When dealing with unknown sources of radiation, this is a very useful feature. Ionization Ion Chamber— This is an air-filled chamber with an electrically conductive inner wall and central anode and a relatively low applied voltage. When primary ion pairs are formed in the air volume, from x-ray or gamma radiation interactions in the chamber wall, the central anode collects the electrons and a small current is generated.
This in turn is measured by an electrometer circuit and displayed digitally or on an analog meter. These instruments must be calibrated properly to a traceable radiation source and are designed to provide an accurate measure of absorbed dose to air which, through appropriate conversion factors, can be related to dose to tissue. In that most ion chambers are "open air," they must be corrected for change in temperature and pressure. Note: For practical purposes, consider the roentgen, rad, and the rem to be equal with gamma or x rays.
Neutron REM Meter, with Proportional Counter— A boron trifluoride or helium-3 proportional counter tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when a neutron radiation interacts with the gas in the tube. The absorption of a neutron in the nucleus of boron or helium-3 causes the prompt emission of a helium-4 nucleus or proton respectively. These charged particles can then cause ionization in the gas, which is collected as an electrical pulse, similar to the GM tube.
These neutron-measuring proportional counters require large amounts of hydrogenous material around them to slow the neutron to thermal energies. Other surrounding filters allow an appropriate number of neutrons to be detected and thus provide a flat-energy response with respect to dose equivalent.
The design and characteristics of these devices are such that the amount of secondary charge collected is proportional to the degree of primary ions produced by the radiation. Thus, through the use of electronic discriminator circuits, the different types of radiation can be measured separately.
For example, gamma radiation up to rather high levels is easily rejected in neutron counters. Radon Detectors— A number of different techniques are used for radon measurements in home or occupational settings e. These range from collection of radon decay products on an air filter and counting, exposing a charcoal canister for several days and performing gamma spectroscopy for absorbed decay products, exposure of an electret ion chamber and read-out, and long-term exposure of CR plastic with subsequent chemical etching and alpha track counting.
All these approaches have different advantages and disadvantages which should be evaluated prior to use. The smoke detector has two ionization chambers, one open to the air, and a reference chamber which does not allow the entry of particles. The radioactive source emits alpha particles into both chambers, which ionizes some air molecules.
The free-air chamber allows the entry of smoke particles to the sensitive volume and to change attenuation of alpha particles. If any smoke particles enter the free-air chamber, some of the ions will attach to the particles and not be available to carry the current in that chamber. An electronic circuit detects that a current difference has developed between the open and sealed chambers, and sounds the alarm.
Geiger counters are mainly used for portable instrumentation due to its sensitivity, simple counting circuit, and ability to detect low-level radiation. Although the major use of Geiger counters is probably in individual particle detection, they are also found in gamma survey meters.
They are able to detect almost all types of radiation, but there are slight differences in the Geiger-Mueller tube. For alpha and beta particles to be detected by Geiger counters , they must be provided with a thin window. The efficiency reduction for alpha is due to the attenuation effect of the end window, though distance from the surface being checked also has a significant effect, and ideally a source of alpha radiation should be less than 10mm from the detector due to attenuation in air.
Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety. They are widely used because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation.
Scintillation counters can be used to detect alpha, beta, gamma radiation. They can be used also for detection of neutrons. For these purposes, different scintillators are used:. Alpha Particles and Heavy Ions. Due to the very high ionizing power of heavy ions, scintillation counters are usually not ideal for the detection of heavy ions. Where needed, inorganic crystals, e. Pure CsI is a fast and dense scintillating material with relatively low light yield that increases significantly with cooling.
The drawbacks of CsI are a high temperature gradient and a slight hygroscopicity. Silicon-based detectors are very good for tracking charged particles. A silicon strip detector is an arrangement of strip like shaped implants acting as charge collecting electrodes. Placed on a low doped fully depleted silicon wafer these implants form a one-dimensional array of diodes. By connecting each of the metalized strips to a charge sensitive amplifier a position sensitive detector is built.
Two dimensional position measurements can be achieved by applying an additional strip like doping on the wafer backside by use of a double sided technology. Such devices can be used to measure small impact parameters and thereby determine whether some charged particle originated from a primary collision or was the decay product of a primary particle that traveled a small distance from the original interaction, and then decayed.
Due to the distribution and reflection of photons, it also does not depend on a line of sight to the alpha source. This image could be analysed for intensity to provide numerical data as well as an image. Due to the short range of the alpha particles, the photon emissions are relatively local to the source allowing accurate location of the contamination. This also allows differentiation between alpha and other forms of ionising radiation, which occur over a longer range and therefore cause less intense radioluminescence [ 12 ].
Although theoretically desirable, there are also considerable difficulties with using the radioluminescence approach.
The main issue that needs to be overcome is separating the alpha-induced air-radioluminescence from background UV radiation, i. Therefore, background light can strongly affect the ability of detectors to identify the relatively weak signal produced by alpha emissions within these wavelength ranges. This inhibits and restricts the use of many of the detectors trialed to date to darkness or carefully controlled lighting conditions, which is unfeasible for most practical decommissioning purposes where a wide range of different environments will be encountered.
Fluorescent lighting also emits very little UV light as this cannot be seen by the human eye and is therefore unwanted. Some fluorescent lamps may emit UVC at nm which is the wavelength at which mercury fluoresces, as this is the mechanism through with fluorescent tubes operate [ 24 ]. So there is likely to be some background UV from interior lighting, but little of this will be in the UVC wavelength range for a properly operating lighting system.
Due to the low intensity of the UV radiation from the nitrogen radioluminescence, a high signal-to-noise ratio is required in order to differentiate the signal from any background, and long collection times are needed, in the order of minutes to hours, to reliably detect the signal. Conversion efficiency is the ratio of the energy of the particle transferred to the air during ionisation and the energy converted to radioluminescence.
There are also issues with calculating the exact yield of this radioluminescence. Energy lost to the air has been used to calculate the yield, but due to internal absorption of the source and the complex mechanism of ionisation, it is not always possible to predict yield from a specific isotope via energy lost. As the nature of the radioluminescence does not depend on the isotope emitting the alpha particle, but depends on the energy levels of the gas atmosphere, this makes isotope identification at best complex and at worst impossible using this technique.
In , Roberts [ 22 ] looked at the feasibility of using alpha-induced air-radioluminescence for the detection of radiation sources. Through a series of calculations and a number of Geant4 simulations, it asserted that a source with 10 10 decays per second at a distance of 10 m, would produce a signal of 10 s or s of photons per centimetre square.
To verify the presence of the signal suggested by the calculations and simulations, experiments were carried out to detect a polonium source, using a photon counting module and bandpass filter.
This verified the emission of photons in the solar blind range by an alpha source, but did not quantify the number of photons in this wavelength range. Although limited in its results in terms of quantifiable experimental data, this work was able to verify the presence of UVC photons and demonstrated an ability to detect these, albeit in dark conditions due to the photomultiplier having some ability to detect photons of above nm.
It concludes that it may be feasible to use this method to detect alpha or other ionising radioactive sources, however this would depend on the situation, and that further research would be required, including determining the yield efficiency more accurately for this wavelength range [ 22 ]. One other consideration when trying to detect alpha induced radioluminescence photons is the transmittance of UV photons through visible light translucent materials.
This is important in both optical elements of any detection system, for example lenses, filters and detector windows, as well as those found in field conditions, for example glove boxes or hot cell windows.
The transmittance of a material will depend on the properties of that material and the wavelength of light trying to pass through. All forms of translucent materials have a transmission spectrum which determines how much of each wavelength of light is absorbed or allowed to pass through.
This can be tuned by the addition of transition metal, rare-earth ions or nano-crystals to produce band pass filters, which can be useful in blocking out unwanted light. Although limited in scope and the number of samples used, Lamadie et al. They determined that 1 mm thickness of Plexiglas would have a transmission of 91 percent relative to air, 1 mm thickness of polycarbonate would have a 92 percent transmission relative to air and that 1 mm thickness of triplex would have a 91 percent transmittance relative to air.
However they do not take into account any specific wavelength differences. Others have shown various successes at imaging UV photons through Plexiglas, although the images were in the main indistinct [ 5 , 25 ]. In the case of in situ materials, such as glove boxes and detector windows, the attenuation of UV photons can be a significant issue. As part of the research into a stand-off detector several of the researchers have looked into this issue and these results are included in this review.
For full characterisation, not only the presence but also the isotope is required. Although it is theoretically possible for the activity, or at least the emission rate, to be calculated from the intensity of the radioluminescence signal, the wavelength of the optical photons emitted are determined by the gas in which they occur, as opposed to the energy of the alpha particles.
As yet, work has not been undertaken on isotope identification, and hence Section 5 looks at alpha particle detectors rather than systems which characterise the isotope. This section explores the benefits and drawbacks of traditional detectors which are commercially available, and looks at the prototype and test detectors designed to detect and locate alpha sources through air radioluminescence. Some novel further ideas are also presented. The detectors included are designed to identify the location of an alpha emitter and not to characterise that source, hence carrying out part of the characterisation required for nuclear decommissioning, but not all.
Currently characterisation of sites in regard to alpha contamination is carried out by taking samples which are then analysed in order that the contamination can be identified and characterised. This process takes significant time as samples must be collected and recorded, sent to a suitable laboratory, analysed, and the results returned in a suitable format [ 9 , 26 ].
Therefore it is desirable to have a less time consuming and labour intensive process to locate and identify alpha contamination. The detection of the alpha contamination is traditionally carried out using hand-held alpha radiation detectors. Although hand-held alpha radiation detectors are readily available, these are in general intended for the immediate detection of alpha radiation for health physics purposes and not characterisation [ 27 ].
As these alpha particle detectors, which use a Geiger-Muller tube or more recently a scintillator, work through direct interaction with alpha particles the detector-source distance must be less than that of the range of the alpha particles [ 3 ]. This means that the detector must be positioned within a few centimetres of the source in order for alpha radiation to be detected.
The benefits of these kinds of detectors are: fast results through the immediate detection of the presence of alpha particles typically within seconds ; good localisation of sources through close proximity requirement; portable; readily available; mature technology.
Although for certain detection purposes this is acceptable, there are drawbacks: proximity to the source provides a hazard for test personnel and requires the use of PPE; detectors may become contaminated if they inadvertently touch the source in hand-held applications; complex plant geometries may make contamination by touch more likely and scanning harder to achieve; time consuming to scan large areas; access issues limiting penetrations to areas which require characterisation ; use in areas of high radioactivity including safety of personnel, levels of PPE required and contamination of equipment ; limited collection of data not suited to isotope identification; no associated automatic mapping of contamination onto an image or map for location purposes.
Therefore, a new way to detect alpha radiation has been sought through secondary effects of alpha particle emissions. Alpha-induced air radioluminescence detectors may provide a way forward in overcoming the shortcomings of traditional detectors and there has been significant research in this area in devising a prototype system. Table 1 shows the results of various alpha particle detection research and is included to provide some comparison between the results of different research projects.
As can be seen from the table the differences in distances, sources, exposure times, conditions and detector methods makes comparison of the methods and results difficult in determining the most efficient system to date, but some broad conclusions can be made by a comparison in this manner.
As of yet, these detectors are designed to locate an alpha source with various success, but identification of the source isotope has not as yet been achieved which would be required for full characterisation.
The remainder of Section 6 looks at this research in more detail, dividing the detectors by technology type. In order to address the main obstacle to detecting radioluminescence, solar-blind detectors, those sensitive only in the UVC wavelength range, have provided the basis for prototype detector systems shown to be operable in normal indoor lighting conditions. In Ivanov et al. This removes the interference of the stronger background light, allowing detection of the much weaker air radioluminescence in daylight conditions.
They also present images of background spots generated by noise, as a single frame and a sum of frames. This shows an apparently random distribution of these background spots over time, which the researchers were able to filter out to some degree for better sensitivity.
They also presented a filtered image taken with a s integration time. Their use of cameras that are available off-the-shelf and are therefore mature technology is beneficial in terms of the reliability. As yet no one has put forward a tested method to quantify the intensity of the light captured by these images, however this could potentially be used to determine the activity levels.
This work shows that the approach of using solar-blind detectors in detecting air radioluminescence is viable in addressing the issue of background UV radiation interference, although Ivanov et al. In Crompton et al.
This sensor is designed to detect the UVC emissions from flames for fire detection purposes and is sensitive in the — nm wavelength range. An average pulse rate of 0. A fused silica window was inserted between the sensor and source to prevent alpha particles directly impacting on the sensor. Although the distance between sensor and source is small, they assess that in this configuration the maximum detectable distance could have been mm.
Crompton et al. Interestingly they found that nitrogen had little effect on the cps. However, they note that these results require replication for verification, especially in light of the difference between the increase in radioluminescence reported in a nitrogen purge Hannuksella et al. Although the sensor used in Crompton et al. This is due to its low background count and insensitivity to indoor lighting conditions. Also, that using a flow of gas which could be achieved through the deployment of a thin flexible pipe, which may be more easily provided in field conditions due to not requiring a gas-tight enclosure and the purging of air, could enhance radioluminescence for detection purposes.
This presents a far from developed detector system, but does show a possible sensor which could be used as a foundation for the development of such. Shaw et al. This semi-conductor based alternative may make alpha induced air radioluminescence easier to detect than using CCD or PMT. In their tests this shows a better quantum efficiency at a wavelength of nm just inside the UVC range.
Although their work does not include any testing for alpha detection, this provides an alternative detector technology which may prove useful in the detection of alpha induced radioluminescence. They also explore a number of possible applications of this technology, including the imaging of deep-UV UVC.
The use of UVC detectors seems to somewhat overcome the issue of background interference from other light sources, however the low signal strength due to the smaller number of photons emitted in this wavelength range is an issue in terms of the distance at which these may work. Other detectors trialed to date specifically focus on the main peaks in the nitrogen radioluminescence spectrum, which occur at wavelengths between and nm, as 95 percent of the intensity falls into this range [ 3 ].
Although in this range the number of generated photons is greatest, the intensity of UV radiation from other sources is much higher, i. Therefore, these detectors must be used in complete darkness or with artificial lighting of specific wavelengths, even when filtering or background rejecting methods are used. This limits their practical application. Work using camera-based systems has mainly focused on locating alpha sources rather than characterising them, with an overlaid image of the radioluminescence over a conventional image being the preferred method of demonstrating the presence of an alpha emitter.
Lamadie et al. This is in comparison to Sand et al. They were also able to detect bulk contamination, showing that internal absorption that did not fully restrict the emission of alpha particles did not prevent detection. They developed two equations to calculate the activity of the sample based on the signal intensity and the number of photons per alpha emission, both of which were verified by their experimental results.
The limitation of Lamadie et al. It does however provide advancement in the quantification and characterisation of the radioluminescence phenomenon. In , Sand et al. They were able to image two mixed fuel pellets uranium and plutonium , with a 60 s exposure time.
The experiment was most likely carried out in darkness as they cite this as being beneficial. They tested both the differences between the two cameras and also the effect of detecting several sources of different activity at the same time.
Their samples were of various alpha emitting materials, and activities ranged from kBq to 4. Both Sand et al. Testing was carried out in a modified glove box where one of the glove ports had been replaced with a quartz glass window to allow a 90 percent transmittance of photons, as compared to approximately 80 percent attenuation by standard glove box Plexiglas.
Their optical results are overlaid on a conventional image. These images show that although the higher activity sources were detected, those emitting similar radioluminescence intensities to the low background light were undetectable to both systems.
They were able to achieve a resolution of better than 1 cm between sources. They also found that high intensity sources could mask lower intensity ones and suggested re-imaging after the removal of high intensity sources to check for sources of lower intensity, using longer exposure times or reduced background lighting. Pineau et al. Their main assertion is to fill the environment containing the source with a scintillating gas, which may contain nitrogen.
As nitrogen has been shown to be the main radioluminescence emitter in the UV range, this is consistent with other findings.
This could be in an enclosure which is placed over the area to be investigated, which will retain the scintillation gas and has a window transparent to UV photons. However, the flow of gas used in other work [ 18 ] could be easier to apply in the field than the need for a gas-tight enclosure to be deployed in potentially difficult to access or contaminated areas.
Due to the small number of photons produced, the system will integrate a number of images, therefore increasing the detection time. They suggest using a wavelength range of — nm. The device may also have a camera able to take a visual image over which to overlay the image of the alpha induced photons.
Due to the possible interference of light in the visual spectrum, they suggest using the system in darkness or using filters to attenuate light outside of the UV spectrum. No results are presented in the effectiveness of this system, however, for a patent to be applied for it may be assumed that they were confident that this system would work and therefore that tests had been successfully carried out.
Haslip et al. A telescope is used to collect the signal, which is amplified by mirrors and focused on six UV-sensitive cameras. This is achieved through the use of beam splitters and wavelength selective filtering.
Images from these 6 cameras are collated by a microprocessor proving an aggregated image to the operator which is in almost real time. Although this system is not able to reject daylight, it can be used at night where these is still a significant amount of background UV radiation, or under street lighting.
In Giakos proposed a stand-off alpha detector architecture using a spectrometer and ICCD camera, with a focusing assembly of lenses and reflectors [ 28 ]. Their calculations indicate that two 3.
The calculations are presented in the research paper to show how the architecture was devised, but there is no evidence that this system was tested and therefore if it was successful or not, or any limiting factors found during any experimental trials. Due to the ability to more easily quantify the signal intensity, other prototypes utilise a PMT to detect the radioluminescence. In Leybourne et al. Using optical filtering, telescope optics for collection, and a PMT photo-multiplier tube , they were able to detect the presence of an alpha emitting source on the surface of any one of three, gallon drums spaced 10 m apart at approximately m distance.
This was achieved in less than 1 min of data acquisition time for each source. However, even at night there is significant UV radiation outdoors. Leybourne et al. They also noted an inverse squared relationship between the intensity of the UV photon signal and distance, as would be anticipated in a spherical or hemispherical isotropic photon emission zone around a point source. The result of Leybourne et al. However there are several drawbacks and limitations to the work.
A relatively crude approach was taken for identifying the alpha source, in terms of a resolution of 10 m between sources i. It is possible that the experiments were carried out at night, to reduce the background UV that the device was required to reject. There is little information on the equipment specification or models used to carry out the experiment, meaning that it could not be replicated to check the accuracy of the work.
This includes the bandpass of the filtering system. However, whilst limited this work does show that there are approaches to this method of alpha particle detection which may prove viable in the field. Baschenko used a monochromator and PMT in photon counting mode to determine the spectrum, and low light sensitive film to image the source [ 3 ].
This has two implications. The first being that this technique can be used to combat exposing personnel to beta and gamma radiation, which may also be present within the range of traditional alpha particle detectors. The second is that the different types of radiation do not interfere with the alpha detection, making it suitable for mixed radiation environments normally seen within the nuclear industry. They calculated that there were approximately 30 UV photons emitted per alpha event, with 2.
Although this conclusion is not supported by other literature which finds the emission of photons is isotropic [ 19 ] and therefore is likely to be a misinterpretation or anomaly in the results. Baschenko used these results to calculate a possible detector set up.
From calculations of the effectiveness of this system, they were able to determine that this would not be suitable for use out of doors as background UV would always exceed the required level, even at night. Other work of Sand et al. In Sand et al. Noting that cameras require relatively long integration times, Sand et al.
This was achieved under artificial background lighting conditions which did not produce UV. Using a 40 nm bandpass filter, the signal was first filtered into the peak air radioluminescence wavelength range, — nm where nm is the most intense peak of the spectrum. The signal was then split, with the background portion being passed through a further 15 nm bandpass filter giving a to nm wavelength range. Using two PMTs and a time correlated single photon counting unit Sand et al.
This time period was sufficiently short to make a background count event at the same time as an alpha induced photon improbable. Using coincidence filtering, they were able to detect radioluminescence against background light which was times more intense than the radioluminescence.
At this stage in their work, they quote a value of photons per 5 MeV alpha emission. They also found a rapid drop in signal intensity when the source was moved 20 mm to the side, giving a positive indication for source location possibilities. By using a nitrogen-only atmosphere and a 10 kBq Am source, Sand et al. They attributed this increase to the removal of the quenching effect of oxygen.
Building on their earlier work, in Sand et al. Using the same set up with two different equipment options, they were able to distinguish a 4 kBq source at 1 m in 10 s under UV free lighting, and kBq under bright fluorescent lighting. The general set up for Sand et al. This light passes through a filter stack before being focused onto the window of a PMT.
The PMT is used in photon counting mode to determine the intensity of this signal. Two different filter stacks and PMTs are used.
The associated filter stack is sensitive at a central wavelength of nm. This was tested under yellow lighting conditions. The other set up utilises a solar blind PMT which has a caesium-telluride photocathode, with a filter stack centred at nm, which was tested under fluorescent lighting conditions.
Due to the differing field environments, each site would have to be surveyed in advance to determine if these detector systems were suitable for that particular site. They also note that solar blind camera detection methods can only be used in open spaces, however, the reasoning behind this statement is not qualified.
0コメント