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A) Pulse Height Spectroscopy

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.

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A) Pulse Height Spectroscopy

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  1. 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.

  2. 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.

  3. 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?

  4. 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

  5. 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 Egammais gamma ray energy you used for calibration for this peak.

  6. 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 < 10 V, then the amplification setting is proper. If V, 2 10 V, the amplification should be reduced. (If Vm < 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 < 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 > 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

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