Suppression of low energy Background by rise-time discrimination in silicon drift detector

Complete text of publication follows. Best Si(Li) and HP Si detectors posses high quality, thin metal Schottky-barrier (SB) entrance windows. Due to the high electric field and low surface recombination, the charge collection efficiency (CCE) for thermalized carriers in these detectors is rather high even in the vicinity of the SB. Thus, for low energy X-rays the background continuum (BC) is related mostly to energetic electrons created in the primary interaction of the detector material with the incident X-rays, through escaping out of detector volume into SB layer, and vice versa. These processes result in more or less flat BC with steps at characteristic detector and SB matrial energies [1]. Typical BC to peak height ration (B/P) for 6 keV X-rays are 1/10000 and 1/30000 above and below SiK edge. Best pn junction entrance window silicon detectors have to date somewhat inferior B/P performance [2]. In these type of detectors, and lower quality SB Si(Li) detectors, as well, there is an incomplete charge collection (ICC) layer in which the CCE is significantly lower than unity. Such an ICC region will, necessarily, increase background, by spreading events according to CCE amplitude range within ICC. The exact shape of BC will depend on the particular CCE depth function. While the effect of a 'thin' ICC region is a mere increase of the BC with some extra 'rounding' of the characteristic steps beyond Fano statistics, in the case of a 'thick' ICC region with linear CCE dependence from 0 to 1, the BC will be high and flat with no characteristic structures at all. Since it is expected that collection of charge carriers from an ICC region may cause certain time delay, this fact can, at least in principle, be utilized for discriminating against such events. An early experiment [3] was, however, unable to resolve such tiny time lags at 6 keV. Unlike Si(Li) detectors, having extremely long (up to, or even above 0.1 ms) noise corner time constants, for SDDs, thanks to their extremely low capacitance (cca. 0.1 pF), it is below 1 ms, i.e. much closer to charge collection time (cca. 100 ns). The related lower jitter makes timing much more accurate, and rise-time discrimination (RTD) more promising. In a recent experiment with a KETEK SDD of cca. 2.5 mm diameter we have succeeded in getting a significant BC reduction at 6 keV incident energy. Without discrimination the BC was almost flat, with a B/P ratio of 1/800. Using RTD [4], this value decreased to 1/2000 above, and 1/3000 below SiK edge energy. Collimating MnK X-rays to diameter 1 mm in the center of SDD the improvement is more pronounced: the B/P ratios are 1/7000 and 1/17000 in corresponding regions. This means, that timing is rather critical: even small spatial- and related time-broadening [5] of electron packets during their collection from peripheral regions strongly diminishes efficient RTD. (author)