Silicon detector

A silicon detector, placed in vacuum, detected low energy charged particles such as 3.5 MeV $\alpha$ particles from dt fusion and 3 MeV protons from dd fusion. For Run Series 1, a passivated, ion-implanted planar silicon (PIPS) detector of active thickness 150 $\mu$m and area 600 mm² was operated at a bias voltage of 30 V. For Run Series 2, two similar PIPS detectors, but of active thickness 300 $\mu$m and area 2000 mm² (diameter 50 mm) were used. Silicon detectors, such as PIPS detectors, and more commonly used silicon surface barrier (SSB) detectors, are in principle reverse-biased diodes. Charged particles slowing down in a carrier-depleted region of the silicon crystal creates electron-hole pairs, which are swept toward and collected at the electrodes by an applied bias field. PIPS detectors offer advantages over SSB detectors, such as thinner yet more robust entrance windows, and lower leakage currents, which improves energy resolution.

  

Figure 3.22: Electronics diagram for a silicon detector. The detector output was pre-amplified to give a fast timing signal and a slow energy signal. The timing filter amplifier, with the use of an external delay, optimized the pulse shape which gave a sub-trigger SITR if in coincidence with EVG, and opened the ADC gate for the amplified energy signal. In Run Series 2, where two silicon detectors were used, both functioned in an identical manner. A 4-input 8000-channel ADC was shared between silicon detector(s) and a germanium detector.

Figure  3.22 shows the electronics diagram for silicon detectors. Pre-amplifiers were placed immediately next to the feed-through connectors of the vacuum system, in order to reduce the total capacitance. Amplification of energy and timing output of the preamps was done with linear amplifiers and timing filter amplifiers, respectively, before the signals were sent to the counting room.

Considerable efforts were made to improve energy resolution in the tritium environment. This included shielding the pre-amplifier and the cables leading to it with a copper foil and copper braid material respectively, grounding the detectors and electronics system to the target vacuum chamber, and using a noise-filtered power supply.

Mounted on the cryogenic thermal shields, the detectors were kept at about 90 K, which reduced the leakage currents and thereby improved the energy resolution.

For Series 2 the horizontal dimension of the detectors was collimated to 13.9 mm to view only the inner sides of the gold foils, where the reactions of interest took place. Furthermore the collimation restricted the angular path of the alphas and protons entering the detectors, preventing the ones with grazing angles which suffer large energy loss in the layer. This reduced variations in the detection efficiencies when layers with different thicknesses were used. The energy scale of the silicon detectors was calibrated with an ²⁴¹Am source.