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Response of the SFD channels to electron and proton fluxes

We recall herein the SFD calibration procedure and results already decribed in Technical Note A [2]. The SFD characteristics shown in Table 1.1 were deduced from experimental data [1], i.e. from the currents generated when the NSF was irradiated by electron and proton beams (See Figure 1.3 and 1.4). The SFD response to protons and electrons is at the heart of its operation as a flux-meter. This explains why special care was paid to check the validity of the measured response. Our conclusions are summarized as follows:

  • The NSF response to electrons was deduced from a single measurement at 3 MeV only (See Section 3.2 in Technical Note A). The energy dependence of the response should have been investigated more deeply however, as it is likely that secondary phenomena (electron backscattering,...) could seriously influence the actual number of electrons reaching the sensor and induce a strong energy dependence.
  • Even though the slope of the curves representing the currents as a function of the proton flux (see Figure 1.4) depend on the proton incident energy, one notices that the response (in A/(W/cm2) shown in Table 1.1) at 100 MeV is not significantly different from the response at 300 MeV. This lack of sensitivity to the particle incident energy leads us to characterize the SFD as an "energy deposition sensor".
  • The mean value of the response to protons at 100 MeV and 300 MeV will be used in all the calculations presented below. The validity of this mean value was checked in a successfull prediction of the current output by the NSF irradiated by protons with energies ranging from 50 MeV to 300 MeV (see Figure 1.5).
  • It should be noted that the SSF sensor was not tested with particle beams in its flight configuration.
  • The NSF response to electrons and protons is a characteristic of the whole system made of the scintillating fibre, the coupling elements and the photodetector. But the fact that this sensor based on an organic scintillator is more sensitive to protons than to electrons is incompatible with previous data [3]. Such a behaviour is conceivable only if the normalized fluorescence spectrum induced by electrons differs from the corresponding spectrum triggered by protons.
Figure 1.3: NSF channel output current as a function of electron flux
at constant energy (3 MeV) [1].
 
Figure 1.4: NSF channel output current as a function of proton flux
at constant energies (100 MeV and 300 MeV) [1].
 
Table 1.1: Response of the NSF and SSF channels to electrons and protons beam.
Particle Energy Energy loss Slope Response
  (MeV) (MeV/J) (A cm2 s) (A / (W/cm2)
Electron 3 0.16/0.26E-13 1.1E-8 4.2E-7
         
Proton 100 0.61/0.98E-13 2.1E-6 2.1E-5
  300 0.30/0.48E-13 0.9E-6 1.9E-5
 
Figure 1.5: NSF output current as a function of incident proton energy
at constant flux (7.106 p/cm2 s)
 

The mean values of the SFD characteristics described in this chapter are summarized in Table 1.2. These values will be used in Chapter 2 to get rough estimates of the SFD output currents and particle fluxes.

Table 1.2: Mean values of the main SFD characteristics.
Property
NSF
SSF
  e p e p
Response (A/W) 4.2 10-7 2.2 10-5 4.2 10-7 2.2 10-5
0.2 6.34 0.22 6.14
0.4 8.6 2. 30.
Emax (MeV) 5. 300. 5. 300.
0.19 0.37 0.21 0.39
0.19 0.89 0.18 0.85
1.64 10-2 1.73 10-2 2.77 10-3 1.98 10-2
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