XMM Users' Handbook

   
EPIC internal background

The internal background is dominated mainly by $\gamma$-rays generated within the spacecraft in response to the cosmic ray flux. An industry study indicates that for a nominal particle flux, and a best estimate of the spacecraft mass distribution, the event rate in EPIC MOS would be of order $1.5\times10^{-4}$ counts cm^-2 s^-1 keV^-1. The spectrum is expected to be virtually flat. No detailed model was performed for the EPIC pn camera, but it was estimated that with an active layer some 4-5 times thicker than the MOS CCDs, the rate might be typically a factor of 2 higher. The uncertainty due to variation of the cosmic ray rates around the orbit and with time is probably also a factor 2 at best.

For some basic assumptions on the pulse height invariant channel widths and other factors, the following estimates for the PHA file characteristics of the particle background component in the XMM cameras were estimated by us:

• EPIC MOS (channel width = 3.4 eV): $4.4\times10^{-9}$ + (B $\times\ 3.32\times10^{-13}$)

• EPIC pn (channel width = 4.25 eV): $8.7\times10^{-9}$ + (B $\times\ 8.3\times10^{-13}$)

• RGS MOS (channel width = 1 eV) $8.82\times10^{-10}$ + (B $\times\ 6.6\times10^{-14}$)

where the units are counts mm^-2 s^-1 channel^-1 and B is the channel number, giving the weak energy dependence.

If it proves possible to correlate a particle-induced background component with the event rates measured in the EPIC Radiation Monitor (ERMS, § 3.6.2), then it should also be possible to include this as part of a background correction calibration file.

The dark currents of both types of EPIC CCDs are negligible sources of background noise under nominal operating conditions.