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Fortunately, this process is neither difficult nor time consuming.
The process for determining appropriate hardware settings consists of placing a known radiation source of
modest activity in front of the detector, adjusting one or more of the hardware settings, and beginning an
acquisition. Repeat this process until a satisfactory spectrum is achieved. Each of the components of this
process is discussed in slightly more detail below.
Radiation Source and Detector
For most gamma scintillation detectors that will be used with URSA-II, an approximately 1 μCi 137Cs source
is probably the best to use in performing the hardware setup. It provides two distinct peaks; a low-energy
multiplet centered around 32.89 keV and a higher energy gamma at 661.65 keV. Another isotope can be
used provided that the expected spectrum is known, but the procedures in this manual assume that 137Cs
will be used. Whatever source is used, be cautious of using one with too great an activity. Many scintillation
detectors, especially those originally designed for hand-held battery powered instruments have a high
resistance dynode string (e.g., 120 Mohms) to minimize the current draw from the HV power supply.
Unfortunately, this also has the effect of causing peaks to shift as a function of the incoming count rate. If
you are purchasing a new detector for use with your URSA-II, it is recommended that a dynode string of
about 6 Mohms be specified. The general thumb rule for spectroscopy is that lower is better, but do not use
a detector with a dynode string less than 4 Mohms. If you are using a detector with a high resistance dynode
string, the best way around this is to perform energy and efficiency calibrations at approximately the same
count rate as the samples you will be analyzing.
Input and Polarity
The URSA-II has two detector inputs. Input 1 is the series “C” connector right next to the DB9 serial connector.
This input has the signal and the high voltage coupled just like on most hand-held instruments. If your
detector has only one connector, Input 1 is the one you should use. If your detector has two connectors, one
for high voltage and one for the signal, you should use Input 2. The SHV connector carries the high voltage
for Input 2 and the BNC connector accepts Input 2’s signal. Only one of the inputs should be connected to a
detector at any one time.
Most single-connector detectors produce negative pulses, so negative polarity should be chosen. Many two-
connector detectors (especially those with internal preamplifiers) produce positive pulses, but this cannot be
taken for granted.
You can either consult the detector manufacturer as to the pulse polarity or just try both polarities and see
which one works.
If your detector requires voltage for an internal preamplifier, the URSA-II can provide positive and negative 12
volt DC through the mini DIN-7 connector and the (included) adapter. The pin out of this adapter follows the
NIM standard [QQQ put in pin voltages] and should couple directly to the detector.
The URSA-II can also accept pre-shaped positive pulses through either input. Note: pulses exceeding about
5V in magnitude (whether positive or negative) will be clipped. If you use this feature an attenuator may be
required.
High Voltage
If you have past experience with the detector, the voltage at which it was operated is a good starting point
for the URSA-II. This is also often referred to as “bias voltage.” Alternately, you can check with the detector’s
manufacturer for an approximate bias voltage or at least a not-to-exceed voltage. Failing that, you can just
increase the high voltage to the detector until you get a decent looking spectrum.
The detector bias voltage used for spectroscopy is generally a bit lower than the “plateau” voltage used in
many grosscount instruments.
