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IONISING RADIATION LABORATORY DEMONSTRATION
AIMS
- To demonstrate the detection of ionising radiation with a Geiger-Müller (G-M) tube and with scintillators.
- To demonstrate the use of a sodium iodide scintillation counter linked to a channel analyser to observe the energy spectrum of radiation.
- To demonstrate the interactions of β and γ radiation with matter and to demonstrate (for γ radiation) the concept of “thickness density”.
BACKGROUND: IONISING RADIATION
Real sources are often of mixed nature and perhaps are unknown. Even a single radioactive isotope may give off several types of radiation.
β radiation interacts with matter by direct ionisation, which removes the kinetic energy of the β. A β particle is light enough to be scattered appreciably in different directions as it slows down. Furthermore, βs are emitted with a maximum energy, E max, but a spectrum up to this
maximum. Keeping this spread in mind it is likely that βs will be shielded against in matter by an approximately exponential law.
In contrast, α particles are emitted with a precise energy, and, in view of their heavy mass, are not deviated by ionisation (occasionally, however, they are deviated by a nuclear reaction). Thus they exhibit more or less precise ranges in matter (including air) and not an exponential distribution.
γs interact with matter through several mechanisms and Figure 1 illustrates the dominant mechanism at different γ energies for matter of different Z numbers.
Z
E / MeV
photoelectric effect dominant
Compton scattering dominant
pair production dominant
Figure 1: Domains of γ interaction with matter.
In the photoelectric effect (Figure 2), the energy of the γ photon is completely transferred to an orbital electron which is ejected from its atom. The γ no longer exists after the collision. The ejected electron then causes ionisation until it loses its energy and is captured by an atom. The photoelectric effect is more likely to occur when the photon energy is low, i.e. below 0.5 MeV and the absorber is a heavy material.
Figure 2: The photoelectric effect.
Higher energy γs may lose only part of their energy to an atomic electron, which is again ejected from its atom. This interaction is known as Compton scattering (Figure 3). The electron goes on to create ionisation as before. The remaining energy is taken up by another γ of reduced energy, which is scattered in a new direction. The new photon will either be absorbed by a photoelectric effect interaction, or, if the energy is still high, by further Compton scattering. Compton scattering occurs in all materials and predominantly with photons of medium energy, i.e. about 0.5 to 3.5 MeV.
Figure 3: Compton scattering.
γ photons with energy greater than 1.02 MeV may interact with a nucleus to form an electron- positron pair. This amount of energy is just sufficient to provide the rest masses of the electron and positron (0.51 MeV each) and the interaction is known as pair production (Figure 4). Excess energy will be carried away equally by these two particles which produce ionisation as they travel in the material. The positron is eventually captured by an electron and annihilation of the two particles occurs. This results in the release of two photons each of 0.51 MeV, known as annihilation radiation. These two photons then lose energy by Compton scattering or the photoelectric effect.
Figure 4: Pair production.
We are chiefly concerned with Compton scattering in this demonstration.
will determine what type of radiation the proportional counter will be able to detect. Argon and helium are the most frequently used fill gases and allow for the detection of α, β and γ radiation. When detection of neutrons is necessary, the detectors are usually filled with boron-triflouride gas.
Figure 6: Gas-filled detector (schematic).
A G-M detector operates in region V. The operation voltage causes a large number of ionisations to occur near the central electrode as the electrons approach (an avalanche effect). The large number of positive ions then form a positive ion sheath which prevents additional electrons from reaching the electrode, stopping the avalanche.
The G-M region has two important characteristics:
The number of electrons produced is independent of the applied voltage.
The number of electrons produced is independent of the number of electrons produced by the initial radiation.
Thus, the detector is very sensitive, but discrimination is not possible, since the pulse height is independent of the type of radiation.
Scintillation Counters
A scintillation counter is a solid state radiation detector that uses a scintillation crystal (phosphor) to detect radiation and produce light pulses.
Figure 7: The principle of operation of a scintillation counter.
As radiation interacts in the scintillation crystal, energy is transferred to bound electrons of the crystal’s atoms. If the energy that is transferred is greater than the ionisation energy, the electron enters the conduction band and is free from the binding forces of the parent atom. If the energy transferred is less than the binding energy, the electron remains attached, but exists in an excited energy state. When the electron ‘de-excites’, a photon is emitted. This photon may naturally be in the visible spectrum, or it can be contrived to be by adding suitable impurities (doping the scintillation crystal during manufacture).
Scintillation counters are constructed by coupling a suitable scintillation phosphor to a light sensitive photomultiplier tube. Figure 7 illustrates the principle and Figure 8 shows a schematic of a scintillation counter using a thallium-activated sodium iodide crystal.
Figure 8: Schematic of a scintillation counter.
There are three classes of solid state scintillation phosphors: organic crystals, inorganic crystals and plastic phosphors.
Inorganic crystals include lithium iodide (LiI), sodium iodide (NaI), caesium iodide (CsI), and zinc sulphide (ZnS). Inorganic crystals are characterised by high density, high atomic number, and pulse decay times of approximately 1 μs. Thus, they exhibit high efficiency for detection of γs and are capable of handling high count rates.
Organic scintillation phosphors include naphthalene, stilbene and anthracene. The decay time of this type of phosphor is approximately 10 ns. They are frequently used in the detection of β particles.
Plastic phosphors are made by adding scintillation chemicals to a plastic matrix. The decay constant is the shortest of the three phosphor types, approaching 1 or 2 ns. The plastic has a high hydrogen content; therefore, it is useful for fast neutron detectors.
The advantages of a scintillation counter are its efficiency and the high precision and counting rates that are possible. These latter attributes are a consequence of the extremely short duration of the light flashes, from about ns to μs. The intensity of the light flash and the amplitude of the output voltage pulse are proportional to the energy of the particle responsible for the flash. Consequently, scintillation counters can be used to determine the energy, as well as the number,
RADIATION SPECTROMETRY
If a γ is entirely absorbed in the scintillator, all its energy releases photons of fixed energy h ν, where h is Planck’s constant and is ν the frequency of the light (which is characteristic of the scintillator). Thus, the number of initial photons and, hence, the final pulse size after amplification are proportional to the original γ energy. Partial interactions, especially through Compton scattering (the most likely interaction), will give further lower energy interactions, which will show up as background, partially obscuring the peak.
A multichannel pulse height analyser is a device that will separate pulses based on pulse height. Each energy range of pulse height is referred to as a channel. As explained above, the pulse height is proportional to the energy lost by a γ. Separation of the pulses, based on pulse height, shows the energy spectrum of the γs that are emitted from a source. Multichannel analysers typically have 100 or 200 channels over an energy range of 0 to 2 MeV. The output is a plot of pulse height and γ activity, as shown in the example in Figure 9. By analysing the spectrum of γs emitted, the user can determine the elements that caused the γ pulses.
Figure 9: An example of multichannel analyser output.
ACCURACY
Recollect that for counting statistics the relative uncertainty in N counts is 1 N. So, if N counts are observed in time t :
Count rate r = N t Uncertainty (one standard deviation) N t in rate
∴ Relative uncertainty( N t )÷ ( N t ) = 1 N
Thus, to improve from 3% to 1% accuracy on statistics alone takes about 10 ten times as long.
The difference of two normal (normally distributed) counts N 1 and N 2 is N 1 (^)! N 2 but the
standard deviation of the difference is given by N 1 (^) + N 2. This fact is important when making corrections for the count rate due to background radiation.
UNITS
Activity is measured in becquerels (Bq): 1 disintegration/s (possibly more than one radiation).
The old unit of activity was the curie (Ci): 1 Ci = 37! 109 Bq.
Energy absorption ( absorbed dose D ) is measured in grays (Gy): 1 J/kg. To allow for the different biological effectiveness of different forms of radiation, the effective dose ( E ) is measured in sieverts (Sv): also 1 J/kg. E = W! D , where W is a radiation-specific weighting factor, the possible values of which may be taken as:
γs 1 (by definition) βs 1 αs 20 neutrons 10 (if of unknown energy)
SAFETY
All radiation is potentially injurious.
Even when the sources are weak they should still be handled correctly, using padded forceps. (This increases the distance between the source and the person handling it, giving some “geometric” shielding.)
Care should be taken not to drop any source and not to damage their windows.
At the end of the demonstration, you should wash your hands. By all means monitor your clothing etc. with the portable G-M tube.
The demonstrator will take you through a “close down drill” at the end of the session.
DEMONSTRATIONS
The demonstration will include:
Discussion of the legislative requirements and how they are met within the “Counting Lab”
Measurement of background radiation levels using a G-M detector
Radiation emission from “every day” objects
Detection of β and γ radiation using a G-M detector
The effects of various shielding materials on β and γ radiation
The effects of shields with similar “thickness densities” on γ radiation
Detection of γ radiation using a scintillation counter
Investigation of the energy spectrum of γ radiation using a scintillator/channel analyser
The “close down” procedure at the end of the session
