Radiation Detectors

The radiation detectors are housed in the Radiological Engineering lab in the NES building, Room G21. Students taking classes with Dr. Xu, such as Radiological Engineering, and Environmental Radiation Safety Controls, use the lab to run experiments. A lab manual from the Radiological Engineering class can be downloaded here.

MOSFET Detectors

The basic Metal Oxide Field Effect Transistor (MOSFET) is depicted in the figure. The type shown is a P channel enhancement MOSFET which is built on a negatively doped (n-type) silicon. When a sufficiently large negative voltage is applied to the polysilicon gate a significant number of minority carriers (holes) will be attracted to the oxide/silicon surface from both the bulk silicon substrate and the source and drain regions. Once a sufficient concentration of holes have accumulated there, a conduction channel is formed, allowing current to flow between the source and drain (Ids.) The voltage necessary to initiate current flow is known as the device threshold voltage (VTH).

When a MOSFET device is irradiated, electron-hole pairs are generated within the silicon dioxide by the incident radiation. Electrons, whose mobility in SiO2 at room temperature is about 4 orders of magnitude greater than holes, quickly move out of the gate electrode while holes move in a stochastic fashion towards the Si/SiO2 interface .At this interface, the holes become trapped in long term sites, causing a negative threshold voltage shift (DVTH ) which can persist for years. The difference in voltage shift before and after exposure can be measured, and is proportional to dose.

MOSFET dosimeters are now used as clinical dosimeters for patients who undergo radiotherapy. The main advantages of MOSFET devices are: the dosimeter is direct reading with a very thin active area (less than 2 mm), the physical size of the MOSFET when packaged is less than 4 mm squared, and the post radiation signal is permanently stored and is dose rate independent. The MOSFET AutoSenseTM (TN-RD-60) is ideal for on-line advanced radiotherapy applications. This system is compatible with all Thomson Nielsen isotropic MOSFET dosimeters.

Proportional Counter

(Tennelec Low Level Alpha/Beta Counting System (Model 5110/5500) and proportional counting gas)

Proportional counters use the principle of gas multiplication to amplify ions created by radiation interactions within a gas. Gas amplification is due to increasing the electric field to an adequate value. At low voltages, the freed electrons and ions created drift to their respective electrodes. However,as the electric field increases ,the average energy between collisions also increases. At a certain critical value of the electric field ,this kinetic energy will be sufficient enought to allow secondary ionization to occur. This gas multiplication becomes an ion cascade, known as Townsend avalanche.

There are several different regions in which a gas filled detector can operate. The detector used here at RPI for research and lab activities operates in the proportional region, where gas multiplication is linear. Proportional counters, like this one, are used to distinguish between certain charged particles such as alpha and beta particles. The alpha and beta particles are distinguished through the use of a discrimination level. The discrimination level is determined by using known sources to find the proper operating voltages for alpha and beta samples and then setting the system to these voltages. A particle is then determined to be a beta until a certain voltage level is reached, and it switched over to an alpha particle.

Plateaus are used to determine proper operating voltages for a counting system. In general, beta plateaus are shorter and have a steeper slope than alpha plateaus due to a broader, less defined height distribution. Counts are determined by the signal that each particle generates as it deposits a sufficient amount of energy to the fill gas of the detector. As the energy increases and the discrimination level is reached, it becomes harder to distinguish between alpha and beta particles. Therefore it is advantageous to operate in the plateau regions of alpha and beta sources. The plateaus act as identifiers for operating voltages, which further means that the count rates in these areas are stable. It is desirable to work within stable counting rates, because this ensures more accurate data.

The detector used by the students and faculty at RPI is a Tennelec Low Level Alpha/Beta Counting System (Model 5110/5500) and proportional counting gas (above left). This is an automatic alpha/beta proportional counting system with a user definable reporting format. The system is designed for ease of operation for the occasional user, yet has unprecedented power and flexibility available for the more experienced operator.

Some key features of this system include automatic plateau generation, selectable analysis modes, time of day/time of year clock, non-volatile memory, disk storage media, two independent output ports, and user definable output formats. This device, and others like it, has many practical health physics applications. One of these applications is to use the detector to test surrounding areas of radiation sites. For example, a swab can be used on a work bench across the hall from radioactive material and tested using this detector to determine what type and the abundance of radioactive particles that workers and or civilians are being exposed to. This information can be used in order to design better shielding apparatuses. Being able to know what type of radiation and how much radiation is being emitted from a source is vital information that must be obtained to properly protect humans, other animals and the environment.

TLD Readers

(Thermoluminescent detector readers)

Materials that endure a detectable change or affect due to radiation are used as an indication of how much radiation was received. These materials are used in radiation detection devices. Unfortunately most detectors are too large for individual use, so a thermoluminescent material is used. This material comes in powder form or in small chips, which are put into a badge for personal radiation monitoring. Thermoluminescent detectors are known as TLDs and are the most common personal radiation detection device.

In order for a thermoluminescent material to emit energy imparted in it by radiation and a dose to be determined, a heating process must occur. It must be heated to a high enough temperature to cause the electrons that have been trapped in the band-gap to become thermally excited. This will cause the electrons to return to the conduction band, producing photons of energy (E= hn) . If the heating process results in a temperature that is too low to free the trapped holes a free electron will combine with the holes. This also results in a photon emission. Ideally, for each trapped carrier, one photon is emitted so the measurement of all light produced is proportional to the dose received.

A TLD reader (above left) is a device that heats the chips within the TLD to a temperate great enough to excite the trapped carriers, causing them to recombine and emit photons. The visible light is counted in terms of nC of current through the use of the TLD reader and the ATLAS computer program. The TLD reader is loaded with a disk that contains the TLD chips leaving the first and last slot of the disk open for background counting. TLD reading is important to the field of health physics because this detection process is used to determine the amount of radiation a person is receiving from a surrounding area.

Survey Meters

(The Ludlum Model 5 Geiger counter and the BF3 proportional counter)

Survey meters can detect a large range of radiation, based on the scale the meter is set on. A Ludlum Model 5 Geiger counter (left), is one type of survey meter that is used to detect gamma radiation. Calibrating such a meter is very important because we must have an accurate knowledge of the amount of radiation a person is being exposed to in order to make good decisions concerning the length of time one should remain in the presence of a radioactive source. It is also important to check all of the meter’s scales. To accomplish such a task attenuators, such as lead shields, with "good geometry" must be used. Good geometry means that the radiation form the source (such as a Cs-137 gamma source) is narrow enough to ensure that all radiation the escapes the shielding will be counted by the detector. Devices with such "good geometry" usually respond with a larger signal than assumed.

The survey meter used for the detection of gamma rays has many purposes. In thefield of health physics, this detector must be used to determine the amount of gamma radiation a worker is receiving form a radioactive source. This can be used in any area where gamma radiation is known to exist. The added feature of the speaker also helps people to realize when they are being over exposed to a specific amount of radiation. This is very important for workers and civilians that come in contact with radioactive materials.

In order to detect neutrons in the fast energy range, many detectors must first slow the neutrons to their thermal energies. Then the detectors use a thermal detector to determine the amount of neutrons. Fast neutrons are slowed to thermal neutrons by elastic neutron scattering, or in other words by moderation. A commonly used moderator is paraffin or polyethylene, which are hydrogenous materials. These are used in order to compensate for the large energy transfer involved with neutron scattering. The survey meter used here at RPI is a BF3 proportional counter (right). This thermal neutron detector is surrounded by a 9" diameter cadmium loaded polyethylene sphere.

Such a detector is very important when working with radioactive material, such as Pu-Be, which emits neutrons. Calibrating a thermal neutron detector is also very important for the same reason that calibrating a gamma detector is. Again it is important to check all of the meter’s scales, beginning with the highest. Calibrations of such devices are vital for all those who come in contact with radioactive material.

Scintillation Detectors

(Ludlum Model 44-98 BGO Scintillator, Ludlum Model 43-3 Alpha Scintillator, and NaI (Tl) detector)

The emission of visible light when radiation is incident upon a certain material is known as scintillation. The most widely used scintillators include inorganic alkali halide crystals and organic liquids. Inorganic scintillators have a higher efficiency in converting the radiation’s energy into detectable light and their response is more linear – the generated light is proportional to the deposited energy over a wide range. Organic scintillators have shorter decay times of the induced luminescence so a fast signal pulse can be generated.

The scintillation mechanism in inorganic materials depends on the energy states determined by the crystal lattice of the material. To increase the probability of a visible photon being generated during the de-excitation process, small amounts of an impurity are added to a pure crystal. These impurities, called dopants, or activators, create special sites in the crystal lattice with a modified energy band structure. In theses regions, energy states are created in the forbidden band, which now constitutes the "band-gap," and now serve as the basis for the scintillation process.

When a charged particle passes through the scintillation material it forms a large number of hole-electron pairs. The positive hole will migrate to the location of the activator and ionize it, while the freed electron will move through the crystal until it encounters an ionized activator. When it does, the electron will fall into an allowed energy level in the activator site, creating a neutral impurity configuration. De-excitation of this electron to the ground state of the activator will occur, with the emission of a visible photon. For a detector to act as a gamma ray spectrometer, it must act as a conversion medium in which incident gamma-rays have a reasonable probability of interacting to yield one or more electrons, and it must function as a conventional detector for these secondary electrons. Essentially, a scintillator is a detector that converts the kinetic energy of an ionizing particle to a flash of light.

Bismuth germanate (BGO) is very dense (7.3g/cubic cm) and has a high atomic number, which results in BGO having the largest probability per unit volume of any commercial scintillator for the photoelectric absorption of gamma radiation. Since BGO does not rely on any activator to promote scintillation, it is considered a "pure" inorganic scintillator. The Ludlum Model 44-98 BGO scintillator (above left) is a beta radiation and low energy gamma detector. It consists of a 1" diameter by 1mm thick BGO scintillator. It has an 11.6 squared cm aluminized mylar window and a 5" diameter magnetically shielded photomultiplier. BGO detectors are more rugged than those using fragile hydroscopic crystals, however, they are still efficient, and easy to use. Another useful scintillation detector is one made of silver activated zinc sulfide (ZnS (Ag)). The zinc sulfide scintillator has a very high scintillation efficiency, but is only available as a powder. It is limited to thin screens used for alpha detection. Thicknesses greater than 25 mg/squared cm are unusable because of the opacity of the multicrystalline layer to its own luminescence. A Ludlum Model 43-2 scintillator (above left) is an alpha survey detector consisting of a zinc sulfide scintillator with a 12 squared cm aluminized mylar window. This is a very useful, portable detector that accurately reports alpha radiation.

The Ludlum Model 2350-1 data logger is a scalar/rate-meter for use with many different detectors including the two mentioned and shown above. As a scalar the count time can be set from 1-65535 seconds in 1 second intervals, and as digital it is corrected for dead time and calibration constants. This also allows the radiation level to be shown in a variety of units such as Rem/hr, Sv/hr, R/hr, CPM, CPS, DPM, Rad, Gray, C/kg, Ci/squared cm, or Bq/squared cm.

The gamma-ray energies emitted by various radionuclides are unique for each species; and they are said to "fingerprint" the material. Determination of the quantity and quality of a gamma spectrum can identify minute quantities of an element in an "unknown sample." NaI (Tl) (left) is an alkali halide inorganic scintillator. NaI (Tl) uses Thallium Iodide as the activator (0.1% by mass). This is a hygroscopic scintillator (it will deteriorate due to water absorption if exposed to the atmosphere), and therefore it needs to be placed in an "air-tight" container. This detector has excellent light yield and linearity. This detector must have a source of high voltage at all times, and must be hooked up to a computer with the proper programs in order to run. Once the set up of this detector is done it is highly efficient, and is operated by placing a sample in the detector, closing the top, and running the computer program.

Semiconductor Detectors

(HPGe Detector)

The HPGe detector (below left) is a semiconductor diode type detector. The detector is formed by setting up two different regions in the semiconductor, "p" (excess hole concentration) and "n" (excess electron concentration), which have a region between them where a charge imbalance exists, called the depletion region. Electron-hole pairs formed in this region constitute the basic electrical signal. Thus, semiconductors provide a way to collect electrical charges created at either boundary of the semiconductor material. By reverse biasing the p-n junction a low current will cross the junction, the width of the depletion region will increase, and the noise properties of the semiconductor will improve. When the reverse bias voltage is sufficient, the depletion region extends through the entire thickness of a wafer, resulting in a fully depleted detector. The depth of the depletion region is what is important to the types of radiation that can be detected.

Because of the small band gap (~0.7eV), room temperature operation of germanium (Ge) detectors of any type is impossible because of the large thermal induced leakage current that would result. HPGe detectors must therefore be cooled through the use of liquid nitrogen to reduce the leakage current. The detector must be placed in vacuum tight cryostat to inhibit thermal conductivity between the crystal and the surrounding air. The HPGe detector must also be connected to a high voltage source, and to a computer with the proper programs. To use it a sample is placed in front of the HPGe detector window at a reproducible geometry for meaningful results. When used properly, high resolution is typical of this detector, resulting in very good statistics.

Whole Body Detector

A nuclear whole body detector is a device intended to measure and image the distribution of radionuclides in the body. This is accomplished by means of a wide-aperture detector whose position moves in one direction with respect to the patient. The whole body counter directly measures the radiation emitted from the internally deposited radionuclides. This information is used to determine the nature and location of the radionuclide, and to quantitatively estimate the amount in the body. This device is not only useful in detecting and measuring human radioactive contamination, but it also can be used to assess the adequacy of the radiation protection policy/measures being taken in a work place.