Specific Principles of Operation

During operation, a region is created within the SSB detector that is similar to a np-junction. This is accomplished by placing a n-type semi-conductor (silicon) in contact with a metal (gold). In particular, a silicon surface is etched at room temperature, and then a thin layer of gold (approximately 40 micrograms/cm squared) is deposited on each side of the silicon wafer by evaporation. These surfaces are then allowed to oxidize slightly before they are mounted in an insulating ring within the detector as shown in the cross-section view in Fig. 21 For electrical contact, the insulating ring has metalized surfaces.

Creating the Depletion Region

Since the Fermi levels in the gold and the silicon are different, a contact EMF is created when the two material are placed together. As shown in Fig. 22, this contact EMF lowers the band levels in the semi-conductor to levels similar to those of a np-junction. The resulting depletion zone is located entirely in the semi-conductor and is also known as a Schottky barrier. In SSBs, the depletion depth can range up to 5 mm.

Biasing

During operation, the SSBs are reverse-biased to increase the thickness of the depletion region to the width and resolution desired for the experiment. As discussed earlier, this is accomplished by connecting a positive potential to the silicon and a negative potential to the gold in the SSB. Biasing voltages for SSBs typically range from 50 V to 300 V. The higher voltages reduce the noise in the detector by increasing the depletion thickness, but there is also the possible risk of breakdown. Ideal biasing voltages should be determined experimentally using an oscilloscope.

By increasing the size of the depletion region, the radiation will have a greater area over which to create electron-hole pairs, and the sweep area of the produced electric field will also be greater. The resolution of the detector can therefore be increased through appropriate biasing.

Creation of Electron-Hole Pairs

It is known that when ionizing radiation strikes the detector, not all of the energy is spent breaking the covalent bonds in the crystal. Some of this energy is released to the lattice in the form of phonons, while some is simply lost. Luckily, the operation of the device only requires the knowledge of what factors influence the creation of the electron-hole pairs and how to interpret the produced current.

When ionizing radiation strikes the detector, it will often excite an electron out of its energy level and consequently leave a hole. This process is known as the creation of an electron-hole pair. The details of the specific processes through which electron-hole pairs are created are not well known, however, it is known that the average energy needed to create an electron-hole pair at a given temperature is independent of the type and the energy of the ionizing radiation. In silicon, this energy is equal to 3.62 eV at room temperature and 3.72 eV at 80 K.

The creation of an electron-hole pair leaves two liberated charge carriers that are opposite in polarity. Since there is an electric field present in the depletion region, any charged particle liberated in the depletion region will be swept out of the area. After the electron- hole pair is created, the positive charge carrier will be swept towards the n-type region by the electric field, and the negative charge carrier will be swept towards the p-type region by the electric field. The movement of these charge carriers constitutes a small current which can be measured and analyzed.

How a Signal is Interpreted

The current produced by the creation of electron-hole pairs is the signal that is measured and analyzed. As demonstrated earlier, this current is proportional to the amount of ionizing radiation that reaches the detector. This current is then converted to a number of electrical pulses which are processed through an electronic circuit and viewed on a computer screen in the control room.