Predicted On-orbit Background Rates

To obtain predicted on-orbit instrumental background rates from the rates measured at XRCF, we scaled the XRCF count rates by the ratio of the on-orbit cosmic-ray flux to the cosmic-ray flux at the Earth's surface. As a first step, we analyzed simultaneous high-speed tap and telemetry data from sequence 1 of science runs 113 and 127 for detectors I3 (FI) and S3 (BI), respectively. To determine the cosmic-ray (CR) flux incident on each detector we plotted the ACIS events frame-by-frame, examined the distribution of events by eye, and decided how many cosmic rays were responsible for creating the events. Using this method the observed cosmic-ray flux, averaged over the upper hemisphere, was $\rm (4.08\pm0.10)\times10^{-3}\ CR\ cm^{-2}\ s^{-1}\ sr^{-1}$ in both I3 and S3.

According to Reinhold Müller-Mellin, principal investigator of the EPHIN instrument on AXAF, the cosmic-ray flux at the Earth's surface is fairly constant over a timescale of a day, with day-to-day variations of less than 1%. Interplanetary transients from solar events (called Forbush decreases) can cause decreases in the flux by 3-10%, while increases of a few percent are sometimes seen due to rare ground-level events. Long-term variations over the 11 year solar cycle result in variations of 15-20%. Thus, we can obtain a reasonably good check on our estimate of the cosmic-ray flux at XRCF by comparing it to the cosmic-ray flux measured by the EPHIN instrument during the AXAF thermal vacuum tests at TRW in 1998 June. Müller-Mellin analyzed 2 hours of EPHIN data on 1998 June 7 and found the cosmic-ray flux, averaged over the upper hemisphere, to be $\rm (4.04\pm0.40)\times10^{-3}\ CR\ cm^{-2}\ s^{-1}\ sr^{-1}$. This is in excellent agreement with the value given above for the cosmic-ray flux measured at XRCF.

The cosmic-ray flux outside the influence of the Earth's magnetic field, (i.e. for eneriges exceeding 50 Mev/nucleon), is approximately $\rm 2\ CR\ cm^{-2}\ s^{-1}$  [Meyer1969]. Assuming this were the particle flux incident on the detectors, the predicted on-orbit background rates in ASCA grades 02346 would be $\rm\sim1.0\times10^{-2}\ ct\ cm^{-2}\ s^{-1}\ keV^{-1}$ in an ACIS frontside device, and $\rm\sim1.8\times10^{-2}\ ct\ cm^{-2}\ s^{-1}\ keV^{-1}$ in an ACIS backside device.

 

Table 4.75: Estimated On-orbit Background. Top row results from canonical cosmic ray spectrum; bottom row from SOHO EPHIN detector rates.

Estimated On-orbit Background (ASCA Grade 02346 events)
Estimate	FI chips	BI chips
	$\rm ct\ cm^{-2}\ s^{-1}\ keV^{-1}$	$\rm ct\ cm^{-2}\ s^{-1}\ keV^{-1}$
Cosmic ray spectrum	$\rm\sim 0.010$	$\rm\sim 0.018$
SOHO EPHIN based	$\rm\sim 0.020$	$\rm\sim 0.036$

 

A second estimate of the AXAF background may be derived from measurements obtained with SOHO and reported by the EPHIN team. The so-called INT channel of EPHIN's sibling on SOHO measures a total flux of $\rm0.38\ ct\ cm^{-2}\ s^{-1}\ sr^{-1}$ in its orbit at L1. This channel integrates all electrons with energies in excess of 8 MeV and all nucleons with energies in excess of $\sim50$ Mev/nucleon. If all of these particles interacted with the ACIS as if they were minimum ionizing muons (the dominant cosmic-ray daughter species observed at sea-level), we would expect about twice the background listed above at the ACIS focal plane (i.e., about 2- $4\rm\times10^{-2}\ ct\ cm^{-2}\ s^{-1}\ keV^{-1}$). We have made no attempt to model the interaction of the actual on-orbit particle spectrum with the instrument. A significant fraction of the high energy protons observed in the INT channel will penetrate directly to the detectors (which are shielded with about $\rm 10\ g\ cm^{-2}$ of Al). If we arbitrarily assume that energetic protons comprise all of the events detected in the INT channel, then the INT rate provides an upper limit to the expected on-orbit ACIS background. To the extent that some INT events are high energy ( $\rm E > 10~MeV$) electrons, this assumption is incorrect.