Fiber Test with SiPM
Testing the various fibers with a silicon PM at JLAB is done using the same experimental setup as when testing the fibers with an XP2020 photomultiplier. The only difference is that the XP2020 PMT is replaced by a silicon PM from Sensl of type A20HD 3x3 mm. The break down Voltage of this device is at 27.9 Volts and was operated at 2 Volts above break down at 29.9 Volts. The supply voltage for the electronics board is set at +/- 5 Volts.
Because the signal from the SiPM has a considerable longer decay time then the XP2020 PMT the ADC gate needed to be increased from the initial 60ns to 140ns. This proved to be sufficient for the SiPM signals. At the same time the signal from the SiPM into the ADC had to be delayed by about 30ns because the SiPM has a much shorter transition time as compared to an XP2020 photomultiplier tube. The additional trigger to the LED in the previous setup with the XP2020 PMT is used as an arbitrary trigger to measure the dark rate. This data is needed to correct the SiPM response the extract the number of photo-electronics from the Sr90 source as will be described below.
As an example a typical response of the SiPM is shown here on three panels explaining the necessary corrections that need to be taken into account
The top right panel shows the response of the SiPM when triggering the DAQ arbitrarily using the former LED-trigger. This shows the effect of the dark rate of the SiPM. The red curve is a fit to the spectrum with three Gauss functions. The first peak is the pedestal and the second peak is the one photo electron peak. At the bottom left panel in black the SiPM response to the trigger from the trigger-scintillator is shown and in red the estimated background triggers caused by cosmic rays instead of the electrons from the Sr90 source. In the case of cosmic rays, such particles usually do not pass through the fibers and therefore behave the same way as the arbitrary LED-trigger. The red histogram is the renormalized spectrum from the LED-trigger using measured fact that cosmic rays cause about 11.3% of all trigger-scintillator triggers. The blue histogram is the resulting background-subtracted spectrum shown once again more in the top right panel with a multi-Gaussian fit in red. The second plot shows a more extreme example of the additional triggers from cosmic rays causing an increased pedestal amplitude and because of the large dark rate of the SiPM also a much increased amplitude of the one and two photo electron peaks. This causes a reduced mean of the raw ADC spectrum resulting in a too low value for the extracted mean number photo electrons if one does not correct for this effect.
To extract the mean number of photo-electrons from the SiPM ADC spectrum is done as follows:
is the mean of the ADC spectrum. Since root does not calculated the mean of a spectrum correctly when subtracting one spectrum form another this number has to be calculated externally. is the pedestal peak position and is the peak position of the one photo-electron peak which is the second Gaussian peak in the ADC spectrum. Instead of using as the denominator the mean distance between neighboring Gaussian peaks is used.
In the following the results of the extracted mean number of photo electrons for different fiber types is shown. The numbers are obtained according to the above description to get from the SiPM response. The first column indicates the approximate position of the Sr90 source from the SiPM. This number is not the same for all fiber types but within about +- 5cm for the 45cm values , +- 2cm for the 100cm and 195cm values. The only exception is BCF60 for the 195cm value. This fiber is shorter than the others and the exact position of the Sr90 source was only 174cm in this case. The uncertainty given is only statistical and a systematic uncertainty of about 5% can be assumed.
|195||2.22 +/- 0.02||1.98 +/- 0.03||2.69 +/- 0.02||1.95 +/- 0.03||2.36 +/- 0.03||3.29 +/- 0.04|
|100||2.75 +/- 0.02||2.62 +/- 0.01||3.42 +/- 0.04||2.69 +/- 0.01||3.32 +/- 0.04||3.82 +/- 0.01|
|45||3.30 +/- 0.03||3.06 +/- 0.04||4.09 +/- 0.02||3.23 +/- 0.02||3.77 +/- 0.04||3.91 +/- 0.03|