Equipment: High-purity Ge detectors with supporting electronics; 4000-channel Tracor-Northern pulse-height analyzer.
Readings: Solid-state radiation detectors, sect. 5.5; Instruction for operating the Ge detectors are at the end of this write-up. For a table of x-ray energies, see the CRC handbook or links from the course’s home page such as http://xray.uu.se/hypertext/XREmission.html. ..
Key concepts: Semiconductor detectors, depletion zone, active detection depth.
Caution: The HPGe detector may be damaged if it is not cooled with liquid nitrogen several hours before use. I nstructions for operating the HPGe detector will be found at the end of this write-up. In particular, understand how the high-voltage supply works and apply a voltage no greater than 500 V to the detector.
8.1 High-resolution gamma-ray energy spectra
Measure pulse-height spectra for a variety of radioactive sources such as 22Na, 133Ba, 60Co, uranium and radium. Plot of energies of photopeaks versus channel in order to calibrate the spectrometer and test its linearity.. Interpret as many features of the spectra as you can (for example, photopeaks, compton distributions, backscatter peaks, photofractions) and compare them with features of pulse-height spectra using scintillation detectors. You might measure a spectrum for radium or uranium overnight and try to idenntify as many of the photopeaks as possible.
8.2 Energy spectra of electrons from 137Cs
A 137Cs source emits beta-rays and conversion electrons. Using the "thin" 137Cs source with the aluminized mylar cover, measure the combined electron and photon pulse-height spectrum from a 137Cs source. Determine the electron spectrum by measurements with and without an electron absorber, such as a piece of plastic about 5 mm thick. Interpret the electron spectrum in terms of the nuclear decay scheme of 137Cs.
Optionally, measure the energy-loss of electrons as they pass through absorbers such as 25 micrometer-thick Al-foils. Measure changes of the electron spectra as more absorbers are placed between the source and detector. Determine the energy-loss per unit absorber thickness. If available, try another type of absorber such as thin plastic sheets to compare energy losses per unit qualtity of absorber. .
8.3 High-resolution x-ray spectra
Using a 57Co or 241Am source, readjust the gain of the amplifier to display photopeaks over a range up to a maximum energy of about 150 keV. You should be able to observe photons down to the ~6 keV x-rays of 57Fe. Interpret as many features as you can in spectra from 57Co, 241Am and 133Ba.
8.4 Moseley's law using gamma-rays to excite the x-rays
Use the same method as in Experiment 5 to generate characteristic x-rays of the elements, but using the HPGe-detector instead of the Kr-gas proportional counter. Measure x-ray energies as before, but try to interpret the more detailed structures which can now be detected. In particular, you will probably be able to resolve K-alpha and K-beta gamma rays. As an advanced project, you might test the validity of Moseley's law for the diffeerent series of K-xray lines. Also, you may be able to test more critically whether the square-root of the x-ray energy varies linearly with atomic number.
Appendix: Using the High-Purity Germanium Detector
1. The detector can not be operated without being cooled to liquid nitrogen temperatures since the thermal current of Ge at room temperature creates too much noise. Also, the preamplifier may be damaged if the dewar is not filled since it also is cooled by the cryogen. In this experiment we will only use the upper detector. At least 3 hours prior to intended use, fill the dewar of the upper detector.
2. After the detector is cooled, apply a bias voltage of -500 volts to the detector using the high-voltage supply model AEC-5000B. Output polarity should be negative (LED indicator).
4. Measurements of radioactive sources. After retracting the "fish coffin" from underneath the detector by gently pulling on the coffin, position sources at the center of the "head" of the coffin. Push the coffin back under the dtector gently, until it comes to the stop. Be careful that no objects stick out of the coffin or they could damage the detector, which is very close to the top of the coffin. You are now ready to start accumulating your spectra.