Simulated and measured Hp(10) response of the personal dosemeter Seibersdorf

The Hp(10) energy response of the personal dosemeter Seibersdorf and its two different filtered LiF:Mg,Ti (TLD-100) thermoluminescence (TL) detectors are investigated. A close-to-reality simulation model of the personal dosemeter badge including the wrapped detector card was implemented with the MCNP Monte Carlo N-particle transport code. The comparison of measured and computationally calculated response using a semi-empirical TL efficiency function is carried out to provide information about the quality of the results of both methods, experiment and simulation. Similar to the experimental calibration conditions, the irradiation of dosemeters centred on the front surface of the International Organization for Standardization (ISO) water slab phantom is simulated using ISO-4037 reference photon radiation qualities with mean energies between 24 keV and 1.25 MeV and corresponding ISO conversion coefficients. The comparison of the simulated and measured relative Hp(10) energy responses resulted in good agreement within some percent except for the filtered TL element at lower photon energies.     

Photon energy response of luminescence dosemeters and its impact on assessment of Hp(10) and Hp(0.07) in mixed fields of varying energies of photons and beta radiation

X and gamma rays continue to remain the main contributors to the dose to humans. As these photons of varying energies are encountered in various applications, the study of photon energy response of a dosemeter is an important aspect to ensure the accuracy in dose measurement. Responses of dosemeters have to be experimentally established because for luminescence dosemeters, they depend not only on the effective atomic number (ratio of mass energy absorption coefficients of dosemeter and tissue) of the detector, but also considerably on the luminescence efficiency and the material surrounding the dosemeters. Metal filters are generally used for the compensation of energy dependence below 200 keV and/or to provide photon energy discrimination. It is noted that the contribution to Hp(0.07) could be measured more accurately than Hp(10). For the dosemeters exhibiting high photon energy-dependent response, estimation of the beta component of Hp(0.07) becomes very difficult in the mixed field of beta radiation and photons of energy less than 100 keV. Recent studies have shown that the thickness and the atomic number of metal filters not only affect the response below 200 keV but also cause a significant over-response for high energy (>6 MeV) photons often encountered in the environments of pressurised heavy water reactors and accelerators.     

Relationship between protection and operational quantities in dosimetry of photon external exposure-deficiencies of Hp(10)

It is commonly assumed that, for a variety of conditions, the E/Hp(10) conversion coefficient is below unity, i.e., Hp(10) can be used as a conservative surrogate of effective dose. The validity of this assumption was checked by Monte Carlo simulation of E and Hp(10) as determined by a practical dosemeter. The calculations concerned irradiation by parallel photon beams with directions varying within a 4pi solid angle and energy ranging from 50 keV to 1 MeV. Observed dependences of conversion coefficients on irradiation geometry, photon energy and dosemeter position demonstrate that in strongly anisotropic radiation fields straightforward application of Hp(10) for assessment of effective dose may lead to significant underestimation of the latter. For photon energy of 80 keV this underestimation may be up to 16-fold. For simulation of real life situations, irradiation by photons coming within broad cones (with solid angle pi) was considered. It was found that even for this irradiation geometry, E/Hp(10) may be as high as 4.3. At the same time, for radiation coming from the frontal hemisphere, the values of the conversion coefficient for a typically positioned (i.e., the left chest pocket) personal dosemeter do not exceed unity. The conclusion was reached that prior to application of Hp(10) as a surrogate of effective dose, the information regarding angular distribution of radiation fields must be taken into account.     

Development of an algorithm for TLD badge system for dosimetry in the field of X and gamma radiation in terms of Hp(10)

In view of the introduction of International Commission on Radiation Units and Measurements operational quantities Hp(10) and Hp(0.07), defined for individual monitoring, it became necessary to develop an algorithm that gives direct response of the dosemeter in terms of the operational quantities. Hence, for this purpose and also to improve the accuracy in dose estimation especially in the mixed fields of X ray and gamma, an algorithm was developed based on higher-order polynomial fit of the data points generated from the dose-response of discs under different filter regions of the present TL dosemeter system for known delivered doses. Study on the response of the BARC TL dosemeter system based on CaSO(4):Dy Teflon thermoluminescence dosemeter discs in the mixed fields of X and gamma radiation was carried out to ensure that the accuracies are within the prescribed limits recommended by the international organisations. The prevalent algorithm, based on the ratios of the disc response under various filters regions of the dosemeter to pure photons, was tested for different proportion of two radiations in case of mixed field dosimetry. It was found that the accuracy for few fields is beyond the acceptable limit in case of prevalent algorithm. The new proposed algorithm was also tested in mixed fields of photon fields and to pure photon fields of varied angles. It was found that the response of the dosemeter in mixed fields of photons and its angular response are satisfactory. The new algorithm can be used to record and report the personal dose in terms of Hp(10) as per the international recommendation for the present TL dosemeter.     

A personal neutron monitoring system based on CR-39 recoil proton track detectors: assessment of Hp(10) using image process algorithms

At the Individual Monitoring Service (IMS) of the ENEA Institute for Radiation Protection (IRP), the Hp(10) fast neutron dosemeter consists of a CR-39 (PADC, poly allyl diglycol carbonate) recoil protons track detector. The tracks across the detector surface are magnified through a chemical etching procedure and counted by a semi-automated system which consists of a microscope, a camera and a PC. A new analysis system, based on the National Instruments vision tools, was developed. The track area distribution for each reading field is recorded and numerical algorithms were developed in order to correct the energy dependence of the response and to recognise the tracks due to the background. This improves the dose evaluation system in terms of accuracy and discrimination or the background.     

Intercomparison measurements of extremity dosemeters in beta and/or photon radiation fields

Beta dosimetry, especially at the extremities, is gaining in importance due to the increasing use of beta particle sources, e.g. in brachytherapy. The dosimetric properties of personal dosemeters to be worn on the extremities and capable of measuring the personal dose equivalent, Hp(0.07), in beta and/or photon radiation fields were investigated within the scope of intercomparison measurements organised by the PTB in two steps. The results were evaluated on the basis of recommendations from the German Commission on Radiological Protection (SSK). In the first step 10 types of dosemeter were investigated in beta particle fields in a range of mean energies from 0.06 MeV to 0.8 MeV. In the second step, five selected beta dosemeter types were exposed to beta particles and, in addition, to photons and to mixtures of both. Three dosemeters fulfill the requirements for the whole range of mean beta energy used for the intercomparison and meet the requirements for photon radiation from 8 keV to 662 keV.     

The use of the Monte Carlo simulation technique for the design of an H*(10) dosemeter based on TLD-100

The thermoluminescence (TL) detector material LiF:Mg,Ti (TLD-100) and appropriate filter materials were combined in order to design a passive dosemeter measuring the operational quantity ambient dose equivalent, H*(10), for monitoring low-dose external photon radiation fields. Using the Monte Carlo simulation technique, optimisations of energy dependent conversion coefficients from air kerma free-in-air compared to ICRU and ISO proposed values. h*K(10), were performed by varying dosemeter detector positioning. geometrical arrangements, and filter materials. Deviations smaller than 5% compared to h*K(10) between 30 keV and 2.5 MeV of primary photon energies were achieved by a dosemeter design consisting of a 15 microm Sn metal layer and a 5 mm PMMA layer surrounding the LiF detector. Subsequently performed free-air verification experiments carried out in well defined standard photon radiation fields showed an obviously TL-specific effect. An underestimation up to -15% of the modelled data at low photon energies was observed.     

A legally approved personal dosemeter for photon and beta radiation based on direct ion storage

The novel DIS-1 dosemeter developed by RADOS is based on ionisation chambers with so-called Direct Ion Storage (DIS). The dosemeter can measure Hp(10) and Hp(0.07) of photon and Hp(0.07) of beta irradiation. The characteristics of the commercially available DIS-1 dosemeter were studied at the Paul Scherrer Institute, particularly in respect to the requirements laid down in the Swiss Dosimetry Ordinance. Detailed tests were carried out in terms of linearity, photon and beta responses, angle dependence, long-term stability of the signal, reproducibility and environmental conditions. The DIS-1 dosemeter has been qualified by the authority to conform to the requirements of the Swiss Dosimetry Ordinance for personal photon and beta dosemeters. It is now used as a legally approved personal dosemeter system at PSI.     

Energy and directional response for the Harshaw dosemeter holders 8814 and 8891, and its effect on the appropriate radiation qualities for absolute calibration

The personal dosimetry laboratory at the Norwegian radiation protection authority utilises a two-element dosemeter card for measuring Hp(10) and Hp(0.07), in a Harshaw dosemeter holder type 8814. Energy and directional responses for photons and betas for this holder have been assessed, as well as for the new Harshaw holder type 8891. The energy response characteristics for the 12-1250 keV photon energy range, in terms of TL output per unit Hp(10) and Hp(0.07) have been evaluated. The maximum over-response to under-response ratio for the Hp(10) element was found to be 1.46 for the new type 8891 holder, as compared to 1.55 for the older type 8814. The new holder also displays a more favourable directional response for this element. For the Hp(0.07) element, no significant differences with regard to energy or directional responses were found. Selecting radiation energy for absolute calibration of the Hp(10) and Hp(0.07) elements are discussed.     

A chamber for determining the conventionally true value of Hp(10) and H*(10) needed by calibration laboratories

A secondary standard chamber for photon radiation developed for measuring directly the conventionally true value of the personal dose equivalent, Hp(10), in a slab phantom is now commercially available. In addition, this chamber can be used for determining the true value of the ambient dose equivalent, H*(10), in monodirectional radiation fields; for example, photon fields generated by X ray facilities. Once the chamber has been calibrated at the facility of the calibration laboratory, the true value of Hp(10) or H*(10) can be measured at other facilities without applying any conversion coefficients. For low energy photon fields the conversion coefficients are strongly dependent on the spectral distribution. For nominally the same radiation quality small spectral differences, caused, for example, by use of different X ray facilities, may lead to differences between the spectrum-averaged conversion coefficients from Ka to Hp(10) and H*(10), respectively, of up to several tens per cent. For this reason, tabulated conversion coefficients for low energy radiation fields cannot be used for calibration purposes, if the standard uncertainty is to be 2-5%. Direct measurement by the secondary standard chamber overcomes this problem.     

ENEA extremity dosemeter based on LiF(Mg,Cu,P) to evaluate Hp(3,alpha)

Recent epidemiological studies suggest a rather low-dose threshold (<0.5 Gy) for the induction of a cataract of the eye lens. Some other studies even assume that there is no threshold at all. Therefore, protection measures have to be optimised and current dose limits for the eye lens may be reduced in the future. ICRP Publication 103 on H(p)(d), in §(136), reads that '… a depth d = 3 mm has been proposed for the rare case of monitoring the dose to the lens of the eye. In practice, however, H(p)(3) has rarely been monitored and H(p)(0.07) can be used for the same monitoring purpose… '. As recommended on the EU 'Technical recommendations for monitoring individuals occupationally exposed to external radiation', a test on the ENEA TL extremity dosemeter is herein reported. The results within the actual EU founded Optimization of RAdiation protection for MEDical staff (ORAMED) Project, whose WP2 is aimed at the quantity H(p)(3) and eye lens dosimetry in practice, are taken into account. The paper summarises the main aspects of the study carried out at ENEA-Radiation Protection Institute (Bologna, Italy) to provide practical solutions (in the use and the design) to evaluate the response of the ENEA TL extremity dosemeter in terms of H(p)(3).     

The electronic neutron/photon dosemeter PTB DOS-2002

During the last few years, PTB has developed the electronic dosemeter DOS-2002. It is of an especially simple design (1 silicon detector) and detects the photon and neutron personal dose equivalent with a low detection threshold of 0.016 and 10 microSv, respectively. Its dosimetric characteristics have been determined in neutron fields with energies ranging from thermal energies up to 15 MeV and in photon fields with mean energies from 65 keV to 7 MeV. It can be used in a wide temperature region from -20 degrees C to +50 degrees C, shows almost no interference in electromagetic fields but is still shock sensitive.     

The applicability of radiophotoluminescence dosemeter (RPLD) for measuring medical radiation (MR) doses

The applicability of radiophotoluminescence dosimetry was determined by assessing various radiophotoluminescence dosemeter (RPLD) properties for measuring medical radiation doses from radiation sources of a continuous spectrum. The RPLD was found to be accurate for measuring doses in diagnostics (50-125 keV) and radiation therapy (6, 10 and 18 MV photons, 6 and 15 MeV electrons). The RPLD shows excellent dose linearity (R(2) > 0.99), reproducibility and batch uniformity, and minimal fading and accurate accumulated dose measurement. The dosemeter material is independent of photon energy in the diagnostic range; however, the dosemeter requires additional calibration in the mammography energy range and also for accurate dose measurement with photon or electron energies in radiation therapy. RPLD measurements with a tin filter show considerable angular dependence at angles exceeding 50° between the photon beam and the normal to the long axis of the dosemeter. The RPLD measurement accuracy at high doses can be improved with optimised pre-heating schemes.     

Design of a finger ring extremity dosemeter based on OSL readout of alpha-Al2O3:C

A finger-ring dosemeter and reader has been designed that uses OSL readout of alpha-Al2O3:C (aluminium oxide). The use of aluminium oxide is important because it allows the sensitive element of the dosemeter to be a very thin layer that reduces the beta and gamma energy dependence to acceptable levels without compromising the required sensitivity for dose measurement. OSL readout allows the ring dosemeter to be interrogated with minimal disassembly. The ring dosemeter consists of three components: aluminium oxide powder for measurement of dose, an aluminium substrate that gives structure to the ring, and an aluminised Mylar cover to prevent the aluminium oxide from exposure to light. The thicknesses of the three components have been optimised for beta response using the Monte Carlo computer code FLUKA. A reader was also designed and developed that allows the dosemeter to be read after removing the Mylar. Future efforts are discussed.     

Direct ion storage dosimetry systems for photon, beta and neutron radiation with instant readout capabilities

The direct ion storage (DIS) dosemeter is a new type of electronic dosemeter from which the dose information for both Hp(10) and Hp(0.07) can be obtained instantly at the workplace by using an electronic reader unit. The number of readouts is unlimited and the stored information is not affected by the readout procedure. The accumulated dose can also be electronically reset by authorised personnel. The DIS dosemeter represents a potential alternative for replacing the existing film and thermoluminescence dosemeters (TLDs) used in occupational monitoring due to its ease of use and low operating costs. The standard version for normal photon and beta dosimetry, as well as a developmental version for neutron dosimetry, have been characterised in several field studies. Two new small size variations are also introduced, including a contactless readout device and a militarised version optimised for field use.     

Response of radiation protection dosemeters in mixed high-energy photon and electron radiation fields

The response of radiation protection dosemeters in terms of the phantom-related operational quantities Hp(10) and H'(10.0 degrees) was measured for personal and area monitoring systems in mixed high-energy electron and photon radiation fields with energies up to 7 MeV. Using mixed radiation fields composed of different fractions of charged particle and photon fluence, three conditions were produced at the point of measurement: charged particle equilibrium (CPE) (a), a lack (b) and an excess (c) of charged particles relative to the conditions of CPE. Personal and area dosemeters of different types were investigated under conditions (a)-(c). A large variability of the response of the different dosemeter types was observed. The results are presented and discussed.     

The implementation in routine of the ENEA new personal photon dosemeter

The ENEA photon dosemeter, introduced in 1995, consisting of two differently filtrated LiF(Mg,Cu,P) detectors, has been modified recently. The ABS (acrylonitrile butadiene styrene) plastic support has been replaced by a new aluminium card supporting the same two detectors (LiF(Mg,Cu,P) GR200). The new card, fully developed at the ENEA-Radiation Protection Institute (which is going to be patented), can now be processed through a Harshaw Model 6600 Automated TLD Reader, a hot gas reader. This paper reports the results of the individual calibration of approximately 60,000 LiF(Mg,Cu,P) GR200 detectors inserted on the new aluminium cards. Before the implementation in routine of the new cards, the reader has been characterised. Steps and tests to be made to use the card in routine (i.e. reader stability, linearity, reproducibility, etc.) are reported. The whole dosimetric system now combines the very good performances of the Harshaw Model 6600 reader and that of LiF(Mg,Cu,P) thermoluminescent material.     

A whole-body dosimetry system for personal monitoring based on hot-pressed thin layer TLD

We are introducing a new high-capacity thermoluminescent dosemeter (TLD) system to measure the whole body values of H(p)(10) and H(p)(0.07) from photons for use in individual monitoring services. Small and light-weight badges allow a convenient application in a wide variety of workplaces with photon radiation from 20 keV to at least 7 MeV. The main advantage of this system will be the large capacity of ～ 100,000 dosemeters per month at costs equivalent to the current film monitoring. The hot-pressed thin-layer TL detector (LiF:Mg,Ti) is welded onto an aluminium substrate and provided with a data matrix code for automatic processing. The detector holder has been optimised, that no additional filter is necessary. The new designed TLD reader with readout times <10 s will allow a large throughput and a considerable degree of automation.     

Development of advanced-type multi-functional electronic personal dosemeter

An advanced-type small, light, multi-functional electronic personal dosemeter has been developed using silicon semiconductor radiation detectors for dose management of workers at nuclear power plants and accelerator facilities. This dosemeter is 62 x 82 x 27 mm(3) in size and approximately 130 g in weight, which is capable of measuring personal gamma ray and neutron dose equivalents, Hp(10), simultaneously. The neutron dose equivalent can be obtained using two types of silicon semiconductors: a slow-neutron sensor (<1 MeV) and a fast-neutron sensor (>1 MeV). The slow neutron sensor is a 10 x 10 mm(2) p-type silicon on which a natural boron layer is deposited around an aluminium electrode. The fast neutron sensor is also a 10 x 10 mm(2) p-type silicon crystal on which an amorphous silicon hydride is deposited. The neutron energy response corresponding to the fluence-to-dose-equivalent conversion coefficient given by ICRP Publication 74 has been evaluated using a monoenergetic neutron source from 250 keV to 15 MeV at the Fast Neutron Laboratory of Tohoku University. As the result, the Hp(10) response to neutrons in the energy range of 250 keV and 4.4 MeV within +/-50% difference has been obtained.     

Fibre remote optoelectronic gamma dosimetry based on optically stimulated luminescence of Al2O3:C

Optically stimulated luminescence dosimetry (OSL-D) used in conjunction with fibre optics enables a remote measurement of dose, for the purpose of radioprotection in the nuclear industry and in medicine (radiology, radiotherapy). Alumina OSL crystals are used because of their low Z, low fading and optical transparency, which improves the sensitivity. An optoelectronic portable dosemeter has been designed and tested that shows a dose detection of 50 microGy with a 20 metre-long fibre. Following irradiation, all trapped electrons are released under light stimulation while the OSL is integrated to provide dose-equivalent measurements. A compensation technique is designed with the help of the MCNP4b code, so that both angular and photon energy characteristics comply with international standards (CEI 61066) for photon dose equivalent Hp(10). Two sensors are described that allow measurements over a wide solid angle (95% of 4piSr), for photon energies ranging from 15 keV to 3 MeV.