D. E. Gruber, G. V. Jung, and J. L. Matteson
Center for Astrophysics and Space Sciences
University of California, San Diego, La Jolla, CA, 92093
The A4 experiment on HEAO-1 was designed at a time when detector activation and the resultant variability were not yet fully appreciated. Enough was understood so that that the dwell time on an individual source in the Medium Energy Detectors was kept short (three minutes) compared to the 45 minute background variability expected from the orbital motion in the geomagnetic field. Moreover, the instrument carried special detectors for monitoring charged particles, and an elaborate telemetry scheme returned very complete information on all detector and active shield counters on time scales of 41 seconds or less. Thus it was possible to make a rather complete analysis of the radiation environment, its variability, and the responses of the various parts of the detector system. This study formed a key role in the formation of a sky map from the A4 detectors: knowledge of the variability permitted the formulation of a scheme of data selection and accumulation which was relatively insensitive to the background changes. Furthermore, the size of residual variability in the sky map could be estimated. We report here on various aspects of this analysis. Much of this work has been reported earlier2.
Figure 1 The A4 instrument on HEAO-1 consisted of seven NaI/CsI phoswich scintillation detectors surrounded by massive CsI shielding. Additional organic scintillators provided for monitoring of charged particles. The instrument was designed for observing cosmic sources of x- and gamma-radiation at energies between 13 keV and 10 MeV.
The HEAO-l satellite was launched into a highly circular orbit of
inclination 23° and altitude 400 km. At this altitude drag from the tenuous
exosphere was sufficient to cause reentry after only 580 days. Total mass of
HEAO-1 was three tons.
ORIGIN AND VARIABILITY OF BACKGROUND
HEAO-1 and its experiments were subjected to two dominant sources of
ionizing radiation: cosmic rays and geomagnetically trapped particles. The
cosmic ray flux was modulated on two time scales as the satellite moved
through the geomagnetic field: twice per orbit the satellite crosses the
geomagnetic equator where the shielding effect of the geomagnetic cutoff is
greatest; and once per day the orbital plane lies closest to the geomagnetic
equator. Geomagnetically trapped particles were encountered only at one
point in the orbit, the so-called South Atlantic Anomaly (SAA), where the
inner belts dip unusually low. This region was traversed six to eight times
per day. Changes of a few tens of kilometers in the satellite orbit caused
large changes in the particle fluxes encountered because of strong gradients
of the population of trapped particles. This is immediately evident in Figure 2, where periodic changes are seen to modulate an
overall strong decline which results from orbital decay. The wobbles about
the general decline were resolvable into two periodicities at 28 and 43 day.
These are probably identifiable with cyclic changes of two elements of the
orbit: the argument of the perigee at 28 days and the right ascension of the
ascending node at 48 days. The 28-day motion moves the satellite apogee in
and out of the SAA region; more intense fluxes are encountered at apogee. A
modulation with this period of 11.5% was measured. The 48-day orbital
precession causes a variation of the local time of maximum daily SAA
encounter with synodic period 43 days. Since the satellite spin axis was
kept fised near the sun, the satellite orientation with respect to the local
vertical at SAA also varied with this period. Proton fluxes on the inner
edge of the belt are strongly anisotropic with apex to the local west. The
particle monitors are not completely isotropic in response, but are
partially shielded by the massive instrument and spacecraft. Therefore a
modulation of the observed proton flux may be expected. In fact, a highly
significant 43-day modulation of amplitude 5.6% was measured, with maximum
at times when the SAA was encountered near the dawn terminator.
After allowance for these two periodic terms, the monitored daily particle
fluences showed no further signs of periodic structure, and evidently the
considerable remaining variability was random. Sizable random variability of
particle populations has been reported by Stassinopoulos3.
In Figure 2 normalized counting rates for two MED detectors are also shown. These rates are measured for a strong internal background line of I123, which results From spallation interactions of protons with the I127 of the detector. This activity decays with a half-life of 13 hours, making it effectively an instantaneous monitor of proton dosage at the detector on the time scales of interest here. Moreover, the decay is predominantly by internal capture, and the energy of 193 keV is a sum peak of a 159 keV gamma from the Te123 daughter and the The Te x-rays, thus the gammas leaking from the CsI shields, where I123 is also produced, are separately counted. The wobbles on the counting rate curves of the two detectors are obviously slower than for the particle monitor. In fact, Fourier analysis shows that the two same frequencies are present, but the relative strengths are reversed: in both detectors the 28-day modulation has an amplitude near 6% and the 43-day modulation has amplitude 15%.
Figure 2 Daily dosage of trapped protons in the SAA region and counting rate of two detectors in a strong internal background line at 191 keV, arbitrarily normalized. The overall decline is due to orbital decay. The wobbles result from 28-day cyclic changes in the eccentricity, and a 43-day precession of the orbital plane.It is also obvious that the wobbles in the two MED curves of Figure 2 are anticorrelated. Analysis shows that the Detector 2 maxima occur when the SAA passages are encountered at the sunset terminator, and the Detector 4 maxima at the sunrise terminator. The explanation is simple, involving only the orientation of the spacecraft and anisotropies in the particle fluxes and detector shielding. Each MED has a direction of minimum shielding (plan view, Figure 1) corresponding to a particle threshold of roughly 80 MeV. Because the spacecraft spin axis is fixed near the sun, this direction of minimum shielding is in the solar direction for detector 4 and antisolar for detector 2; At the sunrise terminator the westward peak of particle flux is pointed at the thin spot of detector 4's shielding, and likewise for detector 2 at the sunset terminator.
In Figure 2 it is seen that the MED count rates drop only by a factor of two in the course of the mission, whereas the particle flux drops by an order of magnitude. An extra, relatively constant activation component is therefore present, which is attributable to cosmic rays. It is clear that at the beginning of the mission activation from trapped particles and cosmic rays occurs in roughly equal amounts.
Figure 3 MED background spectra at two different times of day, showing activation from resulting from exposure to SAA trapped protons and its decay. Except below 100 keV the detector background is dominated by this internal radioactivity.Variability of internal background was, of course, not confined to longer time scales, but was quite pronounced in the course of a day. The SAA region was traversed in six to eight successive satellite passes out of the daily fifteen. Cosmic ray fluxes also varied with time as different geomagnetic latitudes were traversed, but the dynamic range of this variability was probably not more than a factor of two. The net daily variability of MED internal background is indicated in Figure 3. The daily variability of internal background is shown in more detail in Figure 4, which shows the response of the HED detector and one shield segment to trapped particle doses and to cosmic rays, whose flux is parametrized by the McIlwain L-parameter.
Figure 4 A typical daily history of fluxes of activating particle and the responses in selected instrument registers. Fluxes of cosmic rays and equilibrium neutrons are proportional to McIlwain L. The inner shield ULD is sensitive to the incident particles, while the HED events counter shows a delayed response dominated by the decay of I128.
The analysis then was reduced to a very standard problem of multiple linear
regression. For selected data intervals one accumulated the usual weighted
sums of observed counting rates, R(ti, E), their products
with the activity models, Aj(ti)·R(ti,
E), and the cross-products Aj(ti) ·
Ak(ti). The matrix equations:
are then solved for Sk(E), the activation spectra at each half life, plus time-average spectrum So(E). The detector background spectrum is then:
As a practical matter, the various sums were accumulated with the spectral data during the preparation of the sky map for each detector. Great circle scans divided into ninety sky bins were accumulated on each four-day interval of the mission. For each 4-day sky strip a single background spectral file was generated by adding all of the blank sky data, using as selection criterion a distance of 20° or more from the galactic plane, Cen A, NGC 4151 and 3C273. Since the problem is linear, 4-day spectral files could be added in any combination before solution.
Solution, however, was not immediate. Two difficulties were present: statistical precision on shorter data stretches was rather limited for solution into six separate spectra; and the five activity functions were far from independent basis functions. Fortunately, data from the entire mission was sufficient to overpower the first difficulty. Overcoming the second difficulty required some careful manipulations. As a glance at Figure 5 will show, the 13 hour and 100 hour functions are very similar when allowance is made for arbitrary offsets and multiplicative factors Sk. The 13 hour and 2 hour functions are also similarly coupled. The degree of similarity can be quantified in a cross-correlation factor obtained from the error matrix following solution. The procedure employed was to find a subset of the data for which the cross-correlation factor between the 13 hour and 100 hour terms was as small as possible. A reduction from -0.98 to -0.70 was eventually obtained, which was taken as an indicator of a sufficiently stable and reliable solution. The best-fit solution was then used to subtract the 100 hour dependence from the data. Thus the most pathological term in the problem was isolated and removed. The procedure was then re-applied using the remaining terms to find the 2 hour activity spectrum and subtract it. The four terms still included were now sufficiently independent so that a good direct solution was obtained. The fitting was performed unconstrained, and a good indicator of the correctness of the fits was given by the absence of large, unphysical, negative excursions of the spectra, and also by their approach to zero at high energies. A final but very important verification comes from the expected spectral shapes. For example, the 25 min spectrum showed a very convincing representation of the I128 beta spectrum with end point at 2.12 MeV.
Figure 5 Model activity functions at several half-lives for SAA dosages received in a typical day of operation. The curves represent populations of radioactive species induced in the detectors by trapped particles in the SAA region. The "McIlwain L" curve represents prompt response to cosmic ray dosages. The longer half-life terms fail to decay fully in a day.By this method reliable activation spectra, Figure 6 for example, were obtained. The non- independence of the basis functions adds an important benefit: each function collects, in the fit, all the activity at a wide range of related half-lives. If 13 hour and 100 hour activity can be distinguished only with difficulty, then 13 hour and, say, 25 hour activity must appear almost identical. Thus the solution extends to a full range of variability between a few minutes and many days, provided that the initial assumptions of SAA and cosmic-ray activation are correct. It is evident, though, from Figure 2 that the particle monitors gave an incorrect measure, by up to 30%, of the SAA dosage at the detectors, at least for time scales near one month. This difficulty could be overcome, however, by selection of 4-day spectral files only at times of peak activation.
Figure 6 Activation spectrum at 13 hour nominal half life. Line and continuum deexcitation spectra are present from nuclides with half lives between roughly 5 hours and 30 hours. The strong feature at 193 keV results primarily from decay of I123 with a 159 keV gamma and K x-rays following electron capture.The activation spectra thus obtained were examined for spectral features, usually line features but occasionally beta continua, and identifications of radioactive nuclides attempted. The results are in Table 1. Many observed lines are broader than the instrumental resolution, indicating blending. Most of the identifications are already familiar from earlier work4,5, but some new identifications are proposed. Many of the lines result from electron capture and therefore include the K-level energy daughter. A few lines are attributed to inelastic scattering of neutrons. Some observed lines remain unidentified. This work was supported by NASA grant NAG-8-499.