Understanding some fundamentals of radiation counting systems is essential to prudent selection of the equipment that will be purchased and deployed for these applications. This section of the catalog is intended to present some fundamental concepts to non-expert users about radiation measurement systems. It will help you understand the basic theory of radiation measurement and in selecting the type of detectors and options that will best meet your application requirements. Nucsafe is committed to solving problems with our clients. Whether in advising about instrument capability and suitability, after-sales support, service or training, Nucsafe is here to help you get the most out of the equipment and to make its operation simple and reliable for the user.
Radiation is energy traveling in the form of particles or waves in bundles of energy called photons. Some everyday examples are microwaves used to cook food, radio waves for radio and television, light, and x-rays used in medicine.
Radiation is energy that is either ionizing or non-ionizing. Ionizing radiation – invisible waves and particles emitted from radioactive atoms include alpha, beta, x-ray, gamma and neutron radiation. Some atoms (e.g., uranium and thorium) are naturally radioactive, whereas others (e.g., tritium and iodine-131) can be made radioactive in reactors or accelerators.
Radioactivity is a natural and spontaneous process by which the unstable atoms of an element emit or radiate excess energy in the form of particles or waves. These emissions are collectively called ionizing radiations. Depending on how the nucleus loses this excess energy either a lower energy atom of the same form will result, or a completely different nucleus and atom can be formed.
Ionization is a particular characteristic of the radiation produced when radioactive elements decay. These radiations are of such high energy that when they interact with materials, they can remove electrons from the atoms in the material. This effect is the reason why ionizing radiation is hazardous to health, and provides the means by which radiation can be detected.
Background radiation is radiation from our natural environment. It comes primarily from cosmic rays, radioactive material in the earth, naturally occurring radionuclides (such as potassium-40) in food, and radon gas that is in the air we breathe. In the United States, the average background radiation dose is about 300 mrem/year.
Manufactured sources of radiation contribute an additional dose of approximately 60 mrem/year, of which approximately 54 mrem/year is from medical procedures (e.g., x-rays and certain diagnostic tests). Consumer products (e.g., lantern mantles and smoke detectors) contribute roughly 6 mrem/year.
Fallout radiation (still present in our environment from the era of above-ground nuclear testing) contributes less than 1 mrem/year. Figure 2 shows typical annual radiation doses in the United States.
When radiation deposits energy in a person, he or she receives a radiation dose. Radiation doses are measured in units of rem or millirem (mrem). One thousand millirem is equal to one rem (1000 mrem = 1 rem).
The primary risk associated with radiation exposure is an increased risk of cancer. The degree of risk depends on the amount of radiation dose received, the time period in which the dose is received, and the body parts that receive the radiation dose. Although scientists assume that low-level radiation doses increase one’s risk of cancer, studies have not demonstrated any adverse health effects in individuals who are chronically exposed to small radiation doses over a period of many years (e.g., a total of up to 10,000 mrem above the average background dose).
The increased risk of cancer from occupational radiation exposure is small when compared to the normal cancer rate in today’s society. For example, the current risk of dying from all types of cancer in the United States is approximately 25 percent – while a person who receives a whole-body radiation dose of 25,000 mrem over his or her lifetime has a risk of dying from cancer of 26 percent – a one percent increase. This table shows the likely effects of total body radiation doses (measured in rem) to the humans.
Likely effects from total body radiation dose
|1000 rem||An acute dose would cause immediate illness and subsequent death within weeks.|
|100 rem||Acute dose could cause illness such as nausea, and cancer in 5% of persons within several years.|
|5 rem||MPD allowed by the NRC for occupational exposure over one year. No likely effects at this level.|
|0.3 rem||Estimated yearly exposure to all individuals from natural sources such as radon and cosmic rays, commonly referred to as background radiation. There are no likely effects at this exposure level.|
Gross counting is used to determine the presence of absence of radioactivity in an object. Once it has been determined that radiation is present above the ambient background levels, it is necessary to determine the specific radionuclides that are present. Specific gamma emitting radionuclides are determined by gamma spectroscopy, for example. This allows the person monitoring or searching for radiation to determine if it is natural, medical or industrial radionuclides or a radioactive source that should be detained and further investigated. Gross counting measurements are used to screen samples for relative levels of radioactivity. Gross counting is generally performed first to determine the presence or absence of either gamma-ray or neutron radiation. Both portable and stationary systems first measure the background radiation as a count rate. Background may be measured for a preset number of seconds or a continuous moving average of the background may be calculated. If a gross count rate measurement is determined to be above the previously measured background level, an alarm is issued as a visible, audible and/or vibration indication.
Data Acquisition Rates – Over-sampling
Systems used for search or continuous monitoring for homeland security applications must be able to find radioactivity in very short times compared to laboratory counting measurements. Typically, measurements are made in less than a few seconds an in the case of moving objects, one second or less. Over-sampling the counting interval provides fewer missed alarms. In the picture below, two data reporting frequencies are shown, 1 second and 100 milliseconds. If a vehicle, person or other object is moving in front of the detector, it may only be present for a second or less as shown in the previous table and graph of speed versus measurement time. If the system is sampling at 1 second intervals and the interval exactly corresponds to time and the object is in front of the detection system, then an alarm would occur. If, on the other hand, the 1 second sampling interval occurs such that one ends and another begins just as the object is centered on the detector, then 50% of the counts will be in the first interval and 50% in the second interval. This situation may result in the net counts not exceeding the alarm threshold setting. To overcome this possibility, the sampling interval can be reduced to a 100 milliseconds or less. We can then integrate the counts in each sampling interval such that the sum of 10 intervals equals a second of data. 100 milliseconds later we sample again and subtract the data from the oldest interval and add the new counts to the total. This moving average ensures that no more than 10% of the counts will be missed and in general, less than this amount. Over-sampling, by rapid polling of the counting data, thereby reduces missed alarms while allowing the sensitivity and precision of the measurements to be retained.
When a source is moving, the length of time it is in front of a radiation monitor and its detectors. The table here shows the distance traveled in feet per second (fps) related to the speed in miles per hour (mph). As 88 fps corresponds to 60 mph, a 10cm, 30cm and 50 cm wide sensor would have a source in front of them only 4, 11 and 19 thousandths of a second or milliseconds (ms) respectively. At more reasonable speeds such as those required in the ANSI N42.35 standard of 5mph, a vehicle would be moving 7.3 fps. In this case the time in front of a detector would be 45, 134 and 224 ms respectively. Although some radiation would hit the detector before and after the source the was directly in front of the detectors, these times are minimum to collect data. For moving sources, the over-sampling data acquisition of a system is essential. Nucsafe was the first commercial company to implement this method during the IAEA ITRAP assessments.
|3He 10cm = 3000cm2 5000cm2|
|2″ tube with 1″ moderator on each side|
The graph displays the data in the table and indicates that there is a large change in the available measurement time especially at low speeds. Ideally systems measuring moving objects should sample at no more than 100ms.
Selecting Gamma Detector
The choice of gamma ray detector depends on a number of factors including their efficiency (determined in part by their density) and their peak resolution (determined in part by their light outut). In addition, other detector material properties are important to consider such as whether the detector is rugged to thermal and mechanical shock and whether it is hygroscopic (absorbs water). Of course, price may also be a consideration in selecting a detector type. For gross counting only, there are several choices of detectors. Some applications require large area detectors such as large stationary systems. Here, the only choices are PVT plastic scintillator or NaI(Tl). A smaller NaI(Tl) detector can provide equivalent sensitivity to a PVT detector at higher energies because of the NaI(Tl) detectors better energy resolution allowing the use of regions of interest to improve the signal to background. However, at lower energies, the larger area detector will always provide a better sensitivity. As NaI(Tl) is more expensive than PVT, to have the same geometric efficiency may be cost prohibitive. On the other hand, if nuclide identification is needed, the NaI(Tl) detector will be required. Nucsafe can provide recommendations for the type and size of gamma ray detector needed for your application. For portable systems, Nucsafe uses NaI(Tl), BGO and CsI detectors. Each has advantages and disadvantages. Detector resolution and intrinsic efficiency are the most important considerations if size is equal. Energy resolution of a scintillator detector is a function of the intrinsic crystal resolution which is related to its light output, but is also affected by the photomultiplier tube and its photocathode which has a variable probability that a visible photon will produce a photoelectron that can be collected in first dynode stage of the PMT.
Detector Selection Table
|Portable Radiation Search Tool||ü||ü||ü||ü|
|PRST with nuclide ID||ü|
|Transportable Radiation Monitors||ü||ü||ü|
|TRMS with nuclide ID||ü|
|Continuous Radiation Monitors||ü|
|CRMS with Nuclide ID||ü|
|Handheld Gamma Spec||ü|
NaI(Tl) is recommended for all nuclide identification applications because it provides the best currently available energy resolution for gamma rays in a room temperature detector that is relatively inexpensive and available in a wide variety of sizes. BGO is heavier and thereby has higher intrinsic efficiency at higher energies, useful for measuring the 2.6 MeV gamma ray associated with highly enriched Uranium (HEU). CsI has good light output but poorer resolution than NaI, however it can be used with electronics that provide very low power consumption for portable applications. The detector selection table provides some physical parameters for these detectors.
Detector Parameters Table
|Melting point [k]||924||1050||894||894||75|
|Wavelength of emission max. [nm]||415||480||565||420||423|
|Refractive index at emission max.||1.85||2.15||1.80||1.80||1.58|
|Primary decay time 1/e [µs]||0.23||0.30||1.00||0.63||0.0024|
|Afterglow (after 6 ms) [%]||0.3-0.5||0.005||0.5-5.0||0.5-5.0||0.01|
|Typical resolution % FWHM 137Cs||6||10||8||9||180|
|Light yield [photons/MeVy]||38,000||8200||52,000||39,000||10,000|
For gross counting applications only, the following table provides a comparison of NaI(Tl) and BGO for varying gamma ray energies of interest for 1” and 2” thick detectors. Note the biggest difference is in the higher energy gamma rays because both types of detectors are nearly 100% efficient at lower gamma ray energies.
Calculated Photopeak Efficiency for NaI(Tl) vs BGO
|NaI(Tl) 1″thick||BGO 1″thick||NaI(Tl) 2″thick||BGO 2″thick|
Organic Plastic Scintillator
Polyvinyl Toluene (PVT) is a type of organic scintillator. PVT is not the only organic plastic scintillator, but clearly the most commonly employed for radiation measurement systems. It is dissolved in a solvent and subsequently polymerized forming the equivalent of a solid solution. These “plastic” scintillators are easy to shape and fabricate and are often the only practical choice if very large volume solid scintillators are required. When making very large detectors, the self-absorption of light may no longer be negligible and must be accounted for in the design of the system. For good quality PVT, detectors of more than a meter in length can be made as a single detector. Some designs requiring uniform response to radiation over a given height and width may require designs using more than one detector to ensure constant count rates independent of the radiation source position. Photon production in organic scintillator is a molecular process in which the energy deposited by a charged particle or photon, not dissipated as heat, results in an excited state in which the excess energy is carried away as an emitted photon. This fluorescent emission produces approximately 1 photon per every 100 eV of the energy deposited. Because the energy required to produce an excited state exceeds that carried away by photons, the probability for re-absorption of the emitted photon is small; i.e., the scintillator is transparent to the light that it generates. Secondary scintillators are frequently used to “shift” this emitted light to longer wavelengths, near the peak of a Photomultiplier (PMT) spectral-response curve. These secondary fluors have a high absorption cross section at the wavelengths generated by the primary scintillator, and respond to the energy deposited in exactly the same manner as described above, except that all energy levels are somewhat lower (wavelengths are longer) making the emitted light more compatible with the response of the PMT. PVT plastic scintillator has a polyvinyl toluene base with characteristics as follows: Density – 1.032 g/cm3 Electrons/cc – 3.39 x 1022 H atoms/cc – 5.28 x 1022 C atoms/cc – 4.78 x 1022 Index of refraction – 1.581 Attenuation length (l/e) – ~ 43 cm Wavelength of emission – 423 nm Rise time – 0.9 ns Decay time – 2.4 ns Luminescent efficiency – ~15% of NaI(Tl)
Scintillation in inorganic materials depends on the crystal lattice of the detector material.In order to scintillate (create light in response to energy deposited from radiation interactions), electrons in the material must be able to move from their discrete energy bands. In some cases the energy difference between these bands is too large and the emitted photon would not be in the visible wavelengths. The two energy bands are called the valence band in which electrons are essentially bound to the lattice atoms and the conduction band where electrons are free to move through the material. There exist intermediate bands of energies called the forbidden band. Electrons in the inorganic lattice cannot be in these bands in a pure crystal. To enhance the efficiency of light emission and to ensure the emitted photon is of the appropriate wavelength, small amounts of an impurity element can be added to the crystal, these impurities are called activators. Activators create special sites in the lattice at which the normal energy band gap is modified allowing energy states in the forbidden band through which an electron can de-excite back to the valence band emitting a visible flash of light ( a scintillation).
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 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 is clear that especially at low energies, scintillation detectors are low resolution devices unlike Si(Li) or HPGe detectors.
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 criterion 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 / electronics’ 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 of7.0 % at 662 keV.
Sodium Iodide( Nal(Tl)) Detectors
NaI is an alkali halide inorganic scintillator with a relatively high Z from iodine (53). This results in high efficiency for gamma-ray detection. A small amount of TI is added in order to activate the crystal, so that its formula is typically shown as Nal(TI)l. Energy resolution of about 6% for the 662-keV gamma ray from 137Cs is achievable for a 3-in.-diameter by 3-in.-long crystal and slightly worse for smaller or larger sizes. The light decay time constant in Nal is about 0.23 µs. Typical charge-sensitive preamplifiers translate this into an output pulse rise time of about 0.5 µs. Fast coincidence measurements cannot achieve the very short resolving times that are possible with plastic, especially at low gamma-ray energies. Nucsafe offers various sizes of Nal(TI) detectors from 2×2” to 4x4x16” arrays depending on your application.
Bismuth germanate (BGO)
BGO is a scintillation crystal that features high density due to the high Z of Bismuth (83). Its high intrinsic efficiency at high gamma ray energies make it an excellent alternative to NaI for gross counting applications. BGO (bismuth germanate oxide) has a very fast response and little or no afterglow. Because of its speed of response and high density it is ideal for high energy and high rate gamma ray measurements. The only disadvantage of BGO is its low conversion efficiency (i.e. low light output) which results in poorer energy resolution than NaI detectors; making it less desirable for nuclide identification applications.
Cesium (CsI) Iodide
Like NaI, CsI is another alkali halide inorganic scintillator. CsI also requires a small amount of thallium or sodium activator for proper operation. Each activator results in a scintillation detector with varying properties. CsI has a higher intrinsic efficiency than NaI but less than BGO. Csl has poorer energy resolution than NaI. However, CsI has much faster pulse light decay and finds use in timing applications but the wavelength of its light is poorly matched to most PMTs. It is also less brittle than NaI and thereby may be subjected to more severe shock and vibration conditions than NaI detectors. It is less hygroscopic than NaI but more than BGO.
Gross counting systems measure a fraction of the total radiations in such a way as to allow the determination of the presence of radioactivity above ambient background. In a laboratory, where the source to detector geometry is well characterized, the activity of the sample can be determined for many types of radiation. Gross counting of gamma rays will determine the presence or absence of this radiation, but can only be used to determine activity if a single gamma ray is present or if a simple mixture is known and multiple measurements are made at different times. (Gross counting systems consist of the detector, preamplifier, amplifier, discriminator and counter)
The function of the detector is to convert radiation energy into an electrical signal. There are two basic mechanisms for converting this energy: excitation and ionization. In ionization, an electron is stripped from an atom and the electron and resulting ion are electrically charged. These charged particles can be influenced by an electric field to induce a current that can be measured directly or converted into a voltage pulse. 3He neutron detectors, Geiger Mueller, and other gas proportional detectors are examples of ionization detectors. In excitation, electrons are excited to a higher energy level and when the vacant electron is filled, electromagnetic radiation is emitted. Scintillation detectors such as NaI, BGO, CsI, Polyvinyl toluene (PVT) plastic scintillator and the neutron sensitive glass fibers are examples of scintillation detectors.
Gas Filled Radiation Detectors
The most common type of instrument is a gas filled radiation detector. He-3 neutron detectors are an example of a gas filled detector. This instrument works on the principle that as radiation passes through air or a specific gas, ionization of the molecules in the air occurs. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to the negative side of the detector (the cathode) and the free electrons will travel to the positive side (the anode). These charges are collected by the anode and cathode which then form a very small current in the wires going to the detector. By placing a very sensitive current measuring device between the wires from the cathode and anode, the small current measured and displayed as a signal. The more radiation which enters the chamber, the more current displayed by the instrument.
Scintillation Radiation Detectors
The second most common type of radiation detecting instrument is the scintillation detector. Sodium iodide, NaI(Tl), cesium iodide, CsI, and bismuth germanate, BGO are all examples of scintillation detectors. The basic principle behind this instrument is the use of a special material which glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube.
A scintillator is a material that converts energy lost by ionizing radiation into pulses of light. In most scintillation counting applications, the ionizing radiation is in the form of X-rays, g-rays and a- or b-particles ranging in energy from a few thousand electron volts to several million electron volts (keVs to MeVs). 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. The first part of the photomultiplier tube is made of another special material called a photocathode. The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series of plates, called dynodes, through the application of a positive high voltage. When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed or counted by the system. Scintillation detectors are very sensitive radiation instruments and are used in both portable and stationary systems by Nucsafe. 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.
More than any other part of the system determines the overall response function and therefore the sensitivity and minimum detectable count rate of the system. For any detector, there are two important parameters that affect the overall efficiency of the system, geometric efficiency and intrinsic efficiency. By multiplying these values, one can calculate the total efficiency. Efficiency is like a batting average, it is a ratio of how many are hit relative to how many are thrown. In radiation measurements, the geometric efficiency is the ratio of the number of radiation particles or photons that hit the detector divided by the total number of radiation particles or photons emitted from the source in all directions.
Total Efficiency = Geometric Efficiency x Intrinsic Efficiency Counts/Emitted Radiations = (Incident Radiations/Emitted Radiations) x (Counts/Incident Radiations)
The intrinsic efficiency is the ratio of counts detected to the number of photons or particles incident on the detector and is a measure of how many photons or particles result in a gross count. Some radiation is not energetic enough and does not reach the detector because it is attenuated or scattered before it can interact in the detector. Some radiation is so energetic it passes through the detector or is scattered out of the detector without depositing its energy. As a result, the actual counts measured by the system is a fraction of the radiation emitted in the direction of the detector. The intrinsic efficiency of various detectors may range from 100% to very small values such as 0.01%, but are typically around 10 to 50%. Using the example for the plastic scintillator 6×30” detector above, only 1 count might occur from nearly 2 million gamma rays emitted from a source at a distance of 4 meters.
εintrinsic = γcounts / γincident
The product of these two efficiencies is the total efficiency, or the number of counts detected, relative to the total number of radiations emitted from the source.
εtotal = (γcounts / γincident) / (γincident / γemitted)
εtotal = γcounts / γemitted
Once the radiation deposits it energy in the detector and the detector converts the energy into an electrical pulse, the system electronics process the information into a useable form, either as a count rate for gross counting systems or as an energy spectrum for nuclide identification and spectroscopy. The basic components in the electronics include the preamplifier, the main amplifier and either one or more discriminators and counters (for gross counting) or an analog to digital converter and histogram memory (for spectroscopy).
The functions of the preamplifier are:
- conversion from current to voltage
- linear amplification
- providing circuitry close to the diode to minimize the capacitive loading of the diode
- allowing matching of diode impedances.
The purposes of the amplifier are:
- pulse shaping
The input pulse to the amplifier is the voltage output from the preamplifier and the output from the amplifier is an amplified and shaped pulse. The gain of the amplifier is defined as: gain = output voltage / input voltage, so a gain of 100 would result in an output of 10 V from an input of 0.1 V.
The purpose of the discriminator is to create a logic signal output only if the input signal is between a lower and upper discriminator setting corresponding to a low and high reference voltage.
A counter is incremented by one for each output signal from the discriminator. For gross counting of gamma rays, a total count of an energy range may be reported, for example from 25 to 3000 keV for gamma rays. However, because gamma rays associated with a given radionuclide have specific energy and because some gamma ray detectors can measure this energy more precisely than others, it is possible to look at specific energy intervals and improve the sensitivity. Sensitivity is determined by the signal to background ratio. This is true for radiation measurements as it is true for many electronic measurements. Nucsafe gross counting systems for gamma rays utilize at least 6 separate regions of interest (ROI) plus a total of all 6 ROI.