Passive Dosimeter Characteristics and New Developments
Total Page：16
File Type：pdf, Size：1020Kb
XA0053414 PASSIVE DOSIMETER CHARACTERISTICS AND NEW DEVELOPMENTS J. TROUSIL National Personal Dosimetry Service, Prague, Czech Republic F. SPURN Department of Radiation Dosimetry, Nuclear Physics Institute, Prague, Czech Republic Abstract The characteristics of the various personal dosimeters presently used by the national dosimetry services are reviewed. Recent developments in this field are also presented, including the Direct Ion Storage technique. 1. GENERAL REQUIREMENTS Individual monitoring enables individual control of exposure in order to ensure that exposure limits are not exceeded and to support reduction of exposures ever further below these limits. The dose limit for occupational exposure has been reduced to 20 mSv.y"1 averaged over 5 consecutive years with a maximum of 50 mSv within one year (ICRP 60). For a working period of 1800 h.y"1, 20 mSv results in a mean dose rate of about 11 ^iSv.h'1 or a mean dose per month of 1.7 mSv. If we take into account the ICRP 35/60 recommendations an individual dosimeter used for monthly recording of doses should be able to register about 10% of this dose at least, so that the lowest measurable dose (recording level) should be about 0.17 mSv. The new recommendation ICRP 75 which is at the printers (the revision of ICRP 35) considers that the recording level for individual monitoring should be derived from the duration of the monitoring period and an annual effective dose of no lower than 1 mSv or an annual equivalent dose of about 10% of the relevant dose limit. For the monitoring period of one month the recording level is in this case not lower than 1 mSv/12=0.085 mSv. In personal dosimetry the maximum error in the measurement of a dose at the level of the annual dose limit should not exceed a factor of 1.5 at the 95% confidence level. This means that the measured dose should be within the interval from - 33% to + 50% of the conventional true value of the quantity of interest which for individual monitoring is personal dose equivalent Hp(10). The highest measurable dose for a personal dosimeter is not given by the ICRP recommendations, as for the gamma radiation. This value should be at least several Gy, and, in some control areas the accident dosimeter is recommended. The overall accuracy of a dosimetry system is determined from the combined effect of a number of systematic and random errors [1-5]. The following sources are usually considered to cause systematic uncertainties: (1) energy dependence (2) directional dependence 151 (3) fading (4) non linearity of the response (5) effects from exposure to light (6) effects from exposure to types of ionising radiation not intended to be measured by the dosimeter (7) effect from a mechanical shock (8) calibration errors (9) variation in local natural background (10) effect of dose rate. Typical sources of random uncertainties are inhomogeneity of detector sensitivity and zero dose for the batch of dosimeters used and fluctuations in reading parameters, including reader sensitivity and background. The combined uncertainty may be expressed in the form of a standard deviation S=-y/8r +5j where 5r and 5S are the resultant random and systematic standard deviations. It is convenient to differentiate between the systematic uncertainty component related to the energy and angular responses and to all other systematic errors. 2. STATE OF THE ART From the practical point of view personal dosimeters can be divided into four categories (ICRP 60): Basic whole body dosimeter which is worn to estimate the operational quantities Hp(10) and Hp(0.07). It is not required to provide any other information. Examples of this category are two-element thermoluminescent dosimeters and radiophotoluminescent dosimeters. Discriminating whole body dosimeter which provides information on the radiation conditions, for example: • the type, energy and direction of the radiation having caused the exposure, and • contamination of the dosimeter. This kind of information can help to estimate effective dose following accidental exposure, or, in situations where workers may regularly receive doses approaching the dose limits in complex radiation fields. A typical example of a discriminating dosimeter is the film badge type which may be capable of providing a great deal of information on the circumstances of the exposure. A multi-element thermoluminescent dosimeter with different filters and thickness can also give information on the type and energy of the radiation. 152 Extremity Dosimeters An extremity dosimeter is such a one which is worn on an extremity, i.e. hand, fore-arm, foot or ankle, when the extremity may become the limiting organ or tissue. These dosimeters are usually worn in addition to a whole-body dosimeter. Thermoluminescent dosimeters are mostly used for extremity dosimetry. Radiophotoluminescent dosimeters are also used for this purpose. 3. THE CHARACTERISTICS OF PERSONAL DOSIMETERS PRESENTLY USED ON NATIONAL SCALE 3.1. Film Dosimeter Film dosimeters are used for determining individual exposure to photon, beta and thermal neutron radiations. For individual monitoring, the photographic films are commonly placed inside suitable holders. Such assemblies are often referred to as film badges [1,18, 20]. The emulsion of the film is made of silver bromide crystals which are suspended in a gelatinous medium. A thin layer of this emulsion is coated uniformly on a thin plastic base, mostly on both sides. After exposure to ionizing radiation, the latent image is developed, and, the optical density is measured by a densitometer. The optical density depends on the: • film type • developing process (temperature and time) • type and energy of radiation • time after exposure (fading). The optical density does not vary linearly with the dose. The complicating factor, important in practical photon dosimetry, is the energy dependence of the film response relative to human tissue. Compensation of this effect is achieved by using one or more filters, either with a different atomic number, or of various thickness. Selection of films Various films for individual dosimetry are on the market now. Those mostly used are Agfa or Kodak and, in the Czech Republic, Foma. Two films - one of high and one of low sensitivity - are packed in a plastic cover. For film evaluation, the following criteria should be taken into account: • sensitivity and dose range • energy response • fading • homogeneity 153 When selecting the sensitivity range of films which can be influenced by the type of developer and temperature and time of developing, the lower limit as well as the upper one of the dose must be considered. Taking into account the general requirements (ICRP 60), for monthly recording of doses the range of doses should be at present from about 0.17 mSv (-50% +100%) to at least 100 mSv (-33% +50%). The new ICRP 75 Publication which is at present at the printers requires the recording level for one month monitoring period 0.085 mSv. Dosimetric Analysis and Interpretation of Results The non-linearity of the dose-density dependence is simply overcome by the calibration process. In this some films are irradiated in a relevant range of doses (-0.2 mSv ~ 1 Sv) by 137Cs or 60Co sources and/or X rays at maximum sensitivity (~ 45 keV). The set of calibrated films is then developed together with films to be evaluated. A new type test calibration is necessary when a new type of film is used or changes are made to the developing process. The energy dependence is often compensated, either by using one filter with a higher atomic number (Pb or Cd), or by a filtration analysis. While the single filter method is suitable for photon energies higher than about 0.1 MeV, for the lower range of energies, filtration analysis is inevitable. In this case, the energy of incident radiation can be estimated from the ratio of responses behind different filters, and, thus the correction factor for the response of the unscreened part of the film can be found. For the practical application a linear combination method is often used. In this case, each filter element as well as open window can be regarded as an individual dosimeter with its own individual dose measurement value. In general, the individual dose values of one dosimeter vary according to the exposure conditions, i.e. the energy and angle of incidence of the radiation. The final dose value of the personal dosimeter is calculated by means of the linear combination of individual dose values under each element, the evaluation algorithm must be found experimentaly. Recently the linear programming method was proposed [21]. Computers are generally used for dose calculation. A fading correction can be simply made by appropriate time of calibration with regard to the monitoring period. The dependence of Hp(10) on the photon energy for film under the open window is given in Figure 1. The curve is normalized to 137Cs radiation. For the dose calculation by means of filtration analysis and filters 0.05; 0.5; 1.6 mm Cu; 0.5 mm Pb and open window the second curve on Figure 1 is obtained. The ratio of measured Hp(10), and irradiated Hp(10) lies within ± 10% for narrow photon spectra in the range of 15 keV and 6 MeV. For mixed photon energies and routine conditions the accuracy is worse (up to ± 25%). The linear combination method gives similar results for narrow photon spectra, but for mixed photon energies the results are generally worse. From the practical point of view, the filtration analysis method gives additional information about the spectrum of photons, while the linear combination method gives additional information concerning the uncertainty of the evaluated dose. The film badge characteristics, including desirable and undesirable features, are summarized in Table I.