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Personal Radiation Monitoring Service and Assessment of Doses Received by Radiation Workers (2004) Neil D Morris, Peter D Thomas and Kevin P Rafferty

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TECHNICAL REPORT P e r s o n a l R a d i a t i o n M o n i t o r i n g S e r v i c e a n d A s s e s s m e n t o f D o s e s R e c e i v e d b y R a d i a t i o n W o r k e r s ( ) N e i l D M
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TECHNICAL REPORT P e r s o n a l R a d i a t i o n M o n i t o r i n g S e r v i c e a n d A s s e s s m e n t o f D o s e s R e c e i v e d b y R a d i a t i o n W o r k e r s ( ) N e i l D M o r r i s, P e t e r D T h o m a s a n d K e v i n P R a f f e r t y T E C H N I C A L R E P O R T S E R I E S N o Personal Radiation Monitoring Service and Assessment of s Received by Radiation Workers (2004) by Neil D Morris, Peter D Thomas and Kevin P Rafferty Technical Report Lower Plenty Road ISSN Yallambie Vic 3085 June 2004 Telephone: Fax: Notice Commonwealth of Australia 2004 Copyright Notice and Disclaimer This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from the Commonwealth. Requests and inquiries concerning reproduction and rights should be addressed to the Commonwealth Copyright Administration, Attorney General s Department, Robert Garran Offices, National Circuit, Barton ACT 2600 or posted at All care has been taken in the preparation of this work and its conclusions. However, where the data or results presented are utilised by third parties the Commonwealth of Australia shall not be liable for any special, indirect, consequential or other damages whatsoever resulting from such use. Nor will the Commonwealth of Australia be liable for any damages arising from or in connection with any errors or omissions that have inadvertently occurred in this work. Requests for information about the content of this publication should be addressed to the Information Officer, ARPANSA, 619 Lower Plenty Road, Yallambie, Victoria, 3085 or by Reprinted 2008 This second printing of ARPANSA TR139 incorporates some minor corrections to the first printing. Personal Radiation Monitoring Service and Assessment of s Received Page No. 2 Contents PREFACE...4 INTRODUCTION THE ARPANSA PERSONAL RADIATION MONITORING SERVICE MONITORS AVAILABLE WEARING PERIODS HOW THE MONITORS WORK A TECHNICAL DESCRIPTION OF THE MONITORS THERMOLUMINESCENT DOSEMETER (TLD MONITOR) EXTREMITY (OR FINGER) TLD NEUTRON MONITOR SPECIAL TLD ENVIRONMENTAL MONITOR ESTIMATION OF THE RADIATION ENERGY METHOD OF CALCULATING A RADIATION DOSE ACCURACY OF MEASUREMENTS...13 Table 1: Uncertainties for TLD Badges, Special TLD Badges and Neutron Badges...13 Table 2: Uncertainties for other types of Monitor HOW DO YOUR DOSES COMPARE? - OCCUPATIONAL EXPOSURE LEVELS Table 3: Classification of Wearer Occupations...15 Table 4: Classification of Establishment Types...17 Table 5: Annual Photon s to Monitors Worn by Occupationally Exposed Personnel (2004) Diagnostic Radiology Radiotherapy...20 Nuclear Medicine or Pathology...21 Dentistry Chiropractic practice Veterinary Practice Industry Mining Research Education Table 6: Annual Extremity s to Occupationally Exposed Personnel (2004)...30 Table 7: Annual s to Monitors Worn by Personnel Exposed to Neutron Sources (2004) WHAT DO THE DOSES MEAN? - EFFECTIVE DOSE AND PERSONAL DOSE EQUIVALENT Table 8: Effective and H p (10) Conversion Factors REFERENCES Personal Radiation Monitoring Service and Assessment of s Received Page No. 3 Preface This report is an update of ARL/TR121 (Morris 1996) which included data for the year Since 1996, the Personal Radiation Monitoring Service operated by the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) has increased the number of workers monitored by about 6%. It now covers approximately 35,000 occupationally exposed workers throughout Australia and Papua New Guinea. Introduction Exposure to radiation can cause genetic effects or cancer. People who use sources of radiation as part of their employment are potentially at a greater risk than others owing to their being continually exposed to small radiation doses over a long period of time. In Australia, the ARPANSA and the National Occupational Health and Safety Commission (NOHSC) establish radiation protection standards and set annual effective dose limits for radiation workers in order to minimise the chance of adverse effects occurring (Radiation Protection Series No. 1). These standards are based on the recommendations of the International Commission on Radiological Protection Publication 60 (ICRP 1990). In order to ensure that the prescribed limits are not exceeded and to ensure that doses are kept to a minimum, some sort of monitoring is necessary. For exposure to external radiation, personal monitoring is recommended. Personal Radiation Monitoring Service and Assessment of s Received Page No. 4 1. The ARPANSA Personal Radiation Monitoring Service The Personal Radiation Monitoring Service (PRMS) has been in continuous operation since The Service enables people working with radiation to determine the radiation doses that they receive due to their occupation. Since 1986, all persons regularly monitored by the Service have been registered on a database that maintains records of the doses received by each individual wearer. At present, the Service regularly monitors approximately 35,000 persons and maintains dose histories of over 115,000 people. 1.1 Monitors Available The radiation doses received by occupationally exposed workers can be measured by using one of the five types of monitor issued by the Service: The Extremity (or Finger) TLD is used by persons who may be exposed to significant doses to the fingers or other extremities. The monitor consists of a small plastic sachet containing a TLD. It can be chemically disinfected. It measures exposure to beta, gamma or x-rays and is worn for a 4 weekly wearing period. The Thermoluminescent dosemeter (TLD monitor) is the most commonly used with about 175,000 issued annually. The badge is comprised of a thermoluminescent dosemeter (TLD) card, which is placed in a holder that incorporates a filter system. This allows the radiation type and energy to be determined. It is used to determine the whole body exposure of people who may be exposed to beta, gamma or X-rays. It can be worn for a 4, 8 or 12 weekly wearing period, depending on the work carried out and any restrictions placed on the wearing period by the Regulatory Authority in each State or Territory. The Neutron monitor is used to measure the doses received by people exposed to a combined field of fast neutrons, beta and gamma rays. It is worn for a 4, 8 or 12 weekly wearing period. The badge consists of a neutron sensitive plastic (CR39) and a TLD card loaded inside a plastic holder prior to issue. It is used by persons potentially exposed to fast neutrons produced from certain radioactive sources such as americium/beryllium or californium-252 or those emitted from certain high-energy particle accelerators, neutron generators or moisture density gauges. Personal Radiation Monitoring Service and Assessment of s Received Page No. 5 The Special TLD is a monitor constructed within the Service prior to issue. It is comprised of a TLD Badge that is sealed in a plastic envelope. The badge is used in special situations such as in uranium and mineral sand mining operations where conditions may be dusty and in laboratories where there is a significant risk of radioactive contamination. In uranium or mineral sands mining areas, the monitors may be stored in areas that have elevated background radiation levels. These monitors require a special assessment procedure. The monitor is normally worn for periods of between 1 and 3 months and may also be used as an Environmental area monitor. 1.2 Wearing periods The choice of wearing period depends on a number of factors. As radiation has the potential to cause adverse health effects, it is necessary to try to minimise the doses received. An investigation of the causes of radiation doses may be hampered if the wearing period is too long. The Licensing Authority may also restrict the length of the wearing period. Wearing monitors longer than the recommended wearing period can also affect the accuracy of the reported doses. This is due to the build-up of background radiation on the monitors and the fading of the radiation doses with time. The annual background radiation dose is approximately 1 to 2 millisieverts (msv) each year. An extra monitor is always issued to each establishment to act as a control. This monitor is always labelled Control A. The control is used in the assessment of doses received by the other monitors and serves as a means of determining the background radiation level or transit dose to which the batch as a whole is exposed. It will also assist in determining if the batch is accidentally exposed to radiation at any time. Any reading from a worn monitor that is statistically different from the control is assumed to be due to occupational exposure provided that certain consistency checks are also satisfied. The integrity of the doses reported is of the highest importance. The PRMS is accredited by the National Association of Testing Authorities (NATA Accredited Laboratory Number 14442). All measurements are performed in accordance with the NATA requirements which include the requirements of the Standard General requirements for the competence of testing and calibration laboratories (ISO/IEC 17025). All doses are traceable to Australian Primary Standards. A comprehensive quality assurance program is also maintained in order to ensure the reliability of the Service. Personal Radiation Monitoring Service and Assessment of s Received Page No. 6 2. How the Monitors Work A Technical Description of the Monitors When certain crystals are exposed to radiation, some of the radiation energy is trapped within the crystal structure. If the crystals are subsequently heated, the trapped energy is released from the crystal structure in the form of light. The amount of light emitted is dependent on the radiation exposure that the crystal has received. If the amount of light emitted is measured it will be proportional to the radiation dose received. This method of radiation detection is called thermo-luminescent dosimetry (TLD) 2.1 Thermoluminescent dosemeter (TLD monitor) The TLD monitor consists of a TLD card sealed into a Tyvek (DuPont) envelope, which is then placed inside a plastic TLD holder. The doses are determined by the measurement of the light output from the TLD card when it is heated through a specific temperature cycle. The TLD card comprises 8 elements of CaSO 4 :Dy in a PTFE matrix mounted in an aluminium frame and has a uniquely numbered barcode that can be read automatically. The radiation dose can be measured in four discrete primary areas of the card with another four areas acting as a backup. The card is shown in Figure 1. After readout, the card can be annealed to get rid of any residual reading and then reissued. A TLD card can go through this readout/anneal cycle more than 500 times. Figure 1 CaSO 4 :Dy is a sensitive TLD material and this allows for a low minimum detectable dose but has the disadvantage that its response is extremely energy dependent. A graph of the energy response of TLD cards that were exposed to the same dose of radiation is given in Figure 2. The peak of the graph coincides with diagnostic X-ray energies and is approximately ten times the response that would result from the same dose of high-energy gamma rays. Figure 2: Energy Response of CaSO 4 :Dy Dental X-rays Diagnostic X-rays Relative Response Industrial X-rays 2 Tc99m Co Energy (kev eff ) Personal Radiation Monitoring Service and Assessment of s Received Page No. 7 It is also possible to use the TLD cards to assess beta ray doses provided that the beta ray energy is greater than 70 kev. Beta rays below this energy will not penetrate the Tyvek envelope and reach the TLD card. In order to determine the actual dose that the monitor received, it is necessary to take into account the difference in response of the TLD to different types of radiation and different radiation energies. Knowledge of the type of radiation and radiation energy to which the TLD card was exposed is therefore important and the user is always asked to specify the radiation sources used. An estimate of the type and energy of the radiation to which the TLD card was exposed can be made if the card is placed in a holder that contains filters of different materials to attenuate the radiation beam to differing extents. The holder used in the Service is illustrated in Figure 3. Open Window Teflon (845 mg.cm -2 ) 0.25 mm Cu mm Al 2.7 mm Cu mm Al Figure Extremity (or Finger) TLD The extremity monitor consists of either a disc of LiF:Mg,Ti in a PTFE matrix or a small square chip of LiF:Mg,Ti mounted on a kapton strip, known as a chipstrate, which is inserted into a flexible plastic sachet. The sachet can be wrapped around a finger and taped together to make a ring. The LiF:Mg,Ti disc and chipstrate that are used in the extremity monitors are shown in Figure 4. Extremity monitors are generally worn by people handling high activity radioactive sources, such as those used in the treatment of patients, or by people using X-ray diffraction units, where there is the possibility that the wearer may get a high dose to the fingers. Figure 4 LiF:Mg,Ti is used as the TLD material as it is relatively energy independent compared to the response of CaSO 4 :Dy; the sensitivity at 20 kev is only about 30% higher than it is for high energy photons. The minimum detectable dose is greater than that of CaSO 4 :Dy, particularly for X-rays, however this is not a significant disadvantage as the dose limit for the extremities is 25 times greater than for the Personal Radiation Monitoring Service and Assessment of s Received Page No. 8 whole body. A graph of the energy response of LiF:Mg,Ti dosemeters that were exposed to the same dose of radiation is given in Figure 5. Figure 5: Energy response factors for LiF Relative Response Dental X-rays Diagnostic X-rays Industrial X-rays Tc99m Co60 and Beta rays Energy (kev eff ) The Extremity (or Finger) TLD is also sensitive to beta rays that have an energy greater than 70 kev and to gamma and X-rays, but as it has no filtration system the accuracy of the assessment depends on the user correctly identifying the radiation sources used. 2.3 Neutron monitor The neutron monitor consists of a TLD card for the detection of photon and beta radiation and a plaque of CR-39 plastic, which is sensitive to fast neutrons. The card and the plaque are placed in a specially modified holder similar to that used with the TLD monitor. The neutron monitor is constructed prior to issue. Each CR-39 plaque is uniquely numbered for identification purposes. A CR-39 plaque is shown in Figure 6. The photon and beta ray doses are measured in the same manner as described previously for the TLD monitor. The neutron monitor is routinely calibrated using an Americium/Beryllium source and is sensitive to neutrons of energies ranging from 150 kev to 15 MeV. Figure 6 Personal Radiation Monitoring Service and Assessment of s Received Page No. 9 The fast neutron dose is measured using the CR-39 plastic plaque. When neutrons interact with CR-39, proton recoil produces events in the plastic that are visible through a microscope after the plastic has been etched overnight by a concentrated potassium hydroxide solution. The events can be recognised by their shape, which may be elliptical or conical. The number of events per unit area is proportional to the neutron dose received by the plastic. A magnified view of an etched CR-39 plaque is shown in Figure 7. The neutron events are circled. The other events are not positively identifiable as being due to neutrons and are, therefore, not measured. Figure Special TLD The Special TLD monitor consists of an assembly of a TLD card and a TLD holder. The monitor is assembled prior to issue. It is sealed into a plastic envelope so that, when the monitor is used in a dirty or dusty environment, the monitor itself cannot become contaminated. The monitor is often issued to uranium or mineral sand mining establishments, which have the added difficulty that the monitors could be stored in a high background area when not being worn. Consequently, more than one control monitor may be issued. The extra controls are labelled B, C, etc. The Control A is used to measure the normal background radiation level or doses received in transit and must be stored in a low background area. The Control B measures the high background area where the worn monitors are stored. It helps to separate the dose that the wearer actually received from that which their monitor received while it was stored in the high background area. Rate monitors sent out arrive at high background area CNTL B Multiple Controls actual wearing periods Control A Figure 8 Time monitors returned work environment high background low background Figure 8 demonstrates the problem. The grey shaded area is the exposure received by Control A, the green area is the extra exposure received by Control B above the Control A exposure, due to the higher background area in which it is stored. The white area is the occupational dose that the wearer received. The total number of hours worked in the wearing period by the wearer is also required. Personal Radiation Monitoring Service and Assessment of s Received Page No. 10 The wearer dose is calculated in the following manner: DOSE = ( R C ) ( C C ) A B A ( WP 168 TSH ) WP 168 where: R is the total dose recorded by the worn monitor C B is the dose recorded by the Control B monitor C A is the dose recorded by the Control A monitor WP is the wearing period in weeks (168 hours per week) TSH is the total number of hours worked during the wearing period 2.5 Environmental Monitor The Special TLD monitor can also be used to measure the environmental dose at a site. The environmental dose is the total photon dose recorded at the site for the period that the monitor is in place and includes the normal background radiation. The Control A is used to measure the dose received by the environmental monitor during transit. Figure 9 demonstrates the calculation of the environmental dose, where the green shaded area is the total environmental exposure measured and the grey area is the exposure received by the Control A during transit. monitors sent out Start of environmental monitoring Environmental Monitors End of environmental monitoring monitors returned CNTL A Environmental Exposure Control A high background low background WP ANR The environmental dose is calculated in the following manner: DOSE = R C A ( ANR WP) ANR Figure 9 where: R is the total dose recorded by the environmental monitor CA is the dose recorded by the Control A monitor WP is the wearing period in weeks ANR is the period, in weeks, between the date of preparation of the monitor and the date that it is read out. Personal Radiation Monitoring Service and Assessment of s Received Page No. 11 2.6 Estimation of the Radiation Energy Measurements are made on the TLD card in the four areas that correspond to the filtered areas in the TLD holder. All radiation will interact with the part of the TLD card behind the open window area. The thick plastic area will effectively stop any beta rays getting to the TLD card and will attenuate gamma and X-rays to a small extent. The filter that is predominantly aluminium will reduce the photon radiation reaching the TLD card for energies of less than 60 kev and the predominantly copper filter will significantly reduce the photon radiation reaching the TLD card for energies of less than 150 kev. Consequently, by taking ratios of measurements made on the TLD card which correspond to these four areas, the type of radiation can be distinguished and an effective energy estimated. When the radiation listed by the user differs from the estimated radiation energy as determined from the TLD measurements, a comment to that effect can be included on the dose report. This may help the wearer to determine the cause of the dose received. 2.7 Method of Calculating a Radia
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