A) Pulse Height Spectroscopy

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• A) Pulse Height Spectroscopy
• Identify the equipment such as detector,
  electronics modules and NIM bin.
• Note down detector type, size, operating voltage
  and its polarity.
• Read the manuals of NIM modules particularly
  input requirements and output specifications and
  its principle of operations.
• Connect the circuit diagram as shown in Figure.
• Apply high voltage to the preamplifier.
• Connect the amplifier out put to an oscilloscope.
• Ensure that the detector power supply has the
  same polarity as the detector voltage polarity,
  otherwise change the polarity on the power
  supply.
• Switch on the detector power supply and apply
  the detector voltage.
• Watch amplifier signal on the oscilloscope. You
  may not see the signal on the oscilloscope.
• Now place a gamma ray source near the detector.
  Observe the amplifier unipolar or bipolar pulse ,
  as selected by you, on the oscilloscope.

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• A) Optimization of the Shaping Time of the
  Pulses
• Connect the output of the amplifier to input of
  the linear gate stretcher(LG)
• Select normal mode of the LG and connect to the
  input of the MCA.
• Acquire the pulse height spectrum of the detector
  pulse height in MCA.
• Calculate the FWHM and peak centroid of gamma ray
  peak in the spectrum
• Calculate percentage energy resolution of the
  detector by dividing FWHM by peak centroid and
  multiplying it with 100.
• Record the shaping time and corresponding
  energy resolution of the detector.
• Then change the shaping time and record the
  resolution for three or four more readings.
• Then plot energy resolution as a function of
  shaping time in an excel sheet.
• From the graph determine the optimum shaping time
  of the amplifier.

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• A) Recording of Amplifier signals gated with SCA
  signal
• Observe the unipolarr output of the amplifier on
  the oscilloscope and note down its maximum
  height and time width.
• Connect its unipolar output to a delay amplifier.
• Observe the shape of the output of the delay
  amplifier on the oscilloscope and compare it with
  the shape of unipolar output of the spectroscopy
  amplifier. Do you see any difference between
  them.
• Connect the bipolar output of the amplifier to
  input of the single channel analyzer (SCA)
• Select normal mode of the SCA and connect its
  output to input of the gate and delay generator
  (GDG)
• Adjust the height and width of the GDG output on
  the oscilloscope to a suitable value i.e height
  and width should be large enough to accommodate
  unipolar output of the amplifier
• Connect GDG output to channel 1 of the
  oscilloscope and the external trigger input of
  the oscilloscope. Select external trigger mode of
  the oscilloscope.
• Connect output of the delay amplifier to channel
  2 of the oscilloscope and the external trigger
  input of the oscilloscope. Select external
  trigger mode of the oscilloscope.
• View both GDG and delay amplifier signal
  simultaneously on the oscilloscope .
• Adjust delay of GDG and delay amplifier such that
  the amplifier signal lies between the GDG gate
  signal.
• Connect GDG output to gate input of the linear
  gate and stretcher (LG) while delay amplifier
  output is connected to linear input of the LG.
• Connect output of LG to MCA.
• Acquire the spectrum in MCA. Increase the LL of
  SCA and you should observe the lower level cut
  in the sectrum generated by SCA.
• Acquire the ungated spectrum in MCA with LG mode
  in as normal. Do you see any difference between
  gated and ungated spectra of MCA. Why?

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• Energy calibration of a Multi Channel Analyzer
• Determination of range of energies involved.
  Assume this is Emax (MeV).
• Select a gamma ray source that emits particles
  of known energy with energy corresponding to the
  maximum energy. Select Co60 source. One observes
  the signal generated on the screen of the
  oscilloscope. It should be kept in mind that the
  maximum possible signal at the output of the
  amplifier is 10 V.
• In energy spectrum measurements, one should try
  to stay in the range 0-9 V. It is good practice,
  but not necessary, to use the full range of
  allowed voltage pulses. The maximum pulse Vm can
  be changed by changing the amplifier setting.
• Determination of MCA settings.
• One first decides the part of the MCA memory to
  be used. Assume that the MCA has a 512-channel
  memory and it has been decided to use 512
  channels, full memory. Calibration of the MCA
• . Calibration of the MCA means finding the
  expression that relates particle energy to the
  channel where a particular energy is stored. That
  equation is written in the form
• E a1 a2C a3C2 , where C channel number
  and a1 , a2, a3 , ... are constants.
• The constants a1 , a2, a3 ... are determined by
  recording spectra of sources with known energy.
  In principle, one needs as many energies as there
  are constants. In practice, a large number of
  sources is recorded with energies. You just
  choose two gamma rays with known energies one
  near the maximum energy and other near the
  minimum energy for example Na22, Bi or (Cs
  Co60) sources covering the whole range of
  interest.
• Most detection systems are essentially linear,
  which means that energy calibration of the MCA
  takes the form E a1 a2C
• With

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• Record the pulse height spectrum of your selected
  source in MCA.
• Record the channel number C1 and C2 for energies
  E1 and E2 respectively
• Calculate coefficients a1 and a2 of energy
  calibration of MCA measured by you.
• Store your calibration spectra in excel sheet and
  plot energy calibration spectrum.
• Make a lest square fit to your energy calibration
  data.
• Compare the values of coefficients calculated
  using excel sheet and your manual calculation.
• Discuss the deviation between the results of the
  two data sets.
• Now record pulse height spectrum of a gamma ray
  with an unknown energy.
• Calculate energy of the unknown gamma ray source
  using your calibration scheme.
• Calculation of energy resolution of the
  detector
• From the excel plot of you calibration spectrum ,
  determine CL and CR channels , which corresponds
  to channels on left and right side of the peak
  centroid at half of the maximum height.
• Energy resolution () a2(CR-CL)/Egamma, where
  Egamma is gamma ray energy you used for
  calibration for this peak.

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he calibration of an MCA follows these steps 1.
Determination of range of energies involved.
Assume this is 0 I E I Em (MeV). 2. Determination
of preampliJier-amplifier setting. Using a source
that emits particles of known energy, one
observes the signal generated on the screen
of the oscilloscope. It should be kept in mind
that the maximum possible signal at the output of
the amplifier is 10 V. In energy spectrum
measurements, one should try to stay in the range
0-9 V. Assume that the particle energy El results
in pulse height Vl. Is this amplification proper
for obtaining a pulse height Vm I 10 V for energy
Em? To find this out, the observer should use the
fact that pulse height and particle energy are
proportional. Therefore, If Vm lt 10 V, then the
amplification setting is proper. If V, 2 10 V,
the amplification should be reduced. (If Vm lt 2
V, amplification should be increased. It is good
practice, but not necessary, to use the full
range of allowed voltage pulses.) The maximum
pulse Vm can be changed by changing the amplifier
setting. 3. Determination of MCA settings. One
first decides the part of the MCA memory to be
used. Assume that the MCA has a 1024-channel
memory and it has been decided to use 256
channels, one-fourth of the memory. Also
assume that a spectrum of a known source with
energy El is recorded and that the peak is
registered in channel C,. Will the energy Em be
registered in Cm lt 256, or will it be out of
scale? The channel number and energy are almost
proportional, i.e., Ei - Ci. Therefore If Cm 1
256, the setting is proper and may be used. If Cm
gt 256, a new setting should be employed. This can
be done in one of two ways or a combination of
the two 1. The fraction of the memory selected
may be changed. One may use 526 channels of 1024,
instead of 256. 2. The conversion gain may be
changed. In the example discussed here, if a peak
is recorded in channel 300 with conversion gain
of 1024, that same peak will be recorded in
channel 150 if the conversion gain is switched to
512. There are analyzer models that do not allow
change of conversion gain. For such an MCA, if C,
is greater than the total memory of the
instrument, one should return to step 2 and
decrease Vm by reducing the gain of the
amplifier. h he correct equation is E a bC,
but a is small and for this argument it may be
neglected proper evaluation of a and b is given
in step 4 of the calibration procedure. 312
MEASUREMENT AND DETECTION OF RADIATION 4.
Determination of the energy-channel relationship.
Calibration of the MCA means finding the
expression that relates particle energy to the
channel where a particular energy is stored. That
equation is written in the form where C channel
number and a, , a,, a, , ... are constants. The
constants a,, a,, a,, ... are determined by
recording spectra of sources with known energy.
In principle, one needs as many energies as there
are constants. In practice, a large number of
sources is recorded with energies covering the
whole range of interest, and the constants are
then determined by a least-squares fitting
process (see Chap. 11). Most detection systems
are essentially linear, which means that Eq.
9.12 takes the form E a, a,C