KJM5911 Lab Exercise 3 - Gamma Spectroscopy

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Learning Goals

After completing this exercise you should:

  • Understand the processes by which gamma radiation is absorbed in the detector.
  • Understand how the detector (and it's electronics) transform the absorbed energy from the gamma rays into electrical pulses which is continuously sorted according to pulse height and presented as a histogram (number of events vs. energy) - a "spectrum".
  • Know the basic parameters of a spectroscopy system (spectrum, real time, life time, dead time, channels, amplifier gain, lower lever discriminator, etc.).
  • Know the relationship between channel number and energy, how to derive this relationship ("energy calibration") and be able to evaluate it's quality and uncertainty.
  • Recognise the different parts of a gamma-ray spectrum.
  • Know how "peaks" in the spectrum are analysed and understand the results (you should understand terms like: ROI, gross count, net count, peak fitting, FWHM, etc.). You should understand how the software calculate and subtract the background under the peaks.
  • Know the relationship between number of counts and gamma-rays emitted by the source (and the disintegration rate). I.e. how to perform an efficiency calibration.


Experimental Procedure

This is a large exercise and will be divided between two working days.

  • Part 1: Measurements
    • Get familar with the Ge-detector system and Maestro
    • Make energy calibration for Maestro
    • Measure various calibration point-sources (sealed)
    • Measure various large-volum calibration sources
    • Measure some unknown sources
    • Measure some of the sources with a 2" NaI scintillation detector
  • Part 2: Energy calibration
    • Analyse the calibration spectra, for each "good" gamma-peak write down fitted centroid (channel number), net count, FWHM, fitted energy (from Maestro), and library energy (from Berkeley/Lund database).
    • Make (in Origin) a plot of library energy as function of gamma-peak centroid (channel number). Then make a linear fit to the data.
    • Use the linear fit parameters to calculate the energy your calibration yields for each of your gamma-peaks.
    • Calculate the difference between the library energies and the energies from your calibration. Do the same for the Maestro energis (compared to library energies). Plot the differences as function of gamma-peak energies. Evalute the plots!
    • Use your energy calibration and try to determine the unknown sources.
  • Part 3: Efficiency calibration
    • Make an efficiency calibration curve (in Origin) based on as many of the calibration sources as possible. Remember that the sources must have the same geometry.
    • Evaluate the quality of your efficiency calibration.
      • If the point sources is included in the efficiency calibration plot, how does it fit?
    • Use the efficiency calibration to calibrate the 152Eu source with unknown desintegration rate. Then assume that this rate is "true" and include the 152Eu data as if it was another calibration source. Is 152Eu a good source for efficiency calibration?
  • Part 4: Shielding and shielding effects
    • Investigate how the shielding affects your spectrum


You do not need to write a report from this exercise, but you should hand in the following plots:

  • Deviation plot for your own energy calibration curve
  • Deviation plot for the Maestro's energy calibration curve
  • Efficiency calibration curve (in log-log scale)
  • Efficiency calibration curve including the point sources
  • Efficiency calibration curve including the 152Eu gamma-lines.

Safety Aspects

  • We will work in the OCL chemistry lab. Most of you are not on the SAs for the OCL. Thus, you are not allowed to work in these labs on your own but must be supervised.
  • We will prepare calibration sources of natural thorium, natural uranium, 226Ra, and 152Eu. In particular the 226Ra and 152Eu sources are to be handled very carefully. All work preparing the sources should be performed in a hood.
ALI values and Excempt levels
Nuclei ALI Excempt Level (kBq)
kBq kBq mg
226Ra 9 10 -
232Th 0.090 10 2462
238U 0.60 10
natU - 1 81
  • As can be seen from the table, only about 0,8 g of natural uranium can be handled in a C-lab (10x the exempt level). As only very simple operations are to be performed on it, 10 times this should be permissible for a simple calibration source, But keep in mind that we have a great mix of sources.  
  • The sealed gamma sources are between 100 and 500 kBq each - keep them well shielded when not in use. A 1 MBq 60Co source generates a radiation field of 0.36 uSv/h at 1 m distance, substantially more if you are really close to it. Similar for the other sources. Actively use the radiation monitor to check the doserate during work with these sources. 
  • Never handle the sealed sources (even a "sealed" source can develop a leak) with your bare hands - use gloves and a pincher. We keep the sources inside glass vials, these should be ok to handle with your bare hands. There should be no need to take the sources out of the glas vials.
  • There are no dangerous chemicals involved in this exercise. Provided the lab is tidy and all chemicals are stored in the safety cabinet you do not need saftety glases during this lab. But make sure to check that there are no chemicals around!
  • All normal radiation safety precautions should be observed when working with the sources and make sure you check your hands and feet on the monitor before you leave the OCL lab.