Text Reading
Melissinos, Sec 8.4 and Sec 9.2
I . Introduction
In this experiment you will use radioactive sources with emissions of known energy to calibrate a NaI(Tl) detector and measure the position of the Compton edges resulting from known gamma ray lines in Na^22, Cs^137, Co^60 and Co^57.
The experiment consists of four parts:
- Calibration with known lines.
- Measurement of Compton edge and comparison with calculation.
- Measurement of energy resolution δE/E vs E for a NaI(Tl) crystal and a plastic scintillator.
- Measurement of attenuation coefficients for Al and Pb with a number of absorbers of known thicknesses.
II. Theory
The theory for scintillation counters is presented in Melissinos, Sec 8.4.2 and Compton scattering in Sec 9.2
III. Experiment (1-3)
a. Connect the photomultiplier tube with the NaI(Tl) scintillator attached to the preamplifier (Canberra 2007B) and then into the spectroscopy amplifier (Canberra 2012).
[Do not exceed -2000 V on the NaI(Tl) phototube !].
Observe the output pulses at each stage. Check
- the raw PMT signal
- the preamplified signal
- the amplified signal on the oscilloscope [Note that the amplifier should be set to accept positive pulses.]
Adjust the pole zero time constant with a small screwdriver to obtain symmetric pulses with no under- or overshoot.
Connect the output to the MCA and start the MAESTRO software package on the PC. Observe the energy spectrum for each element.
b. Adjust the high voltage and gain to position a prominent X-ray line so that a factor of two adjustment of the amplifier gain will be visible. Verify that the pulse height changes by about a factor of two.
c. Using several sources (Na^22, Co^60, Cs^137 and Co^57), identify the x-ray lines from photopeaks or e+e- annihilation (Na^22), and then construct a plot of MCA channel number vs. gamma energy in MeV. This is your calibration curve. Do not change the high voltage or this plot will need to be repeated !
N.B. The ORTEC “EASY MCA” is now available. Please use it for pulse-height analysis. Save the spectra as ASCII files for later analysis.
d. Record the spectrum from one or more sources with clear gamma lines and measure the energy of the Compton edge. Convert to MeV using your calibration curve and compare with the calculated maximum energy given to an electron in a Compton scattering of a gamma ray of the observed energy. (note there is a “pedestal” or offset in the calibration)
e. From the monochromatic gamma lines, measure the fractional energy resolution (δE/E) as a function of energy. Why does the resolution change as a function of energy ?
f. For comparison, examine the pulse height spectrum from the plastic scintillator. How does it compare to the NaI crystal ? (Are the mono-energetic lines visible ? Explain why or why not.)
IV. Measurement of attenuation coefficients
Using the monochromatic energy lines that you calibrated and identified in the first part of the lab, measure the attenuation coefficients mu where
I(x) = I(0) exp(-mu x)
Typically, the attenuation coefficient mu = K Z^n E_gamma^m, where m is usually around -3, Z is the atomic number, E_gamma is the photon energy and K is a constant.
Using different thickness absorbers, measure mu at two energies for the different materials. Note that you will have to increase your counting time as the absorber thickness increases in order to keep the statistical uncertainties small.
Note that textbooks often quote the mass attenuation coefficient mu^’ = mu/ rho where rho is the density. The quantity mu^’ has units of cm^2/g.
Technical Notes:
- The preamplifier is a small box above the NIM rack. It is connected to the back of the Canberra amplifier and receives power through this connection.
- Compton scattering has an edge when the electron and photon scatter at 180^0. Use the formula for the energy in Compton scattering as a function of angle to verify there is a maximum energy.
- Use lead shielding to enhance the backscatter peak
Last modified: November 9, 2018.
Tom Browder