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 6x30” 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