BNC 970 Portable Multi-Channel Analyzer (MCA) - Manual (Page 40)

Model 970 Instruction Manual 37
6.2.2 Input and Polarity
The 970 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 as 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 970 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 and should couple directly to the detector.
The 970 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.
6.2.3 High Voltage
If you have past experience with the detector, the voltage at which it was operated is a good
starting point for the 970. This is also often referred to as “Bias Voltage.” Alternatively, 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 gross-count instruments.
With scintillation detectors the Bias Voltage also provides Gain, so essentially the same spectrum
can be obtained using a lower Bias Voltage and higher Gain or a higher Bias Voltage and a lower
Gain. If the Bias Voltage is higher than optimal, the spectrum will appear distorted.
6.2.4 Threshold
The Threshold, sometimes called input sensitivity or discrimination, is the minimum pulse height
in millivolts (mV) that will be recorded by 970 as a count. Since the 970 can accept pulses of
either polarity, the Threshold is entered as an absolute value. This can range from 0 to 2047 mV.
Photomultiplier tubes, cables, AC power, and the power supply and other circuitry within the 970
all generate electronic noise that can be erroneously interpreted as counts. This noise tends
towards the low energy region of the spectrum and can be removed by setting the Threshold
sufficiently high to block it. If a large peak occurs toward the left of the spectrum, (i.e., one that
overpowers “real” peaks in the spectrum), the Threshold should be adjusted to a higher value. If
you desire to observe the lower energy portion of the spectrum, the Threshold can be brought
closer to zero until the noise becomes apparent. Changes to the HV or Gain may cause the
location of the noise line to shift, requiring a readjustment of the Threshold. The two highest Gain
ranges will usually require a Threshold to be set in the 100 to 200 mV range unless the detector
is unusually “quiet.”
6.2.5 Coarse Gain and Fine Gain
The Coarse Gain and Fine Gain controls are used to increase the signal from a detector to the
point where it can be displayed usefully as a spectrum. These two controls are used together to
create an overall Gain. The overall Gain is generally the value you should follow.