gamma ray spectrometer
Ken Cheney
5/8/2006
Pictures:
ABSTRACT
A gamma ray spectrometer is used to investigate nuclear energy levels, anti-matter, and shielding.
overview
Neutrons and protons in the nucleus have energy levels much like the more familiar energy levels of electrons in atoms. However there are a number of possible results when an excited neutron or proton moves to a lower energy level. These “decay modes” are named after the “particle” that emerges from the nucleus: alpha decay, beta decay, positron decay, etc. However there can be just a gamma ray emerging just like the visible photons that come from excited electrons in atoms and give bright line spectra. Or there may be a gamma ray in addition to any other decay mode.
These gamma rays have very specific energies. The isotope producing the gamma ray can be identified if the energy of the gamma ray can be measured. The energies of typical gamma rays from nuclear events are hundreds of thousands of times as energetic as visible photons produced by electrons changing energy levels. This huge energy makes the gamma rays “easy” to detect. With common equipment the gamma rays can be detected one at a time. In contrast generally millions of visible photons may be used to make a spectra using visible light.
Fantastically small quantities of radioactive isotopes can be detected leading to endless applications in engineering, physics, life sciences, and medicine. For example cancers are routinely located by detecting the radiation from radioactive tags on molecules that the cancer has concentrated.
WHAT TO DO
- Follow the Set Up/Calibration Instructions to connect and calibrate.
- Measure the gamma ray spectra of as many isotopes as possible.
- Print out each plot and calculate the percent error in the energy of each peak on the plots.
- Look for the Compton edge. Discuss
- Look for the 511kev peak for electron – positron annihilation. The gamma must have enough energy to produce the electron and positron: 2 x 511Kev. Discuss your results.
- Explore shielding and reflection with heavy and light elements.
- Investigate what isotopes are in the room by letting the spectrometer run for an hour or so and trying to identify the peaks observed.
electron production
The gamma rays produce electrons, which are then amplified, and the pulses (one per gamma) are counted, and the amplitude (proportional to the gamma energy) is measured.
Gammas produce electrons by three processes:
- Photoelectric effect: All of the gamma energy goes into the electron
- Compton Scattering: The gamma acts as a particle and scatters off the electron, sharing its energy and momentum. The energy of the electron is a function of the angle of scattering but, of course, reaches a maximum if the gamma is scattered directly backward.
Other angles of deflection give a broad continues background at energies lower than the edge.
(0.1)
Where:
= The energy of the scattered electron
= The energy of the original gamma
= The angle of deflection of the gamma ray
m0 = The rest mass of the electron
Gammas initially going away from the detector can be “Back Scattered” into the detector. They will have energy
Only gammas back scattered at around 180 degrees will enter the detector so a peak may appear at , the backscatter peak.
Compton Scattering produces a background of counts up to the maximum energy of the scattered electrons, which occurs for 180 degrees in Eq. (0.1).
(0.2)
The maximum electron energy is taken to be half way between the start and end of the Compton Edge.
- Pair production: The energy of the gamma goes into producing an electron and a positron. The positron soon meets an electron and annihilates producing a pair of 511Kev gamma rays. If one of these secondary gamma rays enters the crystal detector it will produce electrons proportional to 511Kev.
what is the equipment intended to do?
The equipment is designed to produce a plot of energy versus intensity for gamma rays. The “intensity” is literally the number of gamma rays detected within a “window” of some small range of energy centered on the plotted energy.
WHAT’S INSIDE?
NaI(Tl) Crystal
Starting from the gamma ray source the first part of the equipment is a crystal, which produces a number of low energy electrons (about 3ev) proportional to the energy that the gamma ray deposits in the crystal. The exact process seems to be a bit magical but the results are very good. The number of electrons produced is almost linear with energy and is matched very well with a quadratic fit.
The crystal is NaI with some Tl. The Tl improves the efficiency. More mystery.
Photo multiplier
The next step is to amplify the number of electrons produced, perhaps a million times. This amplification is done with a photo multiplier tube. The tube contains many electrodes with a fairly high voltage between them. The electrons are accelerated into each electrode and are given enough energy so they produce several additional electrons. These daughter electrons are accelerated into the next electrode and . . .
The photo multiplier tube has two connections: one connection is for the high voltage (perhaps 900 volts) and the other connection is to carry the final current resulting from the many steps of amplification.
Amplifiers and ADC
Within the Amplifier/ADC box (labeled “Spectrometer”) there are two more stages of amplification: Course and Fine Gain. There are controlled by the computer through software.
Of course the computer is not interested in analog signals, it requires digital signals. The last part of the Spectrometer box is an Analog to Digital Converter, which produces (very quickly) a number from 0 to 1023 proportional to the voltage of the last amplifier, which is (one hopes) proportional to the energy of the original gamma ray.
Computer
The computer will divide the available voltage range (adjustable to fit the isotopes used, maximum 8v) into 1024 “channels” and count the number of gamma rays with voltages following in each channel.
BEWARE: The computer labels the channels in Kev even when they are just arbitrary numbers. The Kev doesn’t mean anything until you calibrate.
To convert the channel number to gamma energy a quadratic function will be used by the computer. To find the coefficients of the quadratic equation we will give the computer the energies corresponding to three channels. We will know these energies because we have the computer measure the spectra of some known isotopes, in this case Cs-137 (with a peak at 662 Kev) and Co-60 (with peaks at 1173 Kev and 1332 Kev).
The initial spectra (histogram) will be of the number of counts for each channel versus the channel numbers (0 to 1023). After we point at each peak and give the energy the computer can find the coefficients to convert from channel to energy. The plot will then be of number of counts versus energy!
We don’t actually see the function; we just see that the horizontal axis of the plot changes from arbitrary “channels” (corresponding to electrons counted and hence energy) to energy units.
setup / CALIBRATION
Spectrum Techniques Inc UCS 20
Commonly used menu items:
File:
Print and Save
Settings:
ROI’s: Regions of interest
This lets you select a region (probably about a peak) on the plot with your curser. The computer then gives you lots of useful information about that region.
You can also “clear” a ROI or “clear all” ROIs
Energy Calibration: Must be used before reading any gamma energies. See instructions below.
Uncalebrate:
To undo existing calibration
Calibrate:
2 Point
3 Point: probably use Cs-137 (662 Kev) and Co-60 (1173 and 1332 Kev)
Auto Calibration
Amp/HV/ADC:
Amplifier gain, High Voltage setting, Analog to Digital Settings
Useful Icons (under the menu)
GO: starts measuring spectra
STOP: (appears during the taking of spectra)
Rectangular block: Erase
Y log: to show a wide range of y values
Initial Procedures
DON’T TURN ON THE COMPUTER YET!
To turn on:
Gray interface box (UCS 20): ON
Computer: ON
Computer desktop: click on the “UCS 20” icon
From Menu select Settings | AMP/HV/ADC
High Voltage: about 540,
“On, Off” select On,
Course Gain 4,
Fine Gain 1.2,
The default is probably ok on the others.
To make a spectrum:
- Click “Erase” if you want a new plot.
- Put your isotope in the holder, e.g. Co-60
- Click “Go” on the menu
- Wait and enjoy the development of the spectra as the gammas are detected one at a time!
- If you see good peaks proceed.
- If your plot is too flat try changing the “Course Gain” to 2
You can add another isotope while the spectrometer is running, the results will just be added to the first spectrum
To calibrate: “3 Point” recommended, easy when you know how!
- Make a spectrum with Cs and Co; let it run until it is easy to see the peaks.
- Tell the program what the energies of the peaks are:
Settings | Energy Calibrate | 3 Point:
- Use Kev energy units,
- For each of the three little boxes (asking for channel and energy) select a peak (the program will fill in the channel for you) then enter the Kev for Cs-137 (662 Kev) or Co-60 (1173 and 1332 Kev).
You make your selection with the mouse, clicking as close to the peak as possible. You can then find the peak for sure by using the arrow keys to move right and left one channel at a time. At the lower left of the screen are the Channel, and Counts. Find the channel with the highest number of counts.
To change the range of energies on the plot:
You want the spectrograph to fill as much of the graph as possible. If your spectrum has only relatively low energy peaks or unusually high energy peaks you should adjust the range of energies.
Under Settings | AMP/HB/ADC try changing the Course Gain, if you change High Voltage do so by 25 volt increments. I can’t figure out how the Fine Gain works.
references
1. Good on practice and theory.
2. “Spectech Quick Start Guide, UCS 20”, Spectrum Techniques, Inc., 2003 Just what it says!
3. “Experimental Ray Spectroscopy and Investigations of Environmental Radioactivity”, Randolph S. Peterson, Published by Spectrum Techniques, 1996 An excellent source of experiments, theory, based on Spectrum Techniques equipment, so very convenient for us.
C:\KEN\WP\Handouts\P1D\Lab Manual\Gamma Ray Spec 5 8 06.DOC Ken Cheney 1