The radiation monitor on board XMM-Newton registers the total count rate and basic spectral information on the background radiation impinging on the spacecraft and its scientific instruments. The radiation monitor issues a warning when the intensity of the radiation exceeds certain level.
Three types of information are provided by the Instrument:
Counting of pulses for a selected energy band and above the low thresholds. Seven counters (which meaning will be explained below) are provided every 4 seconds
Spectral information: energy spectra with 256 channel resolution. A complete spectrum is provided every 512 sec or every 4 sec, depending on the Instrument mode (SLOW or FAST mode respectively).
A WARNING FLAG signal obtained from the seven counters here above, when the relevant threshold, which is set from ground TC, is exceeded. The threshold currently in use is 500 cps for any of the seven counters.
The Instrument is fully redundant (cold redundancy concept) as shown in fig 1.
The XMM-Newton Radiation Monitor consists of 2 detectors (see fig 1):
the Low Energy (LE) proton and electron unit
the High Energy (HE) particle unit.
All above units are based on Silicon diodes, which record the energy loss in the material (deposited energy), depending on the particle type and incident energy.
Fig. 1.
Description of the detectors and output data
Low Energy Detector
Characteristics:
Silicon detector, 500 µm of thickness, 0.85 cm2 of surface with a little window of Beryllium (20 µm thick) that gives free access to low energy electrons (E>50 keV) and protons (E>1.15 MeV) to the diode.
Sensitivity: due to the electronic threshold, actually only electrons with energy greater than 130 keV and protons greater than 1.3 MeV can be measured by this unit.
Outputs:
The output is digitized in the range from 0 to 3 V full scale with resolution of 256 channels. The maximum counting rate that can be measured is less or equal to 106 count per second.
The conversion factor between the deposited Energy and the channel is 15.8 keV / channel
Three counting rates are evaluated and sent to the telemetry every 4 seconds:
LE0 Count rate: represents the counting of pulses exceeding the low threshold, accumulated over all 256 channels, i.e.;
The energy range of incident particles measured by LE0 is:
Electrons: from 130 Kev to 1.5 MeV
Protons: from 1 MeV to 4.5 MeV
LE1 Count rate: represents the counting for the channels from channel 4 to channel 65 inclusive of the spectrum, i.e.:
The energy range of incident particles measured by LE1 is:
Electrons: from 160 Kev to 1 MeV
Protons: from 1 MeV to 1.5 MeV
LE2 Count rate: as LE1 but for channels from 66 to 255 inclusive. LE2 represent the e- and p+ of a medium energy:
The energy range of incident particles measured by LE2 is:
Electrons: from 1 Mev to 1.5 MeV
Protons: from 1.5 MeV to 4.5 MeV
High Energy Detector
Characteristics:
It consists of two Si junctions (500 µm thick), 0.85 cm2 of surface, placed in a 5 mm thick shielding and protected by a 500 µm window. These two junctions are with special electronics which detects:
single events (a particle passing though one junction, but not the other), this mode is called single and the result is then HES (High Energy Single),
coincident events (normally the same particle which deposits energy in both the junctions); this mode is called coincident (HEC: High Energy Coincidence).
Outputs:
The output is digitized in the range from 0 to 4.4 V full scale with resolution of 256 channels. The maximum counting rate that can be measured is less or equal to 5x105 count per second, 10% of which are in coincidence.
The conversion factor between the deposited Energy and the channel is 44.5 keV / channel for both HES and HEC
Four count rates are evaluated and sent to the telemetry every 4 second, i.e. 3 counts HES0, HES1, HES2 for single events and one count HEC for coincidence events. A detailed description is provided below:
HES0 Count rate: this counting represents the number of counts over the low threshold, accumulated over all 256 channels, i.e.:
The energy range of incident particles measured by HES0 is:
Electrons: from 1 Mev to 1.75 MeV
Protons: from 8 MeV to 40 MeV
HES1 Count rate: this counting represents the number of counts from the channel 5 to the channel 25 inclusive of the single spectrum.
The energy range of incident particles measured by HES1 is:
Electrons: from 1 Mev to 1.3 MeV
Protons: from 8.7 MeV to 9 MeV
HES2 Count rate: this counting represents the number of counts from the channel 26 to the channel 255 of the single spectrum.
The energy range of incident particles measured by HES2 is:
Electrons: from 1.3 Mev to 1.75 MeV
Protons: from 9 MeV to 37 MeV
HEC Count rate: It represents the full counts of the spectrum "coincidence" accumulated on all 256 channels:
The energy range of incident particles measured by HEC is:
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