JINST Instrumentation Theses Archive

Ph.D. degree thesis
accepted by University of Coimbra, Portugal, in 2005

Luis Manuel Panchorrinha Fernandes

Supervisor: Joaquim Marques Ferreira dos Santos

Characterization of large area avalanche photodiodes for detection of X-rays, vacuum ultraviolet and visible light

 Keywords:

• Avalanche photodiodes
• X-ray detectors,
• Spectrometers
• Photon detectors for VUV and visible light(solid-state)

 Abstract:
In this work, original studies of the properties of large area avalanche photodiodes (LAAPD), used as X-ray detectors as well as photosensors for vacuum ultraviolet (VUV) and visible light, are presented.

The operational characteristics of four LAAPDs manufactured by Advanced Photonix Inc. (API) were investigated at room temperature for detection of X-rays. The energy resolution for 5.9 keV X-rays, obtained in the four investigated photodiodes, is found in the 10-18% range for the optimum gain of each LAAPD due to the dark current differences between the several used LAAPDs. The dark current contributes to the electronic noise, affecting the energy resolution and the minimum detectable energy. Photodiodes with lower dark current can detect minimum energies of about 1 keV. Despite the LAAPD limited area, previous characteristics together with its simplicity, compact structure, absence of radiation window and high counting rate capability (up to about 105/s) turn it out to be useful in varied applications, mainly in low energy X-ray detection. LAAPDs with lower dark current are able to achieve better performance than proportional counters.

The non-uniformity of the silicon resistivity originates gain variations due to the local absorption of X-rays, degrading the energy resolution. The non-uniformity effect was investigated and gain relative variations of 2-3% (standard deviation) were measured. Further than this effect, the peaks in the energy spectra present some asymmetry due to a low-energy tail. This background results from X-ray absorptions in different regions of the LAAPD, generating pulses with different amplitudes. The application of digital discrimination techniques, based on pulse risetime, allows a significant background reduction, improving the energy resolution and leading to better discrimination between low-amplitude signals and noise.

LAAPDs have been used as VUV-light photosensors in gas proportional scintillation counters, where they present advantages compared to photomultiplier tubes. The LAAPD characterization in light detection requires the use of X-rays as a reference to determine the energy deposited in the silicon. However, for higher gains, space charge effects resulting from X-ray local absorptions originate a non-linearity between the gains obtained for X-rays and light pulses. The gain ratio between 5.9 keV X-rays and VUV light was determined. For a gain of 200, 10 and 6% variations were observed for VUV light produced in argon (~128 nm) and xenon (~173 nm), respectively. The non-linearity was also investigated for visible light and a variation significantly lower was observed (less than 1% for a gain of 200).

The LAAPD gain and dark current dependence with temperature was investigated. The gain was determined for X-rays, VUV light and visible light, being observed relative variations with temperature of about -5% per �C for the highest gains. The obtained values are significantly higher than the ones stated in the literature (-3% per �C). The strong reduction of the dark current when the LAAPD is cooled originates significant improvements on the energy resolution and minimum detectable energy. The excess noise factor, another contribution to the energy resolution, was also determined, having been obtained values of 1.8 and 2.3 for gains of 50 and 300, independently on temperature. The results show that the energy resolution variation with temperature is not related to the excess noise factor, but is mainly related to the dark current. LAAPDs have been used as 1.9 keV X-ray detectors in the muonic hydrogen Lamb shift experiment. The LAAPD was selected because its response in X-ray detection practically does not vary with the presence of magnetic fields of intensity up to 5 T, required for the experiment. Moreover, its compact structure and fast time response constitute additional advantages in comparison with the tested gaseous detectors. The disadvantage of the LAAPD is associated to its limited area. This was partially eliminated using two series of LAAPDs. The several parts of the experimental system are described and the results of the experiment are presented.