atmosphere requires one further correction listed in Table 7. The absolute differential energy spectra are presented in Fig. 7 after the adjustments shown in Table 7 are made for the energy loss in the atmosphere. A sharp rollover of the
Table 5. A compilation of the fractional changes in the number of events for each stage of the correction process for the combined energy interval 0.5-2.0 GeV nucl-’
Element B C N 0 Ne Mg Si Fe
Charge deconvolution correction
1.003 1.058 0.888 1.058 1.082 1.087 I .094 1.559*
Energy deconvolution correction 1.002 0.999 0.969
1.003 1.006 1.000 1.005 0.998*
Fragmentation (instrument) 1.307 1.332 1.369 1.393 1.450 1.496 1.537 1.794
Fragmentation (atmo.sDhere) 1.052 1.180 1.133 1.261 1.233 1.293 1.337 1.513
*These corrections are limited by poor statistics. Fe actually includes the charge group [Mn, Fe, Co]. Table 6. The differential elemental cosmic ray fluxes* in particles per (m2 sr s GeV nucl-‘) Energy channel (GeV nucl-I) Element
(2) 0.54-0.6
(3) 0.6-0.7
(4) 0.7Xl.9
(5) 0.9-l .4
:
6.33 1.87 f 0.08 0.16
5.14f0.11 1.49 f 0.06
0.97 3.37 *+ 0.03 0.06
0.80 2.52 + It 0.02 0.03
: Ne Mg Si Fe
5.69 1.92 f+ 0.86 f 1.09 + 0.68 f 0.03 f
4.52 1.58 _+ f 0.06 0.11 0.68 f 0.04 0.96 + 0.05 0.68 f o.ost 0.26 f 0.037
3.64 1.15*0.04 f 0.07 0.61 k 0.03 0.83 f 0.04 0.67 f 0.03 0.32 f 0.03
0.80 2.29 f 0.39 f 0.51 f 0.41 * 0.20 +
0.09 0.16 0.06 0.07 0.06t 0.02t
WITH
0.03 0.02 0.01 0.02 0.02 0.01
*These fluxes are obtained after all corrections in Table 5 have been applied to the data. Correction for slowing in the atmosphere has nor been applied in Table 6. Fluxes after this correction has been made are shown in Fig. 7. tFluxes suppressed by instrument trigger (see text).
420
J. H. DERRICKSON
Table 7. Correction Numbers show the atmosphere for each to
for energy loss in the atmosphere. bin-center energy at the top of the charge, after this correction is applied each energy channel Energy channel
At the top of the instrument Element B C N 0 Ne Mg Si Fe
(2)
(3)
(4)
(5)
0.570
0.650
0.800
1.150
0.600 0.610 0.617 0.623 0.636 0.649 0.662 0.726
0.679 0.688 0.695 0.701 0.713 0.726 0.738 0.801
0.827 0.836 0.842 0.848 0.860 0.872 0.884 0.943
1.175 1.183 1.189 1.195 1.206 1.217 I.228 1.284
Fe energy spectrum and to a lesser degree the Si energy spectrum at low energies seen in this figure is an instrumental artifact. The reason is the triggering arrangement of the instrument (refer to Fig. 1, noting that the Pilot 425 Cerenkov counter is located at the bottom of the detector stack). While traversing the apparatus the differential energy loss of the low energy, highly inclined Si to Fe events was great enough to cause the Pilot 425 response to fall below the discriminator level. In Table 8 we compare the relative abundance ratios normalized to oxygen with other measurements acquired during the same solar cycle. The agreement
h-
C
0
+k N
\
Mg
+-A?_ B
Si
Ne
+i
Fe
I-
0.5
1 .o
2.0 0.5
1.0
Kinetic Energy (GeV/nuc
2.0
)
FIG. 7. The absolute differential energy spectra of the cosmic ray elements B, C, N, 0, Ne, Mg, Si and Fe in the 0.5-2.0 GeV nuclk’ energy interval.
et al.
ABSOLUTE
ENERGY
Oxygen
421
OF GCR
oxygen spectrum measurement.
10.0 l
SPECTRA
would
be steeper
than the IMP-8
This work (1976) .
s
t IMP-8 (1974-76;
s
REFERENCES
P
Adams J. H., Badhwar
+
Q . :cn
.
. t tj 1.c
.
3 3 5! .g t n. x 3 ii
tt Iron l This work (1976)
$+ +t
+ CRISIS
(1977)
0.1 1.0 0.5 Energy (GeVlnuc)
2.0
FIG. 8. A comparison with independent observations of the absolute differential energy spectra of the elements oxygen and iron collected during 197477. Details of the IMP-8 measurement of oxygen are in Garcia-Munoz et al. (1977) and a description of the CRISIS measurements is in Young
et al. (1981). with other measured relative abundances appears reasonable. Possible systematic errors due to the uncertainty in the fragmentation cross-sections, the uncertainty in establishing the energy bin edges, and the uncertainty associated with the estimates of the finite charge and energy resolution have not been defined for our experiment or compared for the other experiments listed in Table 8. In Fig. 8 the absolute abundances for both oxygen and iron are compared with measurements made within 2 yr of our balloon flight (Garcia-Munoz et al., 1977; Young et al., 1981). The absolute flux of iron agrees with the measurement by Young et al. within the statistical errors. The statistical errors in our data and limited energy range prevent an extensive comparison of the iron spectra. Our balloon-borne measurement of the oxygen spectrum is somewhat steeper than the IMP-8 measurements. Our oxygen data were measured during the solar minimum. The IMP-8 data were taken during 1974-76 and suffered about the same degree of modulation as data taken in 1973 (Garcia-Munoz et al., 1977). We have not applied a solar modulation model to the data, but it is reasonable that our
G. D., Mewaldt R. A., Mitra B., O’Neill P. M., Ormes J. F., Stemwedel P. W. and Streitmatter R. E. (1991) Papers of the 22nd International Cosmic Ray Conference, 065.2.7, Dublin, Ireland. Austin R. W. and Selig W. J. (1974) Nucl. Instrum. Meth. 117, 429. Derrickson J. H. (1983) A measurement of the cosmic ray energy spectra of the elements boron to iron in the intermediate energy region 0.5-2.0 GeV/amu. Ph.D. thesis, University of Alabama in Huntsville. Dwyer R. D. and Meyer P. (1981) Proc. 17th Int. Cosmic Ray Con& Vol. 2, p. 54. Englemann J., Goret P., Juliusson E., Koch-Miramond L., Masse P., Petrou N., Rio Y., Soutoul A., Byrnak B., Jakobsen H., Lund N., Peters B., Rasmussen I. L., Rotenberg M. and Westergaard N. (1981) Proc. 17fh Int. Cosmic Ray Conj, Vol. 9, p. 97. Garcia-Munoz M., Mason G. M., Simpson J. A. and Wefel J. P. (1977) Proc. 15th Inr. Cosmic Ray Con& Plovdiv, Vol. 1, pp. 230-235. Gregory J. C. (1974) NASA Contract NAS8-24953, Final Report. Gregory J. C., Parnell T. A. and Watts J. W. (1981) Papers of the 17th Int. Cosmic Ray ConJ, Vol. 9, p. 299. Hagen F. A. (1975) Ph.D. thesis, University of Maryland. Julliot C., Koch L. and Petrou N. (1975) Proc. 14th Inf. Cosmic Ray Con/Y, Munich, Vol. 12, p. 4118. Lezniak J. A. and Webber W. R. (1978) Asfroph,vs. J. 233, 676. Lund N., Rasmussen I. L., Peters B., Rotenberg M. and Westergaard N. J. (1975a) Proc. 14th Int. Cosmic Ray Conj, Munich, Vol. I, p. 263. Lund N., Rasmussen I. L., Peters B. and Westergaard N. J. (1975b) Proc. 14th Int. Cosmic Ray ConJ, Munich, Vol. 1, p. 257. Maehl R. C., Ormes J. F., Fisher A. J. and Hagen F. A. (1977) Astrophys. Space Sci. 47, 163. Parnell T. A., Guenther G. A. and Pollvogt U. (1973) Papers of the 13th Int. Cosmic Ray ConJ, Vol. 4, p. 3029. Rizzo A., Parnell T. A. and Guenther G. (1974) IEEE Trans. Nucl. Sci. NA-19, 440. Shea M. A. and Smart D. F. (1975) Tables of asymptotic directions and vertical cutoff rigidities for a five degree by fifteen degree world grid as calculated using the international geomagnetic reference field for epoch 1975.0, Report No. AFCRL-TR-75-0185, Air Force Cambridge Research Laboratories. Silberberg R. and Tsao C. H. (1973a) Astrophys. .I. (Supplement Series No. 220(I)), 25, 315. Silberberg R. and Tsao C. H. (1973b) Asfrophys. J. (Supplement Series No. 220(11)), 25, 335. Stafford T. P. e/ al. (1991) Report of the Synthesis Group on America’s Space Exploration Initiative, Superintendent of Documents, GPO, Washington, DC 20402. Webber W. R., Damle S. V. and Kish J. (1972) Astrophys. Space Sri. 15, 245. Young J. S., Freier P. S., Waddington C. J., Brewster N. R. and Fickle R. K. (1981) Asfrophys. J. 246, 1014-1030.