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Pulses of light emitted by the scintillating material can be detected by a sensitive light detector, usually a photomultiplier tube (PMT). The photocathode of the PMT, which is situated on the backside of the entrance window, converts the light (photons) into so-called photoelectrons. The photoelectrons are then accelerated by an electric field towards the dynodes of the PMT where the multiplication process takes place. The result is that each light pulse (scintillation) produces a charge pulse on the anode of the PMT that can subsequently be detected by other electronic equipment, analyzed or counted with a scaler or a rate meter. The combination of a scintillator and a light detector is called a scintillation detector.
Since the intensity of the light pulse emitted by a scintillator is proportional to the energy of the absorbed radiation, the latter can be determined by measuring the pulse height spectrum. This is called spectroscopy. To detect nuclear radiation with a certain efficiency, the dimension of the scintillator should be chosen such that the desired fraction of the radiation is absorbed. For penetrating radiation, such as g-rays, a material with a high density is required. Furthermore, the light pulses produced somewhere in the scintillator must pass the material to reach the light detector. This imposes constraints on the optical transparency of the scintillation material.
When increasing the diameter of the scintillator, the solid angle under which the detector “sees” the source increases. This increases detection efficiency. Ultimate detection efficiency is obtained with so-called “well counters” where the sample is placed inside a well in the actual scintillation crystal.
The thickness of the scintillator is the other important factor that determines detection efficiency. For electromagnetic radiation, the required thickness to stop say 90 % of the incoming radiation depends on the X-ray or g-ray energy. For electrons (e.g. b-particles) the same is true but different dependencies apply. For larger particles (e.g. a-particles or heavy ions) a very thin layer of material already stops 100 % of the radiation.
The thickness of a scintillator can be used to create a selected sensitivity of the detector for a distinct type or energy of radiation. Thin (e.g. 1 mm thick) scintillation crystals have a good sensitivity for low energy X-rays but are almost insensitive to higher energy background radiation. Large volume scintillation crystals with relatively thick entrance windows do not detect low energy X-rays but measure high energy gamma rays efficiently.
The last effect only occurs at energies above 1.02 MeV. In practice, all effects have a chance to occur, this chance being proportional to the energy of the radiation and the atomic number (Z-value) of the absorber (the scintillation material).
In the photo(electric) effect, all energy of the radiation is converted into light. This effect is important when determining the actual energy of the impinging X-ray or gamma-ray photons. The lower the energy and the higher the Z-value, the larger the chance on photo effect.
Fig. 2.1 shows a typical pulse height spectrum measured with a 76 mm diameter, 76 mm high NaI(Tl) crystal in which the radiation emitted by a 137Cs source is detected. The photopeak, Compton maximum and backscatter peak are indicated. The lines around 30 keV are Ba X-rays emitted by the source.
The total detection efficiency (counting efficiency) of a scintillator depends on the size, thickness and density of the scintillation material. However, the photopeak counting efficiency, important for e.g. gamma-ray spectroscopy, increases with the Z 4-5 of the scintillator. At energies below 100 keV, electromagnetic interactions are dominated by the photoelectric effect.
The absorption can be calculated from the attenuation coefficient for a certain scintillator (or absorber). Consult the SCIONIX leaflet “Attenuation coefficients” for data on the most common materials.
Electrons (e.g. b-particles) can be backscattered from a material which implies that no energy is lost in the interaction process and the particle is not detected at all. The backscattering fraction is proportional to the Z of the material. For NaI(Tl) the backscatter fraction can be as high as 30%! This implies that for efficient detection of electrons, low Z materials such as plastic scintillators or e.g. CaF 2 (Eu) are preferred. The window material is also of importance.
Charged particles such as electrons, muons or atomic nuclei (e.g. a-particles) lose energy through Coulomb interactions with the atomic electrons in the surrounding matter. When selecting a detector for charged particles, the primary consideration is the type of particle to detect.
This includes low energy electrons, protons, a-particles and heavy ions. The rate of energy loss in matter increases as the charge and mass of the particle increase, but the conversion of particle energy in scintillation light decreases. The number of photons produced by an 5.4 MeV a-particle is only 70-80% of a gamma photon with the same energy. Apart from the emitted energy and the specific scintillator, the energy resolution for particles also depends on the surface treatment of the material.
The following aspects should be considered. The entrance window of the detector should be very thin so that the incident radiation is not absorbed; aluminized mylar windows are normally used. For heavy ions, the detector is best operated in a light-tight environment without a window. The thickness of mylar windows can vary between 1.5 mm and 100 m m.
Minimum Ionizing Particles
Particles in this group are usually single charged with a low mass and a high energy. Their energy loss per unit path length is small. Common examples of minimum ionizing particles are cosmic muons and fast electrons. In a plastic scintillator, minimum ionizing particles lose several MeV per cm material. Applications include calorimetry and electron spectroscopy.
Entrance window material and thickness are usually not that important since the particles normally pass through the window and the entire scintillator.
Scintillator Response To g–rays
Pulse Height Spectroscopy
The basic principle of pulse height spectroscopy is that the light output of a scintillator is proportional to the energy deposited in a crystal. The standard way to detect scintillation light is to couple a scintillator to a photomuitiplier. Furthermore, a g–ray spectrometer usually consists of a preamplifier, a main (spectroscopy) amplifier and a multichannel analyzer (MCA). The electronics amplify the PMT charge pulse resulting in a voltage pulse suited to detect and analyze with the MCA. The schematic is shown in Fig. 2.2. For a typical pulse height spectrum see Fig. 2.1.
An important aspect of a g -ray spectrometer is the ability to discriminate between g-rays with slightly different energy. This quality is characterized by the so-called energy resolution which is defined as the width (FWHM) of the photopeak at a certain energy.
Besides by the g -ray energy, the energy resolution is influenced by :
- The light output of the scintillator,
- The size of the scintillator (light collection),
- Photomultiplier characteristics (quantum efficiency and photocathode homogeneity)
At low energies where photoelectron statistics dominate the energy resolution, the energy resolution is roughly inverse proportional to the square root of the g-ray energy.
The energy resolution of a scintillation detector is a true detector property, limited by the physical characteristics of the scintillator and the PMT or other readout device.
A typical energy resolution for 662 keV g -rays absorbed in small NaI(Tl) detectors is 7.5 % FWHM. At low energies, e.g. at 5.9 keV, a typical value is 45 % FWHM. At these low energies, surface treatment of the scintillation crystal strongly influences the resolution. It may be clear that especially at low energies, scintillation detectors are low resolution devices unlike Si(Li) or HPGe detectors.
The time resolution of a scintillation detector reflects the ability to define precisely the moment of absorption of a radiation quantum in the detector.
The light pulse of a scintillator is characterized by a rise time and by a 1/e fall time t(= decay time see section 3.1). It is obvious that the best time definition of an absorption event is obtained when the scintillation pulse is short (small decay time) and intense. Furthermore, the rise time and time jitter of the PMT and of the electronics are important.
Small cm size NaI(Tl) detectors have typical time resolutions of a few nanoseconds for 60Co (1.2 MeV). Much better time resolutions can be obtained with plastic or BaF2 scintillation crystals. BaF2 is presently the fastest known scintillator with detector time resolutions of a few hundred picoseconds.
A sensitive way to check the energy resolution of a scintillation detector is to define a so-called peak-to-valley (P/V) in the energy spectrum. This criterium does not depend on any possible offsets in the signal. Either the peak-to-valley between two gamma peaks is taken or the ratio between a low energy peak and the PMT / electronic’s noise.
A good P/V ratio for a 76 x 76 mm NaI(Tl) crystal is 10 : 1. This is equivalent to an energy resolution of 7.0 % at 662 keV. At 5.9 keV, a high quality X-ray detector can have a P/V ratio of 40 : 1.
Extreme count rate changes, temperature variations or instable electronics may cause peak position variations in a spectrum. To compensate for these effects it is possible to calibrate the peak position with a so-called Am-pulser . This is a very small radioactive 241Am source mounted inside a scintillation detector. The a-particles, emitted by the 241 Am, cause scintillations in the crystal that are detected by the PMT (or the photodiode) of the detector. For NaI(Tl), the a -peak is situated between a Gamma Equivalent Energy (GEE) of 1.5 and 3.5 MeV and can be specified. Count rates are 50, 200 and 1000 cps. The position of the pulser peak is used as a reference to compensate for the above mentioned variations in detector response. The above way of calibration is not ideal since the response of most scintillation crystals for g-rays and a-particles is different. However, a second order compensation using e.g. a thermistor is only necessary for large temperature ranges.
Also other spot-activated scintillation crystals can be used for the above application. The best choice depends on the type of host crystal, the required GEE and required calibration accuracy.
For occasionally monitoring your system integrity, Light Emitting Diodes (LEDs) or laser ports can also be used. LEDs can be mounted inside scintillation detectors or a window for that purpose can be provided. Apart from these ways of pulse height stabilization, it is of course possible to stabilize electronically on the peak of an (always present) external source. Sometimes the 40K background line can be used for this purpose.