Frequently Asked Questions to CR 110
What are charge sensitive preamplifiers?
Charge sensitive preamplifiers were developed to detect the total amount of charge flowing from a detector as the result of a 'pulse' event, such as the detection of individual particles or gamma-rays. The preamplifiers integrate the pulse of current flowing from the detector over time (by virtue of a small capacitance in the feedback loop) to produce an output which is proportional to the charge into the preamplifier input. A large valued resistor in parallel to the feedback capacitor slowly discharges the capacitor, restoring the preamplifier output to its original state. Unlike voltage sensitive preamplifiers, charge sensitive preamplifiers must have low input impedance so that the preamplifier can easily sink (or supply) charge from the detector.
How are charge sensitive preamplifiers more suitable for use with particle detectors than voltage sensitive preamplifiers?
Ionizing events within detectors generally produce an amount of ionized charge that is proportional to the energy of the incoming particle or gamma ray. For this reason, the detector preamplifier should be configured in a way to produce an output that is precisely proportional to this ionized charge. Voltage sensitive preamplifiers were first used to read out solid state detectors when they were first developed in the '40s. A problem was noted, however, in that the signal voltage at the preamplifier input was not only proportional to the ionized charge, but also inversely proportional to the input capacitance. Because the detector capacitance is usually a weak function of the temperature, temperature drifts were causing drifts in the preamplifier gain and degrading the energy resolution. For this reason the charge sensitive preamplifier was developed, which has a gain equal to the reciprocal of the feedback capacitance, and more importantly independent of the input capacitance. For many decades, charge sensitive preamplifiers have been the standard design for use in detectors where the energy measurement of individual ionizing events is of interest.
The decay time of the preamplifier output pulse is quite long. Do I have to worry that pulses will build on previous pulses and cause a 'pile up' of events?
The point at which to be concerned about the effects of pulse 'pile up' is after the preamplifier output pulse has been filtered through a shaping amplifier. The shaping amplifier (also called 'linear amplifier', 'spectroscopy amplifier', or 'pulse amplifier') dramatically changes the shapes of the pulses, generally giving them a longer risetime and a much quicker fall time, and restores the baseline to prevent pile up as much as possible. Events that appear to pile up before the shaping amplifier often become very clearly separated after the shaping amplifier.
What is the bandwidth of the CR-110?
The term 'bandwidth' is generally not used when discussing charge sensitive preamplifiers - instead one describes their rise time due to a delta current pulse input (which charges the feedback capacitance), and their decay time due to the discharge of the feedback capacitance through the feedback resistance. In general, one seeks a fast pulse rise time, but not necessarily a short decay time. In fact, if the feedback resistor value were substantially decreased in order to quicken the decay time, the added thermal noise due to this decreased resistance would be unacceptable.
How can we check to see whether the preamplifiers are operating within the specified noise level?
The method described here requires the following:
- A test circuit board (such as the CR-150-X) with an appropriate power supply.
- A low noise Gaussian shaping amplifier, having a shaping time of 1 s. The CR-200-1 s Shaping Amplifier used with the CR-160 evaluation board would be suitable.
- A pulse height analyzer.
- A tail pulse generator or square wave generator.
- A silicon p-i-n photodiode (Hamamatsu S1223 or equivalent), and a bias supply (100 volts if using the Hamamatsu S1223).
- A small 241Am isotopic source.
To measure the noise of the preamplifier, the gain of the detection system must first be precisely measured (in keV per channel). To do this, construct the circuit shown in Figure 2 (the CR-150-AC-X test board could be used for this). A p-i-n photodiode should be used as the 'detector', and a bias power supply should reverse bias the detector to its maximum allowed value. The 241Am source should be oriented so that its emissions can irradiate the photodiode. The circuitry and photodiode should be in a well shielded, light tight box. Route the preamplifier output to the Gaussian shaping amplifier (1 s), which should have its output routed to a pulse height analyzer. Acquire a 241Am pulse height spectrum, in which you should be able to clearly detect the 60 keV gamma-ray emission (see Figure 6). Note the channel number at which the 60 keV photopeak is observed. The gain of the detection system is the ratio: peak channel number / 60 keV.
Next, disconnect the input lead of the preamplifier (pin 1) from the test circuit board. This can be done a using a variety of methods, but make sure that pin 1 is left floating and does not touch the circuit board or other components. Connect the tail pulse generator (or square wave generator) to the preamplifier via a small valued capacitor of no more than just a couple pF. Alternatively, you can use a 'dangling wire' connected to the tail pulse generator and rely on the small capacitive coupling between the input and the wire to make this connection (be sure, though, that the wire does not move during the subsequent measurements). Acquire a pulse height spectrum of the tail pulse signal, which should appear as a Gaussian distrubution. Measure the width of this distrubution by measuring, in channels, the full width at half the maximum value (denoted as FWHM).
The noise can then be calculated by dividing by the previously measured gain, to yield a figure having units keV FWHM (Si). To convert this figure to the more generally applicable units of electrons RMS, divide by 0.0036 keV (the ionization efficiency of silicon) and divide again by 2.355 (converting FWHM measurements to RMS).