According to Lexus.us, Dosimetry is defined as “Measurement of radiation exposure, especially X-rays or gamma rays; calculation of radiation dose from internally administered radionuclides”. (2010). Neutron dosimetry is neither as precise nor as complex as gamma dosimetry. In the United States today, neutron dosimetry is among the vital requirements used to carry out comparison of the clinical results gotten from radiotherapy trials done at the fast neutron therapy facilities.
The neutron dosimetry radiation uses specialized devices which includes superheated drop detectors. Gamma rays originating from the surroundings, the phantom or biological object experiencing the radiation, the collimating or field-limiting system, the primary shielding, and the neutron producing neutron from the Neutron fields always accompanies the neutron fields. With the field size at a fixed depth a rise in the penetration depth of the incident beam in a phantom result in a rise in the proportion of the total absorbed dose due to the photon component of the mixed neutron-photon field.
For over two decades now, detectors based on emulsions of superheated liquid drops in visco-elastic gels or soft polymers are being used for neutron detection, neutron spectrometry, and neutron dosimetry. Superheated drops are nucleated to vapor bubbles as a result of the energy deposited in the liquid by the radiation while passing through the emulsion when exposed to energetic radiation.
In our world today, superheated liquids are being used extensively as a neutron detector in neutron dosimetry relative to other conventional methods of neutron detection. The application of superheated liquid in radiation detection dates back to the time of the bubble chamber which was invented by Glaser which earned him the Nobel Prize in 1960. In the bubble chamber, as an energetic particle passes through a suitable liquid situated in the chamber, the chamber’s pressure gets minimized suddenly which makes the liquid become superheated instantly, bubbles are thus produced by the particle along its path. The photograph of the charged particle trajectory is taken and the analysis is carried out. It is required to re-pressurize the chamber once the detection of a particle has been carried out and before the next energetic radiation detection, this is due to the fact that the boiling initiated at any point of the chamber would eventually consume the all the liquid. Consequently, even the fastest bubble chamber in the world today can only work at about 20 cycles per second. In 1979, the use of a more convenient superheated form of radiation named superheated drop detector (SDD) was reported by Apfel. (Roy & Roy pg 516). In this type of superheated detector, all the liquid is homogenously dispersed in a gel like medium in the form of minute droplets, as the energetic particle passes through this detector the droplets which interact with the radiation gets vaporized thereby preventing other droplets from getting affected. Consequently, the re-pressurization procedure needed in the bubble chamber is not required for the SDD. The bubble detector (also known as BD) is a similar device to the SDD and was invented by Ing. In the bubble detector, the superheated drops are distributed homogenously into a rigid polymer matrix. (Roy & Roy pg. 516). In bubble detector, the vapor bubbles remained trapped inside the polymers. The known difference between the BD and the SDD is that the BD is the trade name of the Bubble Detector invented by Bubble Technology Industries Inc., Canada, whereas the SDD is the trade name of superheated drop detector invented by Apfel Enterprises Inc., USA. But today, the word “superheated emulsion” is the common denomination that has been adopted by the International Standard Organization (ISO) and the ICRU (International Commission on Radiation Units and Measurements) to denote any kind of detector that uses superheated drops, including the BD and the SDD.
Theory behind Bubble Detectors
For the superheated bubble detectors, the thermodynamic properties of the active gas such as pressure and temperature are used to determine the superheated detector’s response to incoming radiation or particles. The Seitz theory makes us to fully understand the framework of the bubble detector’s operations. The Seitz theory explains that it is that heat spikes created by the energy deposited when a particle traverses a depth of superheated medium that triggers the formation of bubbles. It asserts that the response of superheated droplet detectors to alpha and neutron particles denotes that all the available data can be fully described with a consistent but unique set of variables that parameterize the underlying model of recoil energy threshold and energy deposition. The data response of mono-energetic neutrons as a temperature function can be traced over varying others of magnitude in count rate. The alpha particles are responsible for triggering the formation of droplets as a result of their specific loss along the track. The alpha emitters are uniformly distributed in the gel which surrounds the droplets.
Superheated Liquids and Superheated Emulsions
The term “superheated liquid” refers to any liquid maintaining its liquid state at a temperature which is greater than the liquid’s boiling temperature. At this state, the energy possessed by the liquid is in a relative minimum energy state and not in its absolute minimum, thus, the superheated state is said to be a metastable state of the liquid. Relative to small disturbances, the state can be said to be stable but relative to larger perturbation it is unstable and tends towards becoming a vapor which is a much more stable state.
Then term “superheated emulsion” is a generic name which is used to refer to any kind of radiation detector that uses superheated liquid drops, these include bubble detectors and super drop detectors which are commercially available.
The basic requirement in producing these detectors is to hold superheated drops in another immiscible liquid. These droplet detectors are usually made up of superheated liquids, example of which are; CCl2F2, CF3Br, C4F10, and C3F8.
Superheated Emulsion as Neutron Dosimeter
The application of superheated emulsions began with the development of a neutron dosimeter (Apfel & Roy). This is due to the fact that the dose-equivalent response available Personnel Dosimeters could not meet the set requirements given by the International Commission of Radiological Protection (ICRP). The only passive neutron dosimeters that meet the requirements of the ICRP relative to the other commonly used dosimeter such as film dosimeter and TLD-albedo dosimeter are the passive devices that were made of superheated emulsions. The other exclusive properties that make the superheated emulsions one of the most suitable tools in neutron dosimetry are the low cost, instant dose-reading capability, small size, isotropic response, tissue equivalent composition, passive operation, and photon sensitivity. Its only disadvantage is its temperature dependent sensitivity, although, this can been compensated by using a thermally expansible material or a material with a low boiling point liquid on top of the sensitive emulsions kept in a sealed tube.
It is impossible to achieve precise measurements of neutron dose using conventional neutron dosimeters because pulse primary photons and extremely intense photons which mask the measurement of neutron dose are present. Superheated emulsion has the ability to produce dose equivalent measurement without prior knowledge of the neutron energy of the spectrum; they are also insensitive to photon, thus, they are natural choice over conventional dosimeters.
The PICASSO Detector VS Personnel Neutron Dosimetry Detector
The PICASSO project relies on the technique of superheated droplet. The picasso detector uses dispersed superheated Freon liquid as its active material and traps it in a polymerizes gel. The picasso detection technique is based upon the phase transition of superheated droplet at an ambient pressure and at a temperature close to room temperature. Nuclear recoils, which occur when an atomic nucleus present in the droplet is made to interact with incoming subatomic particles, are used to induce the phase transition. (Azuelos et al)
The concept of the Personnel Neutron Dosimetry Detector is such that it uses multiple detectors possessing different thresholds for the provision of a simple neutron spectrum. The Personnel Neutron Dosimeter uses bubble detectors and Albedo thermo luminescent dosimeters. Its goal is to provide a precise measure of neutron equivalent by measuring the neutron spectrum that contributes to the dose equivalent. Therefore, if the value of the spectrum is measured, the neutron dose equivalent is determined by applying the corresponding fluence to dose equivalent conversion factors. (Buckner 1993 pg. 8)
The Oak Ridge National Laboratory recently developed a new approach in measuring personnel neutron dose equivalent. BDs (Bubble Detectors) are used in combination with TLDs (Thermo luminescent Dosimeters) as a CPND (Combination Personnel Neutron Dosimeter), this makes it possible to obtain precise dose equivalent results and also obtain simple four-interval neutron energy. (Buckner 1993)
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Feighery W., Genest M-H., Gornea R., Guenette R., Kanagalingam S., Krauss C., Leroy C., Lessard L.V .Zacek V. Simulation of Special Bubble Detectors for Picasso. Department of Physics and Astronomy, Indiana University South Bend
South Bend, Indiana, 46634, USA.
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International Journal of Radiation Oncology, Biology, Physics. 1982 Dec;8(12):2057-9. PMID:6819266
- Buckner, M. A. (1993). “Improving Neutron Dosimetry Using Bubble Detector
Technology”. Oak Ridge National Laboratory. Martin Marietta Energy Systems, Inc., The United States Department of Energy.
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