Uncertainty and detection limit in decontamination measurements of gamma emitting radio nuclides in unshielded environments Tom Koivuhuhta

Transcription

1 Uncertainty and detection limit in decontamination measurements of gamma emitting radio nuclides in unshielded environments Tom Koivuhuhta Master of Science Thesis Supervisor: Mats Isaksson

2 Contents Abstract... 3 Introduction... 4 Dosimetry in radiation protection applications... 5 The Effective Dose E... 5 The Ambient Dose Equivalent H*(10)... 6 Calibration of instruments for area monitoring of H*(10)... 7 Detector considerations... 8 Semiconductor detectors... 8 Scintillation detectors Material and method Ortec Detective EX (100T) Exploranium GR-135 B The Phantom I Cs The Phantom measurements Angular dependence MicroShield 6.22 Simulations I volume source simulations I surface source simulations Comparison between H*(10) values The minimum detectable dose rate MDD Measurements of ambient background radiation Results Phantom measurements Angular dependence MicroShield 6.22 Simulations Comparison of measured and calculated H*(10) The minimum detectable dose rate MDD Emergency waiting room Discussion Conclusion References

3 Appendix A Appendix B Appendix C Absorbed dose and air kerma free in air Appendix D A volume source S A approximated as a point source S

4 Abstract When individuals might have been contaminated by gamma emitting radio nuclides there is a wish for an effective and quick survey method that makes it possible to decide if a person is contaminated or not. After a decontamination measure another scan is required to decide its success. To find the contaminated individuals an instrument that measures ambient dose equivalent rate H*(10) can be useful. The detectors Ortec Detective EX and GR-135 are calibrated to show H*(10). Both these were used to measure the H*(10) at distances 10, 20, 40, 60, 80 and 100 cm from a phantom containing 40 MBq homogenously dispersed 131 I. The phantom consisted of 13 five liter bottles filled with water. The phantom weight and length was about 65 kg and 165 cm, respectively. The measured H*(10) was compared against simulated phantom measurements with the software MicroShield ver. 6.22, calculated for Anterior-Posterior geometry. Conversion coefficients from ICRP 74 was used to convert the effective dose equivalent rate H E from simulated phantom measurements to air kerma and then to H*(10). The aim of this work was to investigate the possibility to measure gamma photons emitted from a person in a standing geometry, a situation that might occur before and after a decontamination of a contaminated person. No direct conclusion can be drawn from the results that reinforce or assure the practicality as intensimeters for low activity measurements in unshielded environments. To be able to make a better analysis an efficiency calibration may be necessary for the given measurement geometry. With knowledge of detector properties as crystal size and surrounding detector medium an efficiency calibration and estimation of the detection limits could be done with Monte Carlo simulation. 3

5 Introduction Accidents involving radioactive substances can cause serious health issues, not only to people directly connected to an accident site. Individuals contaminated by radioactive substances must therefore be dealt with in a swift and controlled fashion to avoid further spread of the contaminant, but also to minimize health risks involving exposure from the contamination. An important first measure is to remove radioactive materials from external parts as quickly as possible since potentially harmful substances might otherwise get absorbed into the body by inhalation, ingestion or wounds. Biological damage can be severe if high LET radiation from decay processes (α-particles) reaches internal parts. Emitted low LET photons, however can cause radiation damage even if the radio nuclides only are located on external parts on the body or on the clothes. Photons may therefore be a great contributor to the risks related to radiation exposure. By removing clothing in an early stage at the decontamination process the absorbed dose from photons is expected to decrease by approximately 90 % (when the radioactive substances have stayed on clothes and is not absorbed by skin). It is of primary interest to minimize additional spread of hazardous materials at the accident site. If several individuals get exposed, complications may get much worse. The rescue and medical personnel has to struggle much harder to retain capacity in giving injured the medical help they need. Before individuals contaminated by radioactive substances can be sent to hospital for treatment a decontamination should have taken place, unless the patient suffers from a life threatening trauma. The general strategy is to interrupt the exposure by removing the clothes and perform the complete decontamination at the hospital. Stationary decontamination facilities capable of total decontamination in the Västra Götaland region are situated at hospitals at Näl, Skövde, Borås and Kungälv. 10 After an accidental spread of radioactive substances an early measure is to locate the individuals that need to be decontaminated. In this situation a handheld spectrometer that registers Ambient Dose Equivalent Rate H*(10) is useful. There are, however, certain conditions that can make this detection more difficult. Measurements at the accident site may suffer from an enhanced background due to the accidental release, but background radiation originating from cosmic radiation and building materials can deteriorate the detection limit significantly, unless the measurements are performed in background shielded environmets. The overall purpose of this work was to decide the possibility to use the handheld spectrometers Ortec Detective EX (Ortec, USA) and Exploranium GR135 (SAIC, Canada) for screening of contaminated individuals. These detectors were used as intensimeters, i.e. registered data was obtained ambient dose equivalent rate. 4

6 Dosimetry in radiation protection applications For applications in radiation protection The International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) have developed protection or limiting quantities (published in ICRP ) and operational quantities (published in ICRU ) that makes it possible to get a connection between the effective dose and the ambient dose equivalent. Intensimeters used in radiations protection are usually calibrated to directly show the ambient dose equivalent H*(10), which is an measurement quantity that is as an estimate of the effective dose. 6 An agreed set of conversion coefficients published in ICRP Publication 74 7 makes it possible to convert measured exposure or air kerma into effective dose and ambient dose equivalent (see Fig. B1 and B2 in the Appendix B). The Effective Dose E Limiting values for environmental dose rate estimations are given by the effective dose E (eq. 2 below). These values are given to regulate or limit the risk for stochastic effects on bodily tissue exposed to a radiation field. Equivalent dose is given by eq. 1 (this concept was already introduced 1977 in the publication ICRP ), [Gy = Sv] (1) here is the mean dose absorbed when an organ it is exposed to ionizing radiation type R, and is the radiation weighting factor for the type R radiation; values are listed in ICRP (for example are given for photons, electrons, protons). The radiation weighting factor for photon radiation is 1. The effective dose E is defined as [Sv] (2) here the recommended tissue weighting factors are listed in ICRP In deriving the effective dose from the air kerma, with Monte Carlo simulations using different kind of models or body like phantoms, energy deposition to each of the organs in the human body can be approximated. These simulations give different results in different irradiation geometries. Therefore conversion tables in ICRP 74 7 for exposure situations are given for geometries: AP (Anterior- Posterior), PA (Posterior-Anterior), LAT (Lateral), ROT (Rotational symmetrical) and ISO (Isotropic radiation field). In this report only the AP geometry situation is considered, because this geometry was the closest match to the geometry during phantom measurements with both detectors. In the ICRP document the AP geometry is described as an irradiation condition where the 5

7 ionizing radiation is incident on the front of the body in a direction orthogonal to the long axis of the body. The effective dose equivalent is the former limiting quantity defined in ICRP Some modifications were made and the quantity was changed to the effective dose E in the ICRP 60 publication 21. Dose conversion factors published for the old system is still useful in the effective dose system. 6 Differences between and E does not exceed 12 % for all irradiation geometries and for photon energies above 100 kev (differences between conversions coefficients for and E in the photon case are generally lower than 5 % in most irradiation geometries). 2 But between 25 and 100 kev the effective dose gets twice as big as effective dose equivalent for the AP geometry 2. This difference grows for smaller photon energies because of the skin tissue weighting factor in E, which was not included in. The Ambient Dose Equivalent H*(10) In radiation protection it is desirable to characterize potential irradiation of humans in terms of a single dose equivalent quantity that would exist in a phantom approximating the human body (like the ICRU Sphere, as seen in Fig. 1). In occasions where there might be a risk for radiation exposure, area monitors for free air measurements could be used. Individual monitors or individual dosimeters that usually are worn on the front side of the body could be another option. For these situations operational quantities are defined in ICRU Report For instrumental measurements of penetrating photons the quantity of choice is the ambient dose equivalent H*(10), where the number 10 correspond to the dose equivalent in the ICRU sphere phantom at a location 10 mm from the phantom surface. The ICRU phantom consists of a tissue equivalent material and has a diameter of 30 cm. Detectors that are intended to measure H*(10) should ideally have an isotropic response 8. Fig. 1: The ICRU Sphere irradiated with an aligned and expanded radiation field, according to the definition of ambient dose equivalent. The dose equivalent point in ICRU sphere is at a depth of 10 mm. Image 8. 6

8 Calibration of instruments for area monitoring of H*(10) Calibration is defined as the quantitative determination, under controlled set of standard conditions, of the indication given by a radiation instrument as a function of the value of the quantity the instrument is intended to measure. 8 A calibration can thus be regarded as a method to transform the basic physical properties of the radiation field to a signal in a measuring device (Fig. 2). Fig. 2: Flow scheme from radiation field to measurable quantities. (IAEA Safety Reports Series No16) 8 A reference instrument is required if the radiation field in which the calibration is intended to be performed is not known. The reference instrument generally does not give the appropriate dose equivalent quantity; instead the reference field is often characterized in terms of air kerma, K a. 8 Appropriate conversion factors give the true dose equivalent H at the calibration point (table in Fig B2 in appendix B gives conversion values for H*(10) from K a ), where h is the conversion factor for H, N R is the calibration factor for the reference instrument and M R is the measured value with reference instrument. (3) Fig. 3: Calibration in a collimated field from a radiation source using a reference instrument (the calibration factor N R for this instrument should be known). Image 8 7

9 The instrument under calibration then has the calibration factor N I (4) in which M I is the measured value for the instrument under calibration. The conversion factor h apply only if charged particle equilibrium CPE exists at the point of calibration (in the detector volume). The reference instrument must be calibrated for the range of energies and air kerma and/or air kerma rates that are intended to use. 8 The instrument under calibration (Fig. 2) is then calibrated for the energy of the calibrations source. Using the instrument at other photon energies may require that a correction factor is applied. Detector considerations The detectors used in this work are Ortec Detective EX and Exploranium GR-135. The main detector material in Ortec Detective EX consists of a high purity germanium crystal (HPGe), which is semiconductor of coaxial p-type construction. The GR-135 is mainly a sodium iodide, Na(Tl), an inorganic scintillator detector. These detectors operate with pulse mode operation, which is the most common operation mode for detectors. 1 The pulse mode makes it possible to discriminate between different energies which is applied in spectrum measurements. Semiconductor detectors Semiconductor detectors are mainly constructed from germanium Ge and silicon Si single crystals and produced in two geometries, planar and coaxial. 22 When ionizing radiation with certain energy interacts with the semiconductor material an electron hole pair is produced. For germanium the amount of energy needed for an electron hole pair is 2.96 ev (silicon requires 3.76 ev) 1. The Ge crystal material has a higher electron hole yield at a given gamma energy than any other material. 2 Photoelectric absorption, compton scattering and pair production are the main interaction processes (Fig. 4). A Ge detector must be cooled below 100 K to reduce the leakage current due to thermal generation of charge carrier to an acceptable level. Cooling with liquid nitrogen is an option but today electromechanical coolers are available (the Ortec Detective EX has a low power Stirling Cooler). 8

10 Fig 4: Compton, photoelectric, and pair production cross section of Ge for different photon energies. (Handbook of Radioactivity Analysis) 2 In gamma ray spectrometry the peak formation is created from the electron hole pairs. The photoelectric effect is important because the gamma photon transfers all of its energy to the electron in the atom, whereby the contribution to the full energy peak is more prominent. Compton events can also contribute to the full energy peak, but the most of scattered particles (photons and electrons) must in this case be absorbed in the detector. The Compton process will then mainly contribute to the continuum, unless the detector volume is large which gives an increased interaction probability and thereby a larger degree of full energy events. The spectrum continuum is regarded as the background created by the source itself and this background influences the detection limit. 2 Single and double escape peaks in spectrum are formed by pair and triplet processes. Important performance benefits for semiconductor detectors are characterized mainly by the energy resolution, detection efficiency and peak to Compton ratio. The energy resolution is determined by the statistical fluctuation of the number of charge carriers produced, by the detector noise and backcurrent and by parameters of the electronics. 22 The energy resolution is measured for some mono energetic source, usually Co-60. A good energy resolution means that the output pulses generated by photons of equal energy have a small variance. This gives a full energy peak that has a small Full Width at Half Maximum (FWHM), ideally a delta function. For HPGe detectors the resolution is usually kev FWHM. 22 The detection efficiency for gamma detectors is often well below 100 % due to the finite size of the detector and the large penetration of the photons. The detection efficiency gives a relation between the number of counted pulses and the number of photons that strike the detector. The peak to Compton ratio is an important characteristic of gamma ray detectors, because the Compton process operates over such a wide energy spectrum (as 9

11 seen in Fig. 4). If a detector performs badly in this respect this might indicate a malfunctioning detector. Scintillation detectors This type of gamma detector consists of the two primary parts, a scintillator and a photomultiplier tube. The photomultiplier tube consists of a semitransparent photocathode, focusing electrodes, several dynodes and an anode. Emitted light from the scintillator material are converted into electrons at the photocathode. The photoelectrons are then multiplied over the dynodes to finally produce an output signal at the anode. The energy resolution of scintillation detectors is substantially poorer than that of semiconductor detectors, this because an average energy of about 200 ev is necessary to create one electron hole pair (in Ge about 3 ev is sufficient). Middle sized NaI(Tl) crystal have a FWHM resolution of 6-8%. 22 The detection efficiency decreases rapidly as the gamma ray energy is increased because the photo electric cross section decreases. The peak to Compton ratio is not as good as for the HPGe detector because the peaks in the spectrum are wider because of the poorer energy resolution. 10

12 Material and method Ortec Detective EX (100T) The Ortec Detective EX (Ortec, USA) is a portable nuclide identifier that contains a high purity germanium detector, HPGe. The HPGe crystal is a coaxial p-type and has a diameter of 65 mm and a length of 50 mm. The crystal is located inside the cylindrical housing in front of the detector (Fig. 5). An electrical cooling system keeps the HPGe crystal at the correct operational temperature. The Ortec Detective EX also contains a moderated neutron detector and a Geiger Müller tube. The latter is chosen by default when gamma dose rates gets above 20 µsv/h. At lower intensities of gamma radiation dose rates are acquired with the HPGe. Here the displayed dose rates are acquired from the 8192 channel MCA (Multi Channel Analyzer) energy spectrum. 17 The details of the algorithm could not be found in the literature but usually the ambient dose equivalent rate H*(10) is given from spectral data by weighting the number of counts for each channel against the mass energy absorption coefficient the specific photon energy and material. 15 The stated uncertainty in dose rate is < (-50 % to 100 %). 12 Energy calibration can be accomplished with low activity calibration sources 137 Cs, 40 K or 232 Th. In this work a low activity 137 Cs source was used for energy calibration. for Fig. 5: Ortec Detective EX 100T Exploranium GR-135 B The Exploranium GR-135 (SAIC, Canada) contains two detector systems, which operates with a Sodium Iodide, NaI, crystal or a GM tube (Fig. 6). The NaI crystal has a diameter of 38 mm and a length of 55 mm. The GM tube is located directly in the front of the detector, while the NaI crystal is situated more towards the center behind the GM tube. It is possible to select either one of these for dose rate measurements. When the NaI is chosen the displayed dose rates are acquired from spectral analysis with the 1024 channel MCA. The instrument is calibrated to ambient dose equivalent H*(10). The uncertainty during measurement is specified within ± 10 % for gamma energies between 100 kev and 3 MeV. 11

13 For the GR-135 energy calibration is done every 24 hours (or less) with a 137 Cs calibration source (detector announced when a calibration was necessary, which happened a couple of times before dose rate measurements). Fig.6: Exploranium GR-135 B. The Phantom The phantom consisted of 13 water bottles and the volume of each bottle was 5 liters. The bottles were placed on a plane surface (Fig. 7) after they were filled with water and 131 I solution to give a total activity of 40 MBq at December 3 th (0.48 ml with 6.42 MBq/ml was inserted into each bottle). The phantom weight is 65 kg and it is about 165 cm in length. Each bottle has the estimated dimensions cm 3. Fig.7: Phantom consisting of 13 bottles containing water and 131 I. 131 I 131 I decays with a half-life of 8.02 days emitting gamma and beta particles. The primary emissions of 131 I decay are beta particles with a maximum energy of 606 kev (89.9 % ) and 334 kev (7.27 %). 11 Gamma rays emitted with the highest decay probabilities are 364 kev (81.8 %), 637 kev (7.2 %), 284 kev (6.1 %) and 723 (1.8 %). 13 The photons were the only contributor for the dose rate measured with Ortec Detective EX and GR-135 during the experiments. 131 I is mainly used in nuclear medicine applications for treating malfunctioning Thyroid gland or thyroid cancer. The volatile state of 131 I makes extra precautions necessary when handling the substance. Low ph solutions should be avoided. It s also important to avoid skin contamination and inhalation of 131 I vapour

14 Data acquired from the phantom measurements were decay corrected to December 3 th with the equation (5) where A 1 is the activity at reference time (40 MBq), of 8.02 days and t is the time since December 3 th. is the physical half life 137 Cs 137 Cs has a half life of 30.1 years. In the decay process to 137m Ba (100 %) β-particles with a maximum energy of 1176 kev are emitted. In the following deexitation process to the ground state gamma photons with energy 662 kev (85.1 % of the number of decaying 137 Cs nuclei) are emitted. 13 The Phantom measurements Fig. 8: Phantom with measurement distances shown. Dose rate measurements were done with Ortec Detective EX and Exploranium GR-135 at distances 10 cm, 20 cm, 40cm, 60 cm, 80 cm and 100 cm from the phantom surface at the angle settings 0 degree, 5 degree up and 5 degree down. At 0 degree setting both detectors were directed towards the phantom (towards the X mark in Fig. 8). At 5 degree up setting both detectors were rotated 5 away from 0 degree setting towards phantom head (rotation to the right in Fig.8). At 5 degree down setting detectors were moved the same angle but in opposite direction as in the 5 degree up setting. The measurements at settings 5 degree up and 5 degree down were made for estimating the uncertainty when the detector is not directed straight against a standing person in a measurement situation. 13

15 The data collected at each distance is a mean value of 10 measurements sampled 10 s apart. By noting data at specific increments gave a more random sampling, since the instruments are continuously showing the dose rate measured. Angular dependence Ortec Detective EX and Exploranium GR-135 were placed on a rotating circular plate graded with angles 0, 15, 30, 45,, 360. A 37 MBq 133 Ba source with approximate diameter of 7 mm and length 2 cm was positioned at B according to Fig. 9 (and according to Fig. 10 and 11). The distance between B and C was 160 cm. By varying the angle on the circular surface (θ in Fig. 9) the photon angle of incidence towards the detector was varied. The angle of incidence is however not given by θ (the only apparent exception from this is θ = 0, ). The distance from B to the front of the detector is also changed from 160 cm when θ 0 Fig. 9: The overall setup for dose rate measurements at different angles θ. Detector (red) placed on circular surface with the detector front at A and source at B. The angle of incidence towards the detector was calculated with the Law of Cosines (geometrical considerations shown in fig. 10). Acquired dose rates for θ 0 were corrected to take account of the inverse square effect. Fig. 10: Geometry during angle dependence measurements. For example the angle setting 45 on circular plate means that detector at position A is inclined 45 towards the radiation source at B. 14

16 Dose rate measurements were done at angle settings θ = 0 and 180, in clockwise and counter clockwise direction according to Fig. 9 (negative θ for the counter clockwise direction). The detector angular dependence up and down were investigated as shown in Fig. 11. In this case Ortec Detective EX and Exploranium GR-135 were put on the circular surface with the left side down (measurement geometry refers to Fig. 10). Dose rate measurements at angle settings up and down were done, here (as in fig. 11) refers to θ = 0, 15, 30, 60 and 90 in clockwise direction according fig. 9 and refers to a counter clockwise angle variation. Fig. 11: Measurement at angle settings up and down in reference to the point-like radiation source at B. With the software TableCurve 2D v (Systat Software Inc, USA) curve estimations were done for the angle dependence measurements. From these curves dose rate estimations could be made at angles 15, 30, 45, 345 Input data here was the angles towards the detector with corresponding dose rates. Data collected at each angle measurement was a mean value of 10 measurements sampled 10 s apart. MicroShield 6.22 Simulations MicroShield (Grove Software Inc., USA) calculates the photon fluence rate at a specific point in space by dividing a source distributed in volume or on a surface into kernels, which are then integrated at a specific dose point. Energy fluence is given by multiplying the differential photon fluence rate with the corresponding photon energy. By applying conversion tables from ICRP the program can convert fluence rate into exposure, absorbed dose in air and effective dose equivalent H E (the exposure rate to air is determined using Table 11 of ICRP 51, using the mass energy absorption coefficient for dry air) 5. 15

17 In the simulations the photon energies from 131 I decay were grouped before calculation for timesaving reasons. Photons below 150 kev were included in these calculations. The effective dose equivalents including build up in the air gap and determined for an anterior posterior (AP) geometry are used in this report. 131 I volume source simulations Effective dose equivalent was acquired for dose points at 10 cm, 20 cm, 30 cm to 100 cm from the phantom surface (see fig. 12) that consists of the four volumes head, torso and two legs. The phantom has to be divided in parts due to limitations in the software. A total activity of 40 MBq 131 I was assumed to be dispersed homogenously in a phantom consisting of a water equivalent material. Fig. 12: The Phantom during MicroShied calculations (volume source). Simulations were made separately for the phantom volumes head, torso and one leg (see Fig. 13, 14, and 15). The head was modeled as a sphere with a radius of 10.6 cm (5000 ml), the torso as a cylinder with radius 13.4 cm and length 53.5 cm (30000 ml) and the leg as a cylinder with radius 7.7 cm and length 81.5 cm (15000 ml). The volumes were placed in a coordinate system so that the dose contributions would be known in each case according to Fig. 12. The amounts of 131 I activity in each volume were: head 3.1 MBq, torso 18.5 MBq and one leg 9.2 MBq. 16

18 Fig. 13: The head approximated by a sphere. The dose point is shown as a black dot. Fig. 14: The torso approximated by a cylinder. The dose point is shown as a black dot. 17

19 Fig. 15: One leg approximated by a cylinder. There was a 5 cm water equivalent top clad included (without 131 I) to fill the air space in Y-axis direction between the source and dose point. The dose point is shown as a black dot. Fig. 16 below shows the grouping of photon energies into 14 groups for the case of phantom volume torso. The same grouping of energies was done for all calculations for 131 I within this report. Fig. 16: Energy grouping of 131 I (in this case for the torso). 18

20 131 I surface source simulations Effective dose equivalent was acquired for dose points at 10 cm, 20 cm, 30 cm to 100 cm from the phantom surface (see Fig. 17). For this simulation a total activity of 40 MBq 131 I was distributed over the phantom surface (cylinder mantle). The material under the mantle surface consisted of a water equivalent material and did not contain 131 I. The amount of activity allocated to head, torso and legs were the same as in the volume source situation. Dimensions for the torso and the legs were also the same, but for these simulations the head was approximated by a cylinder with radius 8.5 cm and height 22.0 cm (5000 ml), due to limitations in the software to assign surface activity to a spherical source. As before separate simulations were made for each of the volumes head, torso and one leg (see Fig. 18, 19 and 20). Fig. 17: The Phantom for MicroShied simulations (surface source). 19

21 Fig 18: A cylinder shaped head. There was a 57.7 cm water equivalent top clad included (without 131 I) to fill the air space in Y-axis direction between the source and dose point. The dose point is shown as a black dot. Fig 19: The torso approximated by a cylinder. The dose point is shown as a black dot. 20

22 Fig 20: One leg approximated by a cylinder. There was a 5 cm water equivalent top clad included (without 131 I) to fill the air space in Y-axis direction between the source and dose point. The dose point is shown as a black dot. Comparison between H*(10) values The Detective EX and GR-135 are calibrated to show ambient dose equivalent rate H*(10). The radiation source that was used during these calibrations was 137 Cs and a correction could be made to account for the measurements using 131 I. If the ambient dose equivalent rate is determined by multiplying the air kerma rate by a conversion factor y, which is energy dependent, the relation between H*(10) at two different energies is given by Eq. 6 It has then been assumed that the air kerma rate at the two energies is equal. Fig. B2 in Appendix B gives the H*(10)/K a conversions at different photon energies that are used in eq. 7. Eq. (6) (7) as = 0.94 < 1 the H*(10) shown on detector when measuring the iodine phantom is underestimated. In deriving the factors y 1 and y 2 the only photon energies considered were 364 kev and 662 kev (for 131 I and 137 Cs, respectively). This was also the case when H*(10) was calculated from H E (that was acquired from the MicroShield simulations, but here also conversion factors from table in Fig. B1 was used). (6) 21

23 With the software TableCurve 2D (Systat Sofware Inc, USA) curve estimations were made to get the conversion factors E/K a and H*(10)/K a for photon energies 364 kev and 662 kev (acquired conversion factors for 284 kev, 364 kev, 637 kev, 662 kev and 723 kev are listed in Table B4 in Apendix B). The minimum detectable dose rate MDD The minimum detectable activity MDA can be expressed as [Bq] (8) where is the working expression for the detection limit L D stated by Currie 23. In this expression B is the amount of (pulse) counts from the spectrum measurement (when the background is a smooth varying continuum below the pulse peak). The yield of the gamma photons is f, ε is the absolute counting efficiency for the detector system and t is the counting time. The above MDA equation above is not suitable in this work because the radiation yield is not known; only ambient dose rates were measured, but the dose can be approximated with the equation, [Sv/h] (9) where is the standard deviation for the background dose rate. According to detector specifications the stated uncertainty in dose rate for Detective EX is < (-50 % to 100 %) 12 and the measurement uncertainty for GR-135 is specified within ± 10 % for gamma radiation. To be able to plot MDA for different background dose rates (for illustrative purposes) the standard deviations in Eq. 9 are approximated with two equations ((10) and (11)) given in GUM 4 : Because the stated uncertainty for Detective EX is not symmetrical approximated with is (10) which is the variance for a rectangular distribution with a full width of. Here the lower and upper bounds are written as and, where is the measured dose rate. For the GR-135 the variance is approximated with the triangular distribution 4, (11) here a is the maximum deviation for the stated symmetrical uncertainty interval. 22

24 Measurements of ambient background radiation For assessment of the background radiation levels at possible locations that might get used during screening for contamination purposes the intensimeter RNI 10/SR (RNI AB, Sweden) was used. The instrument is equipped with a GM-tube, calibrated for 137 Cs and measurement uncertainties for displayed dose rates [µsv/h] are specified within ± 20 %. Data for each location is given as a mean value of measurements during 5 minutes. 23

25 [m Sv/h Results Phantom measurements Dose rate measurements with Ortec Detective EX and GR-135 at distances 10 cm, 20 cm, 40cm, 60 cm, 80 cm and 100 cm the from the phantom surface at different angle settings are shown in Fig. 21 and 22 (measurement data are also shown in Table B1 and Table B2 in Appendix B). Dose rate measurements with the Ortec Detective EX at distances 100 cm, 80 cm and 60 cm gave the same values in relation to measurements done at the 0 degree setting. Measurements at distances 40 cm and 20 cm did not show any differences for the 5 degree up setting. But at distances 40 cm, 20 cm and 10 cm there might be a difference between the 0 degree and 5 degree down measurements, because given p-values for the two tailed Student t-test were smaller than 0.05 (calculated p-values for Students t-tests are included in Table B4 in Appendix C). At 10 cm from phantom surface Detective EX did not deliver any good measurement values. For the 5 degree down measurement the values were consistent, but for 0 degree there were four bad readings (4, 8, 4 and 4 µsv/h), which were omitted from the graph. Ortec Detective EX Distance from phantom surface [cm] 5 degree up 0 degree 5 degree down Fig. 21: Detective EX. Dose rates from phantom measurements in standing geometry. The Exploranium GR-135 also gave dose rate values that did not fluctuate at different angle settings. In a similar fashion as above the Student t-test failed to reject the null hypothesis for measurement distances: 100 cm, 80 cm and 60 cm for the 5 degree up setting; 60 cm and 20 cm for the 5 degree down setting. But according to a Student t-test there probably can be a difference for measured dose rates for angle settings 5 degree up and 5 degree down in relation to the 24

26 [m Sv/h] 0 degree setting. Here the null hypothesis could be rejected because p-values were the same as and below 0.05 at the measurement distances: 100 cm 80 cm 40 cm 10 cm for the 5 degree up setting; 40 cm, 20 cm and 10 cm for the 5 degree down setting GR Distance from phantom surface [cm] 5 degree up 0 degree 5 degree down Fig. 22: Exploranium GR-135. Dose rates from phantom measurements in standing geometry. Angular dependence In Fig. 23 and 24 the dose rates at different detector angles toward a point source are shown. When the radiation source was straight ahead of the detector at the distance 1.6 m (Θ = 0 according to fig. 9) Detective EX had the highest measured dose rate 1.03 ± 0.03 m Sv/h. This is not the case with the GR-135 because the 90 degree setting gave the highest response here. The dose response at different angular settings for the Detective EX will be given as a percentage of the maximum dose. And because the corrected angles were approximated to dose rates at 15, 30,.., 345 standard deviations will be omitted for the dose rates at these angles (this is also done for measurement values for the GR-135). Angles 30 and 330 gave 96 % and 95 % of maximum dose rate, both 60 and 300 gave 83 %, 90 and 270 gave 84 % and 83 %, both 120 and 240 gave 81 %. For the GR-135 the dose rates got higher when the angle was increased (this was true up to about ±135 When the radiation source was at a distance of 1.6 m in front of the detector 0.75 ± 0.01 m Sv/h was recorded. The dose rates in relation to this dose for the angles are as follows: 30 and % more; 60 and % and 20 % more; 90 and % and 26 % more, 120 and % and 24 % more (where 90 and 270 gave 1.03 msv/h and 0.94 msv/h). 25

27 Ortec Detective EX , , ,4 0, Fig. 23: Dose rates [msv/h] for angle dependence measurements where numbers 0, 15, 30 up to 345 is the angle settings [degree]. Maximum dose rate 1.03 msv/h is given at 0. GR , , , ,4 0, , Fig. 24: Dose rates [msv/h] for angle dependence measurements where numbers 0, 15, 30 up to 345 is the angle settings [degree]. Maximum dose rate 1.03 m Sv/h is given at 90 (0 gave 0.75 msv/h). The results from the angular measurements up and down are displayed in Fig. 25 and 26 below. These figures show the dose rate as the 133 Ba source is moved in relation to the front of the detector. As previously for Detective EX the highest dose rate 1.02 ± 0.03 msv/h was given with the source at the same height as the detector in 0 setting. Here the dose rates are given in relation to the dose rate given at 0 setting: 94 % and 96 % at 30 and % and 86% at 60 and % and 96 % at 90 and 26

28 Fig. 25: Figure illustrates how the dose rates [msv/h] varied as the source was moved in height in front of the Detective EX. The situation for the Exploranium GR-135 is again different because the setting 0 did not give the peak dose rate, as shown in Fig. 26 (the peak dose rate 0.97 msv/h was at 270 ) ± 0.01 msv/h was given at 0. Dose rates in relation to the 0 measurement value: 13 % and 14 % more at 30 and 330, 20 % and 25 % more at 60 and 300, 26 % and 29 % more at 90 and 270. Fig. 26: Figure illustrates how the dose rates [msv/h] varied as source was moved in height in front of the GR

29 [msv/h] (With build up) MicroShield 6.22 Simulations The MicroShield simulations in which effective dose equivalent rate was acquired delivered lower dose rate values than was measured with both detectors at similar distances (which was expected because the measured quantity was ambient dose equivalent rate). Results for these simulations are shown in Fig. 27. The dose point at distance 100 cm from the phantom surface gave 1.38 msv/h and the figure shows that that the dose rate gradient is bigger as the distance gets shorter. The dose point nearest to the phantom gave msv/h. Fig. 27 also shows a comparison with the inverse square law for a point source Distance from phantom surface [cm] Dose point Point source Fig. 27: Effective Dose Equivalent Rates [msv/h] at different distances from the phantom surface. The whole phantom contains 131 I and acts therefore as a volume source. A comparison with the inverse square law for a point source is also made. Fig. 28 shows the resulting dose rates from the simulations in which the radionuclides were located on the phantom surface. For comparison the dose rates for the volume source (Fig. 27) were included. The dose rates in the case with the volume source and the surface source were very similar at distances beyond 50 cm. But below 50 cm distance from the surface the difference between volume source and surface source begins to increase (for 50 cm distance the dose rates for volume and surface source were 3.62 msv/h and 3.65 msv/h). Fig. 29 shows the resulting data when 131 I is located on the surface parts of the phantom. For the phantom that consists of the parts head, torso and legs the contributions were plotted for the case when only the surface of head volume contained the radionuclide. At the dose point at 100 cm the effective dose equivalent rate is 94 % lower when only the head contributes. This contribution got even lower when the distance decreased. At 10 cm and 20 cm distances the contributions were lowered with 99 % and 98 %. 28

30 [msv/h] (With build up) [m Sv/h] (With build up] MicroShield simulation Effective Dose Equivalent Rate (ICRP ) Surface source Volume source Distance from phantom surface [cm] Fig. 28: Effective Dose Equivalent Rates [msv/h] at different distances for two phantoms. Both these phantoms contain the same amount of 131 I activity. In the case of surface source the activity is located on the phantom surface, i.e. on the cylinder mantle surfaces that the phantom consists of. The volume source is the case when the phantom has the radionuclide evenly distributed in its volume. MicroShield simulation Effective Dose Equivalent Rate (ICRP ) ,1 Whole body Only head 0, Distance from phantom surface [cm] Fig. 29: Effective Dose Equivalent Rate [msv/h] is shown at dose points at specific distances from a phantom acting as a 131 I surface source. The whole body case includes dose contribution from all different parts of the phantom (head, torso and two legs). The dose contribution from only the head surface is shown in the second case. Comparison of measured and calculated H*(10) The calculated ambient dose equivalent (H*(10) ref in Fig. 30) was lower than the measured H*(10) at all distances. The differences between the calculated H*(10) and the measured with Detective EX was 3.03 msv/h at 10 cm and 0.94 msv/h at 100 cm. This difference is also shown in Fig. 31. From this figure it seems that the Detective EX dropped in dose rate response at shorter distances than 20 cm, in relation to the H*(10) ref, a value that was derived from the 29

31 Difference [msv/h] [msv/h] MicroShield simulation for the volume source case. The relative signal loss got even larger with the GR-135. Here the measured signal at 10 cm distance was below the H(10)* ref H*(10) ref Ortec GR Distance from phantom surface [cm] Fig. 30: The difference between calculated H*(10) ref and measured H*(10) with the Detective EX and the GR-137. H*(10) ref was calculated from H E acquired from MicroShield simulations. The measured dose rates were corrected according to Eq. 7. A deviation for the dose rate H*(10) is approximated from the fact that the 40 MBq I-131 activity in the measurement situation has an uncertainty of about ± 5 % H*ref-Ortec H*ref-GR Fig. 31: The calculated H*(10) ref minus the measured H*(10). H*(10) ref was calculated from H E acquired from MicroShield simulations. The measured dose rates were corrected according to Eq. 7. Fig 32 below shows that the dose rate at 1 m distance from a point source is almost the same as the measured dose rates at a distance of 1 m from the phantom surface (2.55 msv/h for the point source, and 2.59 msv/h and 2.17 msv/h for Detective EX and GR-135, respectively). 30

32 [msv/h] The point dose rate scattered radiation, was calculated by Eq. (C2) without any contribution from [Sv] (12) In the above expression, the factor 1.28 is used to convert Gy into Sv when the source nuclide is 131 I. This conversion factor is derived for the photon energy 364 kev. Table B4 in Appendix B have H*(10)/K a values for photon energies 284 kev, 364 kev, 637 kev, 662 kev and 723 kev. When also accounting to these other higher 131 I decay probabilities the H*(10)/K a factor becomes Point source 10 Ortec GR Distance from phantom surface [cm] Fig. 32: The ambient dose rate [msv] from a point source (40 MBq 131 I) situated on the phantom surface compared against the measured dose rates. The measured dose rates were corrected according to Eq. 7. The minimum detectable dose rate MDD The results from background measurements with the intensimeter RNI 10/SR are listed in Table 1. The values for the minimum detectable dose rates MDD are also shown. The dose rates in this table are mean values of 5 minute measurements and therefore the standard deviation was calculated with Eq. (11). The RNI 10/SR gave a slightly higher dose rate reading for the background in the room where the phantom measurements were done. In this room the RNI 10/SR gave 0.08 msv/h, whilst the Detective EX and GR-135 gave 0.06 ± msv/h and 0.07 ± msv/h. In both these cases the background standard deviation SD was calculated from 20 dose rate measurements. The MDD values for Detective EX and GR-135 were calculated with (9) to be 0.04 msv/h and 0.01 msv/h, respectively. Here the applied SD was derived from 20 measurements. MDD values listed in Table 1 for the RNI 10/SR was instead derived with SD values that was approximated with (11). 31

33 MDD [msv/h] Table 1: Background measurements with the intensimeter RNI 10/SR. Minimum detectable dose rates MDD listed are only applicable when measurements are done with the specific measurement device. MDD in this table was calculated with Eq. (9) and (11). Dose rate [msv/h] MDD [msv/h] Whole Body Laboratory Corridor Waiting room In the room with the whole body counter Sahlgrenska University Hospital Emergency waiting room Ambulance hall (outside) Fig. 33 below illustrates the variation of the minimum detectable dose rate MDD with the background dose rate and its standard deviation. As the background dose rate increases a higher measured signal is needed for a safer detection. If the measured dose rate is lower than MDD there is a big chance that the measured signal only is due to fluctuations in the instrument reading of the background level of ionizing radiation. 0,70 0,60 0,50 0,40 0,30 0,20 Ortec GR-135 0,10 0,00 0,05 0,15 0,25 0,35 0,45-0,10 Background dose rate [msv/h] Fig. 33: The minimum detectable dose rate MDD for varying background dose rates. The standard deviation for each detector was estimated with (10) and (11). When a disc shaped surface is contaminated with 137 Cs, what source activity is required for both detectors to give MDD at distances 0.5 m 0.8 m and 1m? A disc shaped surface source with radius R 0 can be approximated as a point source if the perpendicular distance r between surface source and the measurement point is more than 2.2*R 16 0 (see Appendix C). In this case the dose rate from the disc source is given with Eq. (2), and without considering the scattered radiation this gives an expression for the source activity A, 32

34 [Bq] (13) where r is the distance [m] from the point source, E is the photon energy 662 kev, n is the radiation yield and is the mass energy absorption coefficient in air at the photon energy 662 kev, 3.0*10-3 m 2 kg -1. The required 137 Cs activity for a circular surface contamination is shown in Table 2 for measurement distances r 0.5 m, 0.8 m and 1 m. The circular surface area R 0 is assumed to follow the relation R 0 < r/2.2 (so that the dose rate from activity on a circular surface can be approximated as a point source, as shown in Appendix C). According to these calculations GR-135 can detect a smaller source activity at a similar distance. Table 2: 137 Cs activities [Bq] that fulfill MDA:s at different measurement distances. The MDD values for each detectors were: Ortec EX 0.04 msv/h and GR msv/h. Distance r [m] Ortec Detective EX [Bq] GR-135 [Bq] In Table 3 dose rates are calculated with MicroShield from activities in Table 2. This was done to check the consistency of calculated values with (19) against MicroShield simulations for a point source at similar distances. Equation 19 seems to give the same result as the MicroShield calculations for this simple geometry. Table 3: Dose rates [msv/h] from a point source Cs-137 at distances 0.5, 0.8 and 1 m. Dose rates are calculated with MicroShield Build up is included. Distance r [m] MBq MBq MBq MBq MBq

35 Discussion The Ortec Detective EX gave a higher dose rate at similar distances in the phantom measurements. This difference decreased when the distance was increased. When the angular dependence was investigated it was quite clear that the GR-135 did not show the same angular response as the Detective EX, which gave a maximal signal when the source was in front of the detector. The GR-135 has a GM tube in front of the scintillator inside the detector housing, which attenuates the photons passing from the front side of the detector into the scintillator (more than when photons pass from the side into the detector material). The reason why the GR-135 got a larger relative dose response at greater distances in the phantom measurements depends on the geometry of the phantom. When the detector is close to the phantom photons from more distal parts of the phantom will be more attenuated when passing the bottles filled with water. As the measurement distance is increased this effect will be reduced, as the photons to a larger extent now enter the detector through the sides of the GR-135, resulting in an increased relative dose response. During the phantom measurements the detector angle was varied with 5 degrees in two directions (towards the phantom head and legs). This was done to investigate the difference in measured dose rate when the detector was not exactly directed against the whole body phantom. Only in a few cases there were some small differences, but mostly the readings coincided. This indicates that the instruments are not so sensitive to small angle variations, which easily could be the case when measuring with handheld instruments. However, at distances 10 cm, 20 cm and 30 cm the dose rates decreased slightly with the Detective EX in the 5 degree down setting (the Student t-test for the 10 cm case was positive even though the 0 degree measurement only had six good measurement values). The reason is probably that the detector head in this position was rotated against the phantom legs, a part of the phantom with a smaller thickness - and therefore the distance from the phantom surface to the detector was larger than the distance from phantom surface at the torso to the detector. For the GR-135 the response variation was more inconsistent or somewhat inconclusive. At 10 cm distance the 5 degree down setting gave a smaller response (0.3 msv/h less), but the 5 degree up setting gave o higher response (0.1 msv/h more). At 20 cm distance the 5 degree down setting gave a smaller response (0.1 msv/h less). At the other measurement distances there were not any significant differences in detector response when other angle settings than 0 degree was used (the null hypothesis for the two tailed Student t-test was rejected at these measurement distances as well, but since these differences are very small (around 0.03 msv/h) it is doubtful if they are meaningful in a real world situation. In the angle dependence measurements with the 133 Ba as a point 34

36 source the measured dose rates got higher when the angle of incidence towards detector front was increased. With this in mind it seems little inconclusive that the dose rate decreased when more of the detector side was directed against the higher torso part of the phantom. One explanation could be that the slightly larger signal at 5 degree up might depend on a larger effective phantom surface area that the detector side sees (the leg part of the phantom consists of 6 bottles giving a surface area of 0.26 m 2, while the 6 bottles in the torso part has an surface are of 0.19 m 2 ). The MicroShield simulations were done for two cases, one in which the radiation source, 131 I, was assumed to be evenly disposed in the phantom material and the other in which the same activity was located on the phantom surface. Here the effective dose equivalent rate H E was very similar in both cases. But for a smaller dose point distance the H E got slightly larger for the surface source case. The reasons are geometrical where a less part of photons get attenuated in the surface source case. There was no specific information about uncertainties for the MicroShield calculations, but it was mentioned that build up and scattering effect are the greatest source of uncertainty. 5 MicroShield simulations and phantom measurements at 10 cm distance showed a dose rate and distance dependence that was not the same as if the dose rate was caused by a point source. The dose rate decreased more slowly when the source was distributed in the phantoms. Comparisons of the calculated H*(10) and measured H*(10) regarding the decrease of the dose rate with increasing distance showed a good agreement from distance 20 cm to 100 cm with the Detective EX and from 40 cm to 100 cm with the GR-135. The phantoms in both cases had the same weight and about the same length, but differences in geometrical properties were inevitable, because other building blocks than the 5 liter bottles was not available during the construction of the phantom. At 10 cm distance from the phantom surface the Detective EX showed a rather abrupt drop in the measured signal compared to the calculated H*(10). This drop may be caused by disturbances when detector is reaching its dose rate limit (20 msv/h for the HPGe detector). Four out of ten readings were omitted from the 0 degree measurement, because these dose rates were inconsistent by showing a much lower dose rate than expected. The built in Geiger Müller tube automatically starts measure the signal when the dose rate gets above 20 msv/h. The drop in response in relation to the calculated H*(10) with the GR-135 near the phantom surface could be caused by spill over losses in the NaI scintillator material. The larger deviation in dose response at smaller distances, compared to calculated ambient dose equivalent rates, can be caused from the fact that small bias in distance measurement results in a larger effect at shorter distances. 35

37 In converting the effective dose equivalent rate H E, given from MicroShield simulations, conversions factors from ICRP 74 were used. These conversion factors are for radiological protection purposes and therefore the uncertainties involved may be too large to make any quantitative assumptions about the accuracy of the calculated H*(10). The minimum detectable activity MDD for Detective EX and GR-135 was derived as tree times the standard deviation for the background dose rate. This approximation gives the chance to detect a signal or dose rate, other than the background, within 95 % confidence interval. For the phantom measurement situation the GR-135 had a lower MDD, this because there was a smaller variability for the 20 measurements each background data consisted of. 36

38 Conclusion The aim of this work was to investigate the possibility to measure gamma radiation emitted from a phantom in standing geometry, a situation that might occur before and after decontamination. The results in this work do not clearly show which level of accuracy that can be expected in these kinds of measurements, when the Ortec Detective EX and GR-135 are used as intensimeters. The stated uncertainties for the dose rate measurements are high for both detectors, which makes it hard to compare them against the MicroShield simulations. The MDD values for Detective EX and GR-135were calculated to 0.04 msv/h and 0.01 msv/h, respectively. The Ortec Detective EX seems to give a more conservative estimation of the dose rate because the measured ambient dose equivalent rate H*(10) overestimated the effective dose equivalent rate H E at all measurement distances. 37

39 References 1. Glenn F. Knoll, Radiation Detection and Measurements, Third Edition, John Wiley & Sons inc. USA, Michael F. L Annunziata, Handbook of Radioactivity Analysis, Third Edition, Academic Press, Burlington, Frank H. Attix, Introduction to Radiological Physics and Radiation Dosimetry, John Wiley & Sons inc JCGM 100:2008, GUM 1995 with minor corrections, Evaluation of measurement data Guide to the expression of uncertainty in measurement, First edition 2008, Corrected version MicroShield User s Manual v.6.20, Grove Software Inc Radioisotopes - Applications in Physical Sciences, Environmental Dosimetry - Measurements and Calculations pp , Mats Isaksson,, ISBN , InTech, ICRP Publication 74: Conversion Coefficients for use in Radiological Protection against External Radiation, Ann. ICRP 26 (3-4), IAEA Safety Reports Series No. 16, Calibration Of Radiation Protection Monitoring Instruments, Austria, Sahlgrenska Universitetssjukhustets Katastrofplan, Västra Götalandsregionen Sahlgrenska Universitetssjukhuset, Version Idé och mål för utveckling inom CBRNE-området i Västra Götalands län, En sammanställning av länets resurser, Rapport 2012:08, Länsstyrelsen Västra Götaland, Kungälv, Nuclide Safety Data Sheet, Iodine-131, Detective-EX-100T and Detective-DX-100T HPGe-based Hand-Held Radioisotope Identifiers ( Detective-EX-100T-Detective-DX-100T- HPGE-Hand-Held Radioisotope-Identifiers.pdf ),

40 13. LBNL Isotopes Project LUNDS Universitet Nuclear Data Dissemination Home Page, Table of Radioactive Isotopes, isotopes.lbl.gov/toi.html, ICRU Report 51: Quantities and Units in Radiation Protection Dosimetry, International Commission on radiation units and measurements, USA, September Radiation Measurements 38 (2004), pp , Gamma ray contribution to the ambient dose rate in the city of São Paulo, E.M. Yoshimura, S.M. Otsubo, R.E.R. Oliveira,, Elsevier Ltd., Brazil, Kenneth R. Kase, Walter R. Nelson, Concepts Of Radiation Dosimetry, Prepared for the U.S Atomic Energy Commission under contract No AT(04-3) 515, Slac-153, Stanford University, USA, Ortec Detective-EX Series Portable Neutron and Gamma Nuclide Indentifiers Administrator s manual, Ortec part No Manual Revision K, USA, ICRP Publication 103: The 2007 Recommendations of the International Commission on Radiological Protection, Ann. ICRP 37 (2-4). ICRP, Technical reports Series No. 277, Absorbed Dose Determination in Photon and Electron Beams An international Code of Practice, Second Edition, International Atomic Agency, Vienna, ICRP Publication 26: Recommendations of the ICRP, Ann. ICRP 1 (3). ICRP, ICRP Publication 60: Recommendations of the International Commission on Radiological Protection. Ann. ICRP 21 (1-3). ICRP, International union of pure and applied chemistry, Analytical Chemistry division commission on radiochemistry and nuclear techniques, Determination Of Very Low Levels Of Radioactivity (Technical Report), Great Britain, Lloyd A. Currie, Limits for Quanlitative Detectionand Quantitative Determination Application to Radiochemistry, Analytical Chemistry Division, National Bureau of Standards, Washington DC

41 Appendix A The angular dependence measurements were corrected for the true angle that the detector sees at a specific angle setting on the rotating disk. Distance corrections for dose rates at angle settings were also made. Fig. A1 shows the relation of these calculations. The resulting dose rates at calculated angles are shown in Tables A1 to A4. Fig. A1: Relations for the distance and angle correction calculations, where cosines, sinus and tangents theorems were applied. A point source is located at B and the detector head is at A in a circular path (the case is shown when the detector head is rotating to the right). Table A1: Dose rates [µsv/h] for angle dependence measurements given for Ortec Detective EX and GR-135. Resulting angles [degrees] and dose rates are shown after corrections (Deg. corr, Right corr. and Left corr). Background is included in uncorrected measurements in table, but the corrected data was calculated from measurement data without background. (Measured background with Ortec Detective and GR-135 was 0.10 µsv/h (±0.01 µsv/h) and 0.12 µsv/h (±0.01 µsv/h) (the background is the same for table A3)). Degree Ortec Right 1,13 1,11 1,04 0,84 0,81 0,78 0,67 0,43 0,21 Left 1,10 1,03 0,84 0,80 0,76 0,67 0,40 Deg. corr (net) Right corr. 1,03 1,02 0,97 0,84 0,86 0,87 0,82 0,53 0,22 (net) Left corr. 1,01 0,97 0,84 0,85 0,85 0,82 0,49 GR135 Degree Right 0,87 0,91 0,94 0,97 0,95 0,91 0,80 0,56 0,31 Left 0,91 0,93 0,92 0,89 0,86 0,75 0,56 Deg. corr (net) Right corr. 0,75 0,80 0,85 0,98 1,01 1,02 0,98 0,70 0,75 (net) Left corr. 0,80 0,85 0,91 0,94 0,95 0,91 0,70 Table A2: Calculated standard deviations for dose rates in Table A1. sd Degree Ortec Right 0,03 0,04 0,04 0,03 0,03 0,03 0,03 0,03 0,01 Left 0,03 0,02 0,03 0,03 0,03 0,03 0,02 Deg. corr (net) Right corr. 0,03 0,04 0,04 0,04 0,04 0,04 0,04 0,04 0,03 40

42 (net) Left corr. 0,03 0,03 0,03 0,03 0,04 0,04 0,03 GR135 Degree Right 0,01 0,01 0,01 0,01 0,01 0,01 0,01 0,01 0,01 Left 0,01 0,01 0,01 0,01 0,01 0,01 0,01 Deg. corr, (net) Right corr. 0,01 0,01 0,01 0,01 0,01 0,01 0,01 0,01 0,01 (net) Left corr. 0,02 0,01 0,01 0,01 0,01 0,01 0,01 Table A3: Dose rates [µsv/h] for angle dependence measurements are given for Ortec Detective EX and GR-135. Resulting angles [degree] and dose rates are shown after corrections (Deg. corr, Up corr. and Down corr). Background is included in uncorrected measurements in the table, but the corrected data was calculated from measurement data without background. Degree Ortec Up 1,12 1,05 1,00 0,86 0,49 Down 1,10 1,08 1,01 0,84 0,74 Deg.corr (net) Up corr. 1,02 0,96 0,93 0,86 0,51 (net) Down corr. 1,01 0,99 0,95 0,84 0,82 GR135 Degree Up 0,88 0,91 0,94 0,91 0,86 Down 0,92 0,95 0,95 0,87 Deg.corr (net) Up corr. 0,76 0,80 0,86 0,91 0,96 (net) Down corr. 0,81 0,86 0,95 0,97 0,81 Table A4: Calculated standard deviations for dose rates in table A3. SD Degree Ortec Up 0,03 0,02 0,04 0,04 0,02 Down 0,04 0,04 0,02 0,02 0,03 Deg.corr (net) Up corr. 0,03 0,03 0,04 0,05 0,03 (net) Down corr. 0,04 0,04 0,03 0,03 0,04 GR135 Degree Up 0,01 0,01 0,01 0,01 0,01 Down 0,01 0,01 0,01 0,01 Deg.corr (net) Up corr. 0,01 0,01 0,01 0,01 0,01 (net) Down corr. 0,01 0,01 0,01 0,01 0,01 The software TableCurve 2D v was used for curves estimations (input data is shown in table A5). Table A5: Input data used for the TableCurve 2D calculations. Ortec Detective EX (Angle dep. detector rotating in clockwise direction) Degree [µsv/h] 1,0325 1,0229 0,9750 0,8422 0,8555 0,8710 0,8163 0,5257 0,2152 Degree [µsv/h] 0,4905 0,8163 0,8522 0,8507 0,8444 0,9678 1,0148 1,0325 GR-135 (Angle dep. detector rotating in clockwise direction) Degree

43 [µsv/h] 0,7513 0,8000 0,8546 0,9760 1,0093 1,0230 0,9772 0,7029 0,3461 Degree [µsv/h] 0,6956 0,9072 0,9519 0,9359 0,9130 0,8463 0,8040 0,7513 Ortec Detective EX (Angle dependence up and down) Degree (up) [µsv/h] 1,0175 0,9583 0,9326 0,8635 0,5115 Degree (down) [µsv/h] 1,0050 0,9936 0,9492 0,8444 0,8198 GR-135 (Angle dependence up and down) Degree (up) [µsv/h] 0,7583 0,8030 0,8577 0,9051 0,9556 Degree (down) [µsv/h] 0,7583 0,8101 0,8649 0,9456 0,9668 Fig. A2: Plot of input data from table A5 for Ortec Detective EX angle measurements. Fig. A3: Data shown in figure A2 after Spline Interpolation. 42

44 Fig. A4: Resulting curves fitting of table A3 data for Ortec Detective EX. Fig. A5: Plot of input data from table A5 for GR-135 angle measurements. Fig. A6: Data shown in figure A5 after Spline Interpolation. 43

45 Fig. A7: Resulting curves fitting of table A6 data for GR-135 Fig. A8: Curves fitting of table A5 data for Ortec Detective EX (degree up). Fig. A9: Curves fitting of table A5 data for Ortec Detective EX (degree down). 44

46 Fig. A10: Curves fitting of table A5 data for GR-135 (degree up). Fig. A11: Curves fitting of table A5 data for GR-135 (degree down). 45

47 Appendix B Fig. B1: Table A.17 from ICRP 74 7 Fig. B2: Table A.21, from ICRP