Detector bias supply

When S is open (no ionization in the detector), the potential at point P must be V = Vq, i.e. the potential of the bias voltage. At time r = 0, S is closed (production or charge carriers has occurred in the detector because of a nuclear particle), and the charge of Cj + Cg flows through R- together with a small current from the bias supply through R. The potential in P decreases according to... 

The remaining block in Fig. 4.20 is the detector high-voltage bias supply, which is simply used to apply a reverse bias voltage across the Si(Li) detector diode. The dc voltage is typically in the range of —700 to —1500 V. 

Triple detector system as installed at the NIST Center for Neutron Research, Gaithersburg, MD, USA. (1) HPGe-detectors with 48 h dewars (2) amplifier and bias supplies (3) ADC with LFC module (4) network acquisition interface (MCA) (5) rate meter (6) LN2 fill system controller ... 

Loss of excitation Loss of antenna loading Excess antenna loading Failure of plate supply Failure of grid bias supply No protection required RF output detector and relay RF output detector and relay No protection required Plate overcurrent relay Plate overcurrent relay RF output detector and relay RF output detector and relay No protection required Does not apply No protection required Grid overcurrent relay RF output detector and relay Grid overcurrent relay Does not apply... 

It is not unknown for Dewars and cryostats to spring a leak. If this happens, the loss of vacuum results in a loss of thermal insulation. The increase in evaporation rate will then lead to complete loss of liquid nitrogen over a short time and warm-up of the detector. If a detector is allowed to warm up while the bias supply is connected, damage will be caused to the preamplifier. To prevent this, every detector system should have a temperature sensor and some means of switching off the bias if the detector warms up, whatever the failure - mechanical or human. Most bias supplies suitable for germanium spectrometry now have the appropriate cut-out built in and the detector manufacturers now routinely provide a temperature sensor. However, these systems do not work unless the user bothers to connect them together ... 

Available Bias Voltage. If the available bias supply voltage is less than the optimum or recommended bias voltage, large voltage drops across the load cannot be tolerated. The maximum responsivity in this case will be obtained when the load resistor equals the detector resistance. 

Three Complications When operating FPAs, we generally cannot set the detector bias directly instead, we command a bias supply that causes a detector bias that varies linearly with the bias supply voltage. Further, we measure an output that... 

The detector substrate is a backside-illuminated Si As IBC structure with 30 /um square pixels and a 20 pan thick epitaxially grown IR active layer doped to an As concentration of about 5x10 cm. A common transparent blanket contact is implanted at the active layer/substrate interface to supply the detector bias voltage. The magnitude of the bias field is set by the difference between the reference reset potential and the potential at the transparent contact. 

In this section, bias supplies for detectors are described. We can distinguish between several types of such supplies. 

Fig. 5.6 shows a circuit diagram of a HV-bias supply for a solid state detector. 

A basic radiation measuring system consists of the detector, a high-voltage power supply for bias voltage, a preamplifier, and a power supply for the preamplifier and signal amplifier. A pulse height discriminator is used to decrease the low-energy noise from the detector and the amplifier. 

In addition to the responsivity and the NEP, other parameters are of concern in selecting a detector for a particular application. The time constant or the response to signal modulation frequency is important, and so is the linearity or at least the reproducibility. In a more practical sense mechanical integrity, insensitivity to high-energy particle radiation, convenience in matching preamplifier characteristics and that of a bias power supply, temperature range, and other subtleties need to be considered in a detector selection process. 

Optimum Bias for Arrays We normally use one supply to bias all elements in an array - we cannot set separate biases for each element. To select the optimum bias for the array, we use the methods described above for individual detectors, but work with the array-average 5/V ratio or noise versus bias. In addition, we should also plot and attempt to minimize the array nonuniformity for an FPA. The current, noise, and uniformity should be quite independent of bias for a wide range of biases as long as we avoid the forward-bias region and the extreme back-bias range (where some detectors begin to break down), and we will obtain near-optimum performance. 

---