Timing Spectroscopy Techniques for Radiation Detection Optimization

1. Measurement of Timing Properties Chapter No. 17 Radiation Detection and Measurements, Glenn T. Knoll, Third edition (2000), John Willey.

3. A) Timing Spectroscopy-Walk Optimization of CFD • 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. Ensure that the detector power supply has the same polarity as the detector voltage polarity, otherwise change the polarity on the power supply. • Connect the detector anode out put to an oscilloscope. • Switch on the detector power supply and apply the detector voltage. • Now place a gamma ray source near the detector. Observe the anode signal on the oscilloscope. • Note the rise time of the anode pulse. • Connect the anode pulse to the input of the CFD. Select CFD mode on the CFD. • Connect a delay cable corresponding to 40-90% of rise time in the delay connection of the CFD. • Connect the walk inspect signal of the CFD as per attached diagram to the oscilloscope. • Adjust the walk according to Figure (b) using potentiometer on CFD

4. A) Optimization of Walk of the CFD • Measure the detector anode pulse rise time on the oscilloscope. • Connect the anode output of the detector to input of the CFD. • Select CFD mode of the CFD and connect to the input of the MCA. • Select a delay cable for a delay corresponding to 40-90% rise time of the anode signal. • Connect the walk inspect output of the CFD to the oscilloscope as per attached circuit • Adjust the walk switch to bring the walk pattern similar to figure (b) shown below.

7. A) Recording Coincidence Delay Curve • Connect one the positive output of the CFD to oscilloscope and note its width. • Connect the second output of the CFD also to the oscilloscope and look for their timing correlation. • Check the CFD output count rate by connecting it to the counter and count for one second. Record it. • Now connect one of the positive out put of the CFD to input B of the universal coincidence unit. • Now connect the second positive out put of the CFD to input C of the universal coincidence unit. • Select the coincidence and 2 coincidence mode on the universal coincidence. • Connect the output of the universal coincidence to the input of the counter. • Record the counter reading. This is the maximum count rate when both signals are overlapping • Now connect the second out of the CFD to the input C of the coincidence through a delay box. Select the zero ns delay. This is your maximum count rate with the delay box. If you vary the delay slightly, count rate does not vary significantly. Choose 2-3 sec counting interval. • Now record the count rate registered by the counter as a function of delay time. • You should get a curve similar to the one shown in the following figure but only half one. • The curve width corresponds to coincidence resolving time . • For universal coincidence unit, the resolving time is determined by signal width. In this case it was width of your CFD positive signal recorded by you in step #1 of this study. • Calculate percentage error in coincidence resolving time measurement? • If you put two delays one in each branch instead of one, will your error improve?

10. Time calibration of a TAC • 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

12. Timing Spectroscopy using TAC • 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.