NOAA KLM User's Guide

Section 7.5.2

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7.5.2 MEPED Calibration MEPED Particle Calibration

The MEPED telescopes were calibrated with proton and electron beams at a number of accelerators, covering the energy range from 20 keV to several MeV. The omnidirectional sensors were calibrated with proton beams over the range of 13 MeV to 153 MeV. The calibrated particle response geometric factors for the telescopes are listed in Table Note that the electron channels have a secondary response to protons which has the same geometric factor. The listed geometric factors are the calculated values for the detector and collimator geometry of the telescopes, and agrees quite well with the measured geometric factors for particles in the appropriate energy range. The proton channels also have a small response to high energy electrons, which is primarily in the 0P1 and 9P1 channels. The electron geometric factor of the 0P1 and 9P1 channels varies from about 10-7 cm2-sr at 30 keV to 10-6 cm2-sr at 100 keV to 10-5 cm2-sr at 300 keV to 10-4 cm2-sr at 2 MeV.

Table MEPED Telescope Calibrated Geometric Factors.
Channel Designation Particle Type Detected Energy Range (keV) Geometric Factor
G(cm2 -sr)
0E1 and 9E1 Electrons >30. 0.0100
0E2 and 9E2 Electrons >100. 0.0100
0E3 and 9E3 Electrons >300. 0.0100
0E1 and 9E1 Protons 210 - 2700 0.0100
0E2 and 9E2 Protons 280 - 2700 0.0100
0E3 and 9E3 Protons 440 - 2700 0.0100
0P1 and 9P1 Protons 30 - 80 0.0100
0P2 and 9P2 Protons 80 - 250 0.0100
0P3 and 9P3 Protons 250 - 800 0.0100
0P4 and 9P4 Protons 800 - 2500 0.0100
0P5 and 9P5 Protons 2500 - 6900 0.0100
0P6 and 9P6 Protons >6900 0.0100

The calibrated proton geometric factors for the four omnidirectional sensor channels are given in Table, which lists the measured angular responses integrated over the angular range of 0 to 105. This corresponds to the approximate loss + reflected particle cone over the polar caps, where the omnidirectional sensor measurements of solar proton fluxes is expected to be most important. The omnidirectional sensors have detector energy loss thresholds of 2.5 MeV, which effectively eliminates any response to electrons and electron bremsstrahlung. This produces a nominal upper limit of about 500 MeV for detected protons, which is moderately broad because of energy loss straggling and variations of particle path in the detector with angle and position.

Table MEPED Omnidirectional Sensor Calibrated Geometric Factors.
Channel Designation Particle Type Detected Energy Range (MeV) Geometric Factor
G(cm2 -sr)
P6 Protons 16 - >500 1.50 (See Note 1.)
P7 Protons 35 - >500 1.50 (See Note 1.)
P8 Protons 70 - >500 1.50 (See Note 1.)
P9 Protons 140 - >500 1.50 (See Note 1.)

1. Geometric factors integrated over the angle range of 0 to 105, which includes most of the particle-filled cone over the Polar caps.

The thresholds and combinatorial logic which determine the various particle energy channels are calibrated for each MEPED using radioactive sources with precise x-ray energies. The telescope detectors are calibrated against the 59.54 keV x-ray from 241Am, which produces a strong photo-peak in the measured spectrum. This is used to calibrate a precision Pulse Generator, which is then used to calibrate all the threshold levels. The omnidirectional sensor detectors and thresholds are calibrated using the 661.6 keV gamma-ray from 137Cs, which produces a strong Compton edge at 477.3 keV. This Compton edge is used to calibrate the precision Pulse Generator, which in turn is used to calibrate detector gain and threshold level values. All MEPED detectors are selected to have the same sensitive thickness and detection area, so the calibrated particle responses for all MEPEDs should be identical. MEPED In-Flight Calibration

The MEPED In-Flight Calibration (IFC) is an automatic calibration verification started by command from the ground. Processing the telemetered data gives the energy of each threshold and the full width at half maximum (FWHM) noise for the telescope detectors. After the command, the IFC starts at the next Major Frame sync pulse from the spacecraft. The IFC terminates automatically, but may also be terminated by command. The DPU controls the IFC cycle, which is used to verify all of the electronic gains, threshold values, and coincidence logic for the MEPED.

During the IFC negative pulses at a frequency of 2080 Hz are applied to the front end of each charge-sensitive amplifier. The amplitude (voltage) of the pulses is stepped up in a linear staircase of 0 to 191 steps. Groups of four pulse amplitudes are counted and telemetered, making 48 values received on the ground. The MEPED IFC has two phases, 0 and 1. The pulses are scaled for each detector and IFC phase. Phase 0 measures low energy thresholds and telescope detector noise widths, and phase 1 measures high energy thresholds. The fraction of the maximum count (a count of all the pulses if the pulse height is well above the threshold) for a given IFC output is called F. For Phase 0 the variation of F with IFC pulse amplitude is fit with a Gaussian distribution to determine both the threshold value and the detector FWHM noise. For Phase 1 there is generally only one value of F which is between 0 and 1, and this value is used to measure the threshold.

The threshold values used in the MEPED are listed in Table, which shows which threshold is used for the lower energy determination of the listed particle channels. The IFC Phase used for each threshold measurement is also listed. Note that all telescope thresholds are actually two separate values, one for the 0 telescope and one for the 90 telescope. Thresholds for the "E" channels are for the electron telescopes, while thresholds for the "P" channels are for the proton telescopes.

The actual thresholds are derived from the IFC count data usingcalibration factors obtained from the calibrated threshold values for each MEPED. The MEPED IFC calibration factors are provided in a Calibration Report for each instrument.

Table MEPED Thresholds and IFC Phase for Measurement.
Threshold Value
Particle Channels IFC Phase FWHM Noise Measurement
LS1 (2 levels) 25.6 0E1 and 9E1 0 Yes
LS2 (2 levels) 98.1 0E2 and 9E2 0 Yes
LS3 (2 levels) 299. 0E3 and 9E3 1 No
LS4 (2 levels) 2500. 0E3 and 9E3 1 No
LS1 (2 levels) 21.4 0P1 and 9P1 0 Yes
LS2 (2 levels) 70.7 0P2 and 9P2 0 Yes
LS3 (2 levels) 243. 0P3 and 9P3 1 No
LS4 (2 levels) 796. 0P4 and 9P4 1 No
LS5 (2 levels) 2498. 0P5 and 9P5 1 No
LS6 (2 levels) 50. 0P6 and 9P6 0 Yes
LS1 (omni) 2500. P6 1 No
LS2 (omni) 2500. P7 1 No
LS3 (omni) 2500. P8 1 No
LS4 (omni) 2500. P9 1 No

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