Localization of radioactive fragments in the human body using surgical detectors

Transcription

1 UNIVERSITY OF GOTHENBURG Master of Science Thesis Localization of radioactive fragments in the human body using surgical detectors Afrah Mamour SUPERVISOR: Mats Isaksson Assoc. Prof. DEPARTMENT OF RADIATION PHYSICS 2010

2 Abstract Intraoperative probes have been employed to aid in the detection and removal of tumors for more than 50 years. For a period of about 40 years, fundamentally every detector type that could be invented had been tested or at least suggested for use as an intraoperative probe. These detectors included such as scintillation detectors (NaI(Tl)) and semiconductor detectors (CdTe, CdZnTe and HgI 2 ). Intraoperative probes are now established as clinical devices for example these have been utilized in breast cancer, where surgeons should locate the first lymph node (sentinel lymph node, SLN). The technology can also be used to locate radioactive fragments that have penetrated the skin. For this reason one must be aware of the detector properties if a survey should be necessary in the case if contaminated patients found in the hospital. It is very important to remove the radioactive fragments as soon as possible. Contamination could be by radioactive fragments which emit radiation with high energies, i.e. these haven t similar energy such as radiopharmaceuticals utilized at surgery, and then these must be tested. Several moments was performed in this work to study properties of the detector such as efficiency, angular sensitivity and line spread function and a computer simulation was also carried out to estimate radiation dose rates to those who will work at the detector, if the patient is contaminated. The localization of radioactive sources was also executed in the cavities in the phantom to study the localization. 2

3 Table of contents ABSTRACT INTRODUCTION SENTINEL LYMPH NODE MICROSHIELD THEORY DETECTOR DEFINITION STATISTICS MATERIAL AND METHOD MICROSHIELD SIMULATION EXPRIMENTAL EFFICIENCY ANGULAR SENSITIVITY PHANTOM LINE PROFILE RESULTS SIMULATION EXPRIMENTAL EFFICIENCY ANGULAR SENSITIVITY PHANTOM LILNE PROFILE DISCUSSION SIMULATION EFFICIENCY ANGULAR SENSITIVITY PHANTOM LINE PROFILE STATISTICS DETECTOR CONCLUSIONS ACKNOWLEGEMENT Appendix Appendix Appendix

4 Appendix Appendix References

5 1. Introduction When an accident in which radioactive material is involved, there must be a rapid procedure that determines whether people are contaminated or not. At a first stage to perform a contamination measurement on all involved, a radiation protection instrument is required. Because the presence of radiation is not detectable by human senses, it is notable to check the operability of any instrument before its use. At many hospitals today, small detectors are used in conjunction with surgical procedures to detect the uptake of radionuclides. One example is breast cancer, where surgeons should locate the first lymph node (sentinel lymph node, SLN). The technology can also be used to locate radioactive fragments that have penetrated the skin. If the fragments have high activity, it is necessary that they can be removed as quickly as possible. At localization and removal of radioactive fragments existing equipment should be used at the current hospital by the people who normally use such equipment. The proposed method of application of the National Radiological Protection preparedness is primarily to provide a compilation of available resources. The purpose of this work was to quantify the characteristics of the detector (efficiency, spatial resolution, angular sensitivity, etc.) in the detection of different radionuclides with different energies. Additionally, the microshield simulation was performed in this work to estimate DDER (Deep dose equivalent rate, [µsv h -1 ]). The aim of this was to illustrate how much radiation dose is for those who will work at the detector, if the patient is contaminated. The fact is that the damage will be taken care of first before those start with decontamination. 1.1 Sentinel Lymph Node The concept was proposed by Cabanas in 1977 [1]. The first lymph node to receive lymphatic drainage from a tumor site is the sentinel node, and if there has been lymphatic spread, the sentinel node is the first node to have metastatic involvement. Further, the concept implies that sampling the sentinel node is sufficient for assessing a lymphatic bed. The sentinel node concept applies to the spread of several types of cancer including breast cancer and melanoma. It was first applied in the management of cancer by Cabanas in 1977 and it was first applied in the management of melanoma in 1992 by Morton et al. [2]. Thurston et al [3] reported the basic criteria for the choice of a radionuclide to be used for lymphoscintigraphy. The 99m Tc labeled nanocolloid particles are pre-operatively injected around the primary tumor with a specific activity. Due to the size of the molecules, 99m Tc labeled nanocolloid can be transported only to the node, the first or sentinel node, which will demonstrate the highest node activity in the axilla. Several hours to one day after injection, allowing for the lymphatic transport of the nanocolloid, the patient undergoes surgery, followed by histological examination of the sentinel node. The role of the surgical gamma probe is to localize the sentinel node intra-operatively. 5

6 1.2 MicroShield This is a broad gamma ray shielding and dose estimation program that is commonly used for designing shields, estimating source strength from radiation measurements, minimizing exposure to people, and teaching shielding principles. It is fully interactive and utilizes wide input error checking. Integrated tools provide graphing of results; material and source file creation, source inference with decay, projection of exposure rate versus time as a result of decay, access to material and nuclide data, and decay heat calculations. In some cases, such as the calculation of absorbed dose, additional tools may be needed to complete an assessment [4]. 2. Theory 2.1 Detector In general all semiconductor detectors for ionizing radiation will, through some interaction in the detector volume, create a charge pulse that can be detected. This charge pulse consists of electrons and holes, which are separated under the influence of an applied electric field, and the current is detected by an external circuit. Therefore, in view of detector operation the factors of concern that must be concerned is the nature of the interaction between the incident radiation and the volume of the detector material where the charge is created, the efficiency of the excitation process, the efficiency of the charge collection process, the external circuit that detects the charge pulse that has been created, and finally the background noise of the device. In a semiconductor material the incident radiation can create a very large number of electronhole pairs since the energy necessary to produce one electron-hole pair is about 3 to 6 ev, depending on the bandgap and other properties of the material being employed. The electronhole pairs are created either directly, as might be the case if the incident radiation is a charged particle such as an electron, or indirectly where the incident radiation undergoes any number of the processes and secondary particles produced lose their energy through the production of electron-hole pairs (see Fig 1). The relatively small energy required to produce electron- hole pairs in semiconductors and the high quantum efficiencies are two advantages of semiconductor nuclear detectors [5]. 6

7 Fig 1. Basic design and operating principle of a semiconductor detector Cadmium telluride (CdTe) is a type of semiconductor detector which combines high atomic numbers, Cd and Te have atomic numbers of 48 and 52, respectively, with large enough bandgap energy (1.52 ev) to permit room temperature operation. The probability of photoelectric absorption per unit pathlenght is approximately a factor of 4 to 5 times higher in CdTe than in for example Ge for typical gamma-ray energies. Due to difficulties in charge carrier transport, most CdTe detectors are generally small and are usually planar in design. Energy resolution is best for low-energy gamma-ray and X-ray irradiation on the cathode side. Although CdTe detectors can be generally efficient for low-energy gamma rays, suitable detection efficiency for high-energy gamma rays requires larger volumes, which unfortunately translates into poor spectroscopic performance [6]. 7

8 2.2 Definitions Deep dose equivalent (DDE) which applies to external whole body exposure means the dose equivalent at a tissue depth of 1 centimeter (1000 mg/cm 2 ), also called personnel dose equivalent at a depth of 1 cm. The unit for deep dose equivalent is J kg -1 or Sievert (Sv) [12]. Efficiency is fraction of emitted radiation detected by the probe (cps/ kbq). The efficiency of the detector system, which depends in turn on many material and geometrical factors, will determine how well the system uses the information in the available radiation. The counting sensitivity of detectors varies, depending on size of the detector, energy of incident gamma radiation, source-to-detector distance, source radial position and thickness of the semiconductor. With increasing distance between source and detector, the efficiency will decrease (Inverse square law, the rule that states that the intensity of radiation from a source decreases as 1/d 2 from the source in a nonabsorbent medium, where d is the distance from the source). Efficiency has two separate components such as geometric efficiency and intrinsic efficiency. Geometric efficiency is the fraction of emitted radiations that intersect the detector, which is the fraction of the total solid angle subtended by the detector. It is directly proportional to the radiation-sensitive detector area and, for a point source, inversely proportional to the square of the source-detector distance. Intrinsic efficiency, or efficiency, is the fraction of radiations intersecting the detector that is stopped within the detector [11]. Angular sensitivity, the probe should be sensitive to the photons in a limited solid angle. This ensures good spatial resolution for the localization of the radionuclide. Spatial resolution (FWHM, Full Width at Half Maximum) is the capacity of the detector to determine correctly the location of a source. Spatial resolution is related to the material, size and lateral shielding of the detector. FWHM illustrated in the fig 2 and is defined as the width of the spreading at a level that is just half the maximum ordinate of the peak. 8

9 Fig 2. Schematic determination of FWHM [7] 3. Statistics Radioactive decay is a random process and, therefore, random fluctuations will occur in the measured counts or counts rates arising from decay of radioactivity. For this reason, if an intra operative probe were used to repeatedly measure the counts or count rate from a given activity of radionuclides, a dissimilar value would be obtained for each measurement. Such random fluctuations complicate the true detection and measurement of radioactivity. If a measurement is made on a radioactive sample and N counts is obtained, the standard deviation (s) of the number of counts is the square root of the number of counts s = N (1) The standard deviation was calculated for tow centering of sources in the front of the detector, also mean value calculated for tow centering. Uncertainty in the mean value of two different sources was estimated according to equation (2); 9

10 s = ( s 1 m 1 ) 2 + ( s 2 m 2 ) 2 (2) Were s 1 and m 1 are standard deviation and mean value for the first measuring series, s 2 and m 1 are standard deviation and mean value for the second measuring series. The ratio m 1/ m 2 are the relation between mean values of two different sources. m 2 m 1 ± s Uncertainty in the Reproducibility m1 ± s1 m2 ± s2 4. Materials and methods 4.1 MicroShield simulation The simulation begun with choosing appropriate nuclides in this situation, 60 Co, 137 Cs, 192 Ir, 131 I and 241 Am were used, since they are industrial radionuclides and medical. Simulation was carried out in MicroShield (Grove Software, Inc, Lynchburg-USA, version 6.20) program to calculate Deep Dose Equivalents Rate (DDER). To study the attenuation in the tissue, the simulation was performed in water between the point sources and measuring point in MicroShield for different sources such as 60 Co, 137 Cs, 131 I and 192 Ir between 2 and 15 cm depth with 1 MBq activity, to obtain DDER. 10

11 To simulate an adult human, a three section cylindrical phantom was used that was similar to an adult human phantom (in 1969 Snyder et al [8] developed the first heterogeneous model to represent the adult male). The phantom s head, torso, and leg sections had a cylindrical form which is shown in fig. 3, furthermore was simulated a child phantom with the same parts as an adult phantom, but the length of 140 cm, which was taken from PCXMC program [13], a 10- year-old child was selected and the lengths of each element was scaled in proportion to the adult phantom (fig 3). Fig 3. An adult phantom which represents an adult human in order to calculate DDER in MicroShield [8] A unit surface concentration of 1MBq m -2 simulated for the whole phantom i.e. a uniform distribution of activity was used to calculate DDER from each part. The total DDER was estimated a sum of the three parts. This was simulated for 20 cm and 100 cm distance from the body in the middle of the large phantom (at mid torso) Fig 4. A child phantom which represents a child in order to calculate DDER in MicroShield. 11

12 4.2 Experimental Efficiency The efficiency was measured 1cm in front of the detector and the sources used were the nuclides which have only one major gamma energy peak. Nuclides with different energies were used in order to see how the efficiency changes at a constant depth with varying energy. It is also important when gamma probe detection is performed with more than one radionuclide having different energies. The detector, placed in the holder. Table 1 shows the radionuclides which were utilized in the case of the efficiency. Table 1. Physical properties of the radionuclides which were used to determine efficiency Radionuclide Half-life Type of emission Energy (kev) I γ (%) 60 Co 5.27 y γ rays 1173, , Cs y γ rays I 8.00 d γ rays 364, , m Tc 6.00 h γ rays 140, , Co d γ rays 122, , Mn d γ rays Zn d γ rays Cd d γ rays The CdTe detector (Eurorad SA, Eckbolsheim France, issued in Paris on April 24th, 2009) which was used has been designed for low to mid energy radiation detection ( 125 I, 99m Tc, etc ). The detector and the preamplifier are mounted in a probe of 11mm (detection head) and the collimator is a part of the probe. 12

13 The small probe (SOE311) was used which has a short angled probe (30 º ) to permit easier contact to certain tissues (fig 5). It was connected to the read-out module through a 3.5 m flexible cable [9]. Europrobe electronic system allows the visualization of the counting rates or counts on a digital display (fig 6). Fig 5. CdTe probe (SOE311) which was utilized in this case [10]. Table 2. Characteristics of the CdTe gamma probe CdTe probe Length 175 mm Diameter 11 mm (detection head) Tip angle 30 Shielding High Energy Tungsten Collimation Internal (optional: additional external collimation) Energy range 20 kev to 364 kev Weight 140 g 13

14 Fig 6. Europrobe electronic system which is a gamma detection device for radioguided surgery[10]. For each measurement, the activity of each source was decay corrected. The nuclides used were 60 Co, 137 Cs, 54 Mn, 65 Zn, 99m Tc, 131 I, 109 Cd and 57 Co (Table 1). The detector had two own enengy windows for 99m Tc and 125 I. The own energy window for measurement of 99m Tc was used and the total energy window was used for the rest. Then sources were placed in front of the detector and 21 readings were recorded. Then the detector was moved and centered again and 21 new readings were recorded. Because of technical limitations the sources could only be measured for 50 seconds, therefore, it was measured 21 times, and the individual readings were summed (see Appendix 1). The next step was to assess the efficiency of the sources used. This efficiency is expressed as the count rate (cps) per kbq according to equation (3); ε = C A t (3) Were A is the activity (kbq), I γ is the gamma intensity, t is the measurement time (s) and C is the number of counts. ε = cps kbq The efficiency expressed in percent according to equation (4); 14

15 ε % = C Iγ A t 100 (4) Were I γ is the gamma intensity. To examine if the measured efficiency of the detector is similar compared to the manual of the detector, the intrinsic efficiency was calculated by using the measured efficiency of such as 57 Co. It was appropriate to compare with the manual description due to this has a low energy. The intrinsic efficiency (ε i ) was calculated by the head area of the detector and the distance between the detector and the source (see fig 7). The solid angle was calculated according to equation (5). Fig 7. The figure presents how the internal efficiency was calculated, r is the distance between the source and the detector head and A is the detector head area. Ω = A r 2 (4) 15

16 Ω 4π = A 4πr 2 (5) Were distance (r) between the detector and the source was 1 cm and area (A) was 1cm 2. The solid angle was divided with 4π since the radiation from the source is assumed to be emitted isotropically Angular sensitivity The angular sensitivity of the detector was measured in air, using a disc which could be rotated at different angles and a plastic holder was used to maintain stable sources without moving during measurements. The source placed 4 cm from detector on the plastic holder (see fig 8). The radionuclides used in this operation were 60 Co, 57 Co, 137 Cs and 133 Ba. The sources were positioned coaxial to the aperture of the probe sensitive area. The counts were acquired for source-detector angles between 0 and 90 and also between 0 and -90 in the steps of 15. Because of the low number of the counts, 30 measurements were carried out, each measurement in 50 s, and then summed for each of the angles (see Appendix 2). Fig 8. The plan for studying the angular sensitivity of the detector 16

17 4.2.3 Phantom Measurements were performed at a doll which weighed 50 kg, and had cavities were it was possible to place sources in order to detect them with the CdTe detector (fig 9). The purpose of this was to see it the detector could be used to find hidden sources in the body. The source was placed 3 cm deep in the cavity, and measurements were performed by moving the probe in steps to 5 cm laterally from the source. The measurement time was 50 seconds and the number of counts were recorded (see Appendix 4). To perform the measurements, different sources such as 60 Co, 57 Co, 137 Cs and 133 Ba (gamma ray, 81, 276 and 356 kev) was used to have a spread in energy. The measurements were carried out also with Plexiglas in cylinder form (ρ 1.2 g/cm 3 ) in front of the source to consider attenuation in tissue with similar procedure as without the Plexiglas. Plexiglas is a good approximation for water, because the body contains most water and these have a density and atomic composition (Z eff ) nearly as the water. The audio signal was used to guide the localization of the sources. The measurements could be performed without timing by just using audio signal and this was an advantage in this case i.e. to locate sources in the body at accidents. Fig 9. The phantom used to localize the sources 17

18 4.2.4 Line profile To determine the line profile a capillary tube filled with a solution of water, bovine serum albumin (BSA) and 0.02 sodium aside (NaN 3, it is extremely toxic) was used and nuclide which were 131 I and 99m Tc. The tube was placed at different distances on a PMMA (Polymetyl methacrylate phantom) slab. The detector, placed in a motorized holder, was moved horizontally perpendicular to the line source (fig 10). The counts were determined every 1 mm for positions less than 10 mm distal to the line source and every 2 mm for positions more than 10 mm distal to the line source, extending from -30 to 30 mm from the source. 131 I solution had 2.83 MBq ml -1 for activity concentrations, BSA were used to reduce absorption of nuclide to the capillary tube wall. Capillary tube was used as a line profile which it had 10 µl for volume ( 30 mm length) and then the solution was drawn up by the capillary tube. 137 Cs was also used to see the response at a higher energy. Unfortunately this had a low activity concentration and therefore the measurement was performed at 1 mm distance. Due to the low number of counts, 20 measurements were carried out in the each position from the center of the line source. It was not possible to mix the 137 Cs with BSA due to the low activity concentration (0.015 kbq µl -1 ). The spatial resolution of the detector was determined as full width at half maximum (FWHM) of the response profile for the radionuclides as utilized. Fig 10. The method utilized for studying the line response of the detector 18

19 5. Results 5.1 Simulation Table 3 presents DDER for an adult phantom which was simulated in MicroShield with different radionuclides and different energies. It is seen that for 60 Co the DDER is highest but it is the lowest for 241 Am. DDER is higher at 20 cm than 100 cm from the cylinder surface for all nuclides. Table 3. Deep Dose Equivalent rate (DDER) from an adult phantom with a uniform surface activity of 1MBq m -2 Radionuclide Gamma energy (kev) WMVE (kev) I γ % DDER (µsv h -1 ) DDER (µsv h -1 ) At 20 cm * At 100 cm * 60 Co 1173, , ,0 0, Cs ,52 0, I 284, 364, , 81.7, 7.1 0,38 0, Ir 317, , ,80 0, Am 14, ,06 0,009 *Distance between the cylinder surface and dose point Weighted Mean Value of Energies Table 4 shows DDER for a child phantom which was simulated in MicroShield also with similar radionuclide s as the adult phantom. The same results present in table 3, that the higher energy, except for 137 Cs and 192 Ir, increases also DDER. DDER is also higher for 20 cm than 100 cm from the surface. The child phantom compare to the adult phantom has lower DDER at all nuclides due to the less surface activity. 19

20 DDER (msv/h) Table 4. Deep Dose Equivalent rate (DDER) from a child phantom with a uniform surface activity of 1MBq m -2 Radionuclide Gamma energy (kev) WMVE (kev) I γ % DDER (µsv h -1 ) DDER (µsv h -1 ) At 20 cm * At 100 cm * 60 Co 1173, , ,6 0, Cs ,48 0, I 284, 364, , 81.7, 7.1 0,35 0, Ir 317, , ,74 0, Am 14, ,057 0,0073 *Distance between the cylinder surface and dose point Weighted Mean Value of Energies Simulation in water for different radionuclides shows the attenuation in water, DDER decreases with increasing depth (Fig 11). Attenuation is lowest for 60 Co clearly due to the higher energy and highest for 131 I which has the lowest energy in this situation. MicroShield simulation in water 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0, Co-60 Cs-137 I-131 Ir-192 Depth[cm] Fig 11. Simulation in water which was performed between 2 and 15 cm depth to consider the attenuation of the dose in tissue which has a density close to the water 20

21 Efficiency % 5.2 Experimental Efficiency Fig 12 presents the efficiency for different energies in the air and constant source to detector distance, the efficiency decreases with increasing energy and for high energy the efficiency is approximately constant (from 400 kev). Efficiency was highest for 57 Co (E γ =122 kev) and lowest for 65 Zn. The efficiency for 60 Co was acceptable at this point due to the detector system, i.e. the detector is more effective for the lower energies. The detector makes it possible to detect the sources with the higher energies. 7 Efficiency Energy [kev] Fig 12. The efficiency at varying energy at 1 cm in front of detector with different radionuclides in air In order to see if the measured efficiency of the detector is similar compared to the manual of the detector, the calculations was performed according below. It was approximately correct compare to the manual; Ω 4π

22 relative response % ε i = ε Ω 4π 6,5 0,08 81% for the efficiency measurements of 57 Co Angular sensitivity Fig 13 presents results regarding the relative angular sensitivity. The data are normalized for each radionuclide on the sensitivity at an angle of 0. At high energy such as 60 Co the angular sensitivity is less than lower energy such as 133 Ba and 57 Co, which shows that the detector is more sensitive at low energies and almost independent of angle for high energies i.e. the detector has a good angular sensitivity for low energies and less angular sensitivity for high energies. 1,2 Angular sensitivity 1 0,8 0,6 0,4 0, Angle (degrees) Co-60 Co-57 Cs-137 Ba-133 Fig 13. Angular sensitivity at varying energies at 4 cm from the detector. The responses are normalized to the highest response i.e. at 0. 22

23 Relative responce % Relative responce % Phantom Measurements of different radionuclides in the phantom (see fig. 9) at different lateral distances are shown in fig 14 and 15. The response for high energy decreases more slowly compared to the low energies. The localization without and with Plexiglas are also presented in fig 14 and15. 1,2 Localization without plexiglas 1 0,8 0,6 0,4 0, Lateral distance(cm) Co-57 Co-60 Cs-137 Ba-133 Fig 14. The response of the sources which was placed in the phantom to localize them with the detector. The responses are normalized to the highest response i.e. in front of sources. 1,2 Localization with plexiglas 1 0,8 0,6 0,4 0, Lateral distance (cm) Co-60 Co-57 Cs-137 Ba-133 Fig 15. The response of the sources which was placed in the phantom with Plexiglas. The responses were normalized to the highest response i.e. in front of sources. 23

24 relative response % Line profile As shown in the figure 17 and 18 the resolution (FWHM) deteriorates dramatically with increasing the source-detector distance, and also this is depended of the sources energies i.e. the higher energy leads to worsens in FWHM which can be observed in the fig 17 and 18. The comparison between the figures show that with increasing of the energy, in addition to 131 I was measured in shorter distance (1 mm) than 99m Tc (1 cm) decreases spatial resolution. It applies also that for 137 Cs with high energy (662 kev), according to the fig 16, the resolution is worsens more compare to the 99m Tc and 131 I which have lower energies. Cs-137, 1mm distance on a PMMA slab 1,1 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0, Position[mm] Fig 16. The line profile of 137 Cs at 1 mm distance on a PMMMA slab to study line spread function, measurement was performed every one millimeter inside 10 mm from center of the line source and every 2 mm outside 10 mm, i.e. from -20 to 20 mm from the source. Normalization was to the highest response. 24

25 Relative response % Relative responce % I-131, 1 mm distance on a PMMA SLAB I-131, 5 cm distance on a PMMA slab 1,2 1 0,8 0,6 0,4 0, Position [mm] Fig 17. Line profile of 131 I at 1 mm (blue) and 5 cm (red) distance on a PMMMA slab to study line spread function, measurement was performed every one millimeter inside 10 mm from center of the line source and every 2 mm outside 10 mm, i.e. from -30 to 30 mm from the line source. Normalization was to the highest response. Tc-99m, 1 cm distance on a PMMA slab Tc-99m, 5 cm distance on a PMMA slab 1,1 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0, Position [mm] Fig 18. Line profile of 99m Tc at 1 cm (blue) and 5 cm (red) distance on a PMMMA slab to study line spread function, measurement was performed every millimeter inside 10 mm from center of the line source and every 2 mm outside 10 mm, i.e. from -30 to 30 mm from the line source. Normalization was to the highest response. 25

26 Relative response % I-131, 5 cm distance on a PMMA SLAB 1,2 1 0,8 0,6 0,4 0, Position [mm] Fig 19. Line profile of 131 I at 5 cm distance on a PMMMA slab to study line spread function, measurement was performed every millimeter inside 10 mm from center of the line source and every 2 mm outside 10 mm, i.e. from -30 to 30 mm from the line source. Normalization was to the highest response Figure 19 showes a separate figure of 131 I to present the better line spread function of 131 I at 5 cm. Table 5. Spatial Resolution for different nuclides at different distances from the detector on a PMMA slab Radionuclide 137 Cs 131 I 99m Tc Distance (mm) FWHM (mm) FWHM (mm) FWHM (mm) FWHM = Full width at Half Maximum No measuerements in this distance FWHM is lower for 131 I at 1 mm (the smaller the FWHM, the better the spatial resolution), highest for 131 I at 50 mm (worse spatial resolution), 99m Tc has the best FWHM at 1 cm. The other nuclides wasn t measured at 1 cm. Althought 131 I had the lower FWHM but at 1 mm compare to 99m Tc. 137 Cs had a worse FWHM at 1mm. At 5 cm 99m Tc has the better spatial resolution than 131 I (table 5). 26

27 6. Discussion 6.1 Simulation DDER reduces by increase of the distance between the surface of source and the dose point obviously due to the inverse square law, which the intensity of radiation from a source decreases as 1/d 2 from the source. The dose could be less for children than the adult human with the activity which used in the MicroShield simulation program (1MBqcm -2 ) due to the smaller surface of the children. DDER decreases also with increasing depth in water due to attenuation and this also dependent of energy i.e. the higher energy leads to less attenuation, the radiation passes through the water. Fig 11 recognizes this phenomenon well then DDER for 60 Co which has high gamma energy (1253 kev) decreases more slowly than other radionuclides. 6.2 Efficiency Photoelectric absorption for the photon energies of for example 57 Co is higher than for example 60 Co in the detector such as CdTe which have a high atomic number (see fig 12). 60 Co and 131 I had almost similar efficiency and the results are promising in this case. The thickness of the detector is which results in a decrease of efficiency when the energy of the gamma rays increases. The efficiency affected by the dimensions and composition of the radiation source, and by the absorption of the radiation in the source as well as in the surrounding materials. The efficiency measurements were performed in air in this work i.e. without attenuating and scattering medium, in reality there are attenuating medium such as tissue which leads to lower efficiency of the detector when lowered efficiency leads to a low number of counts and high statistical uncertainty. 6.3 Angular sensitivity The detector may perhaps be able to detect the sources with the higher energies, but by the lower efficiency. However, this can utilize at localization of the radioactive fragments with the higher energies such as 60 Co or 137 Cs. Contamination couldn t happen with the sources which emit radiation with the higher energy than 60 Co. A higher angular sensitivity is required in order to achieve good spatial resolution, but this work shows that the CdTe detector is poor angular sensitive to the higher energies as 60 Co (due to the longer range of the photons by the 27

28 higher energy i.e. the photons can nevertheless reach the detector) and excellent angular sensitivity at lower energies such as 57 Co and 133 Ba. 6.4 Phantom The localization of sources in the phantom by different energies presents that at the higher energy the lateral response decreases more slightly compare to the lower energies due to the longer range of the photons and hence photons undergo more Compton scattering. The localization with Plexiglas shows that due to the attenuation in the Plexiglas the response decreases somewhat than the localization without Plexiglas. 6.5 Line profile In the case of that estimate the line spread function, this work is shown the ability of the detector (CdTe) which used to detect a line source which deep-seated can unfortunately be too difficult according to the fig 17 and 18 for radionuclides as 99m Tc and 131 I. This presents also for the lower energy its capability is much better than the higher energy by reason of the detector is more efficient for the lower energy. The spatial resolution of this detector deteriorates dramatically with increasing distance of the source from the detector, mainly because the probe s field of view increases with increasing distance. The line profile for 137 Cs was too poor at 1 mm, this can caused also by its low activity concentration (0.015 kbq µl -1 ). 6.6 Statistics What is often necessary is a measure of uncertainty that defines an interval about the measurement result M within which the value of the measured Y can be confidently asserted to lie. The measure of uncertainty proposed to meet this requirement is termed expanded uncertainty (U), and is obtained by multiplying s by a coverage factor (k), thus U = k s and it is confidently believed that Y is greater than or equal to M - U, and is less than or equal to M + U, which is commonly written as Y = M ± U. 28

29 Typically, k is in the range 2 to 3. When the normal distribution applies and s is a reliable estimate of the standard deviation of M, U = 2 s (i.e. k = 2) defines an interval having a level of confidence of approximately 95 %. In this work were performed many measurements which the expanded uncertainty was tested and uncertainty was approximately 1% between two measurement i.e. measurements were well done. In determining the distance between the source and the detector a ruler has been used, this contributes to an uncertainty of 1-2 mm because of the manual reading. The very short distance such as 1mm between the detector and the source in the case of line profile can give uncertainties due to the uncertainty in the where the detector sits in relation to the entry window. 6.7 Detector The purpose of shielding and collimation is to use attenuation to stop radiations from unwanted locations from striking the detector and producing counts. To maximize attenuation, collimator should be fabricated out of high- atomic number materials. Among such materials, lead has been the most commonly used shielding and collimation material but other material such as tungsten has even greater attenuation than lead. The gamma probe which used in this work has tungsten as shielding and this provides an advantage. It is essential that the instrument be equipment with an audible signal to enable the user to perform the search without watching the meter, the detector which used in this work has this benefit. For search application light weight and comfortable carrying handle equipment should be used, the gamma probe which utilized, was a light weight probe, hopefully. 29

30 7. Conclusions This work recognizes that the detector was utilized in the aim to investigate its properties with different radionuclides and different energies can be used at contamination but this presents the better response for the radionuclides with lower energies such as 99m Tc and 57 Co. The contamination can include radionuclides which emit radiation with higher energy than 60 Co, this can happen with different radioactive nuclides either medical or industry and the detector is efficient also for higher energy but not so superior compare to the lower energy. The detector isn t good for localization of deep-seated sources for example at 5 cm as were investigated in this work. For search application the detector is comfortable and has a good audio signal. This detector can be used at the contamination of humans in the absence of any other instruments in the hospital. 8. Acknowledgements I would like especially to thank my supervisor Assoc. Prof Mats Isaksson who has helped me accomplish this work and his great helpfulness and advices during my study. I would like to thank all my classmates for their supporting and fine time during my study at the Department of Radiation Physics and the Chalmers University and their help during my work. Furthermore I would like to thank the workshop, especially Mats Sak and Jan Samuelsson for their fine help during my work. Many thanks to the Department of Radiation Physics. Finally I would like to thank my family, my dear husband and my beautiful sons for their wonderful supporting and their great interest for my study. 30

31 Appendix 1 Efficiency 57 Co A= kbq 60 Co A= kbq Measuring series 1 2 Measuring series ,0 4608, ,0 1768, ,0 4709, ,0 1637, ,0 4503, ,0 1696, ,0 4574, ,0 1700, ,0 4497, ,0 1729, ,0 4493, ,0 1678, ,0 4603, ,0 1669, ,0 4508, ,0 1710, ,0 4598, ,0 1798, ,0 4672, ,0 1687, ,0 4558, ,0 1720, ,0 4648, ,0 1812, ,0 4618, ,0 1690, ,0 4658, ,0 1705, ,0 4600, ,0 1739, ,0 4646, ,0 1712, ,0 4655, ,0 1716, ,0 4711, ,0 1712, ,0 4563, ,0 1717, ,0 4501, ,0 1717,0 sum 92507, , ,0 1673,0 std 304,1 303,2 sum 33771, ,0 Cnt-Bg 92129, ,0 std 183,8 189,7 cps 87,7 87,2 Cnt-Bg 33393, ,0 eff 5,5 cps 31,8 33,9 Eff% 6.48 eff 1,0064 Eff% 1,01 Uncertainty in reproducibility Uncertainty in reproducibility M 1 = 4625,4 S 1 = 52,5 M 2 = 4596,2 S 2 = 70,3 M 1 = 1608,1 S 1 = 38,4 M 2 = 1713,6 S 2 = 40,8 M 1 ± S 1 =4625±52,5 M 2 ± S 2 =4596,2±70,3 M 1 ± S 1 =1608,1±38,4 M 2 ± S 2 =1713,6±40,8 Uncertainty in the measurement of two different sources Uncertainty in the measurement of two different sources m 2 m ± s = 2 0,99 ± 0,005 m 1 m 1 ± s = 1,1 ± 0,008 31

32 65 Zn A= 7.92 kbq 54 Mn A= kbq Measuring series 1 2 Measuring series ,0 172, ,0 525, ,0 181, ,0 520, ,0 189, ,0 532, ,0 174, ,0 501, ,0 196, ,0 535, ,0 174, ,0 556, ,0 177, ,0 520, ,0 177, ,0 528, ,0 166, ,0 520, ,0 173, ,0 535, ,0 173, ,0 557, ,0 163, ,0 544, ,0 183, ,0 512, ,0 182, ,0 510, ,0 178, ,0 551, ,0 174, ,0 539, ,0 185, ,0 545, ,0 172, ,0 474, ,0 183, ,0 496, ,0 163, ,0 549, ,0 174, ,0 514,0 sum 3738,0 3709,0 sum 11034, ,0 std 61,1 60,9 std 105,0 105,2 Cnt-Bg 3360,0 3331,0 Cnt-Bg 10656, ,0 cps 3,2 3,2 cps 10,1 10,2 eff 0,4 eff 0,8 Eff% 0,8 Eff% 0,85 Uncertainty in reproducibility Uncertainty in reproducibility M 1 = 178,0 S 1 = 11,6 M 2 = 176,6 S 2 = 8,1 M 1 = 525,4 S 1 = 18,9 M 2 =526,8 S 2 = 21,2 M 1 ± S 1 =178,0±11,6 M 2 ± S 2 =176,6±8,1 M 1 ± S 1 =525,4±18,9 M 2 ± S 2 =526,8±21,2 Uncertainty in the measurement of two Uncertainty in the measurement of two different sources different sources m 2 m 2 ± s = 0,99 ± 0,02 ± s = 1,003 ± 0,01 m 1 m 1 32

33 109 Cd A= kbq 131 I A= kbq Measuring series 1 2 Measuring series ,0 409, ,0 2535, ,0 420, ,0 2511, ,0 416, ,0 2494, ,0 430, ,0 2537, ,0 408, ,0 2480, ,0 441, ,0 2453, ,0 412, ,0 2481, ,0 416, ,0 2472, ,0 440, ,0 2510, ,0 449, ,0 2540, ,0 411, ,0 2537, ,0 414, ,0 2474, ,0 420, ,0 2543, ,0 419, ,0 2557, ,0 420, ,0 2479, ,0 454, ,0 2461, ,0 424, ,0 2402, ,0 390, ,0 2512, ,0 413, ,0 2413, ,0 440, ,0 2483, ,0 376, ,0 2474,0 sum 8725,0 8822,0 sum 53104, ,0 std 93,4 93,9 std 230,4 228,8 Cnt-Bg 8347,0 8444,0 Cnt-Bg 52726, ,0 cps 7,9 8,0 cps 50,2 49,5 eff 0,5 eff 1,2 Eff % 1,33 Eff % 1,44 Uncertainty in reproducibility Uncertainty in reproducibility M 1 = 415,5 S 1 = 23,6 M 2 = 420,1 S 2 = 18,3 M 1 = 2528,8 S 1 = 52,8 M 2 = 2492,8 S 2 = 41,5 M 1 ± S 1 =415,5±23,6 M 2 ± S 2 =420,1±18,3 M 1 ± S 1 =2528,8±52,8 M 2 ± S 2 =2492,8±41,5 Uncertainty in the measurement of two Uncertainty in the measurement of two different sources m 2 m 1 ± s = 1,01 ± 0,02 different sources m 2 m 1 ± s = 0,99 ± 0,006 33

34 99m Tc A= 23 kbq / µl 137 Cs A= 4.61kBq Measuring series 1 2 Measuring series 1 counts 2 counts ,0 2627, ,0 259, ,0 2729, ,0 268, ,0 2598, ,0 259, ,0 2783, ,0 264, ,0 2578, ,0 268, ,0 2627, ,0 292, ,0 2689, ,0 286, ,0 2725, ,0 297, ,0 2658, ,0 292, ,0 2693, ,0 260, ,0 2725, ,0 257, ,0 2643, ,0 283, ,0 2663, ,0 280, ,0 2571, ,0 285, ,0 2577, ,0 287, ,0 2597, ,0 272, ,0 2635, ,0 252, ,0 2586, ,0 256, ,0 2613, ,0 295, ,0 2571, ,0 273, ,0 2514, ,0 264,0 sum ,0 sum 5528,0 5749,0 std 229,3 235,4 std 74,4 75,8 Cnt-Bg Cnt-Bg 5150,0 5371,0 cps 49,7 49,3 cps 4,9 5,1 eff 2,2 eff 1,1 Eff % 2,42 Eff % 1,27 Uncertainty in reproducibility Uncertainty in reproducibility M 1 = 2503,5 S 1 = 61,8 M 2 = 2638,2 S 2 = 67,0 M 1 = 1623,1 S 1 = 263,2 M 2 = 1769,4 S 2 = 273,8 M 1 ± S 1 =2503,5±61,8 M 2 ±S 2 =2638,2±67,0 M 1 ± S 1 =1623,1±263,2 M 2 ± S 2 =1769,4±273,8 Uncertainty in the measurement of two Uncertainty in the measurement of two different different sources sources m 2 m 2 ± s = 1,05 ± 0,006 ± s = 1,04 ± 0,02 m 1 m 1 34

35 Appendix 2 Angular sensitivity 57 Co A= 15 kbq Angle ,0 192,0 149,0 41,0 37,0 44,0 44, ,0 208,0 143,0 48,0 42,0 47,0 38, ,0 213,0 135,0 42,0 30,0 47,0 38, ,0 200,0 148,0 34,0 31,0 27,0 44, ,0 208,0 160,0 38,0 29,0 32,0 40, ,0 209,0 132,0 37,0 26,0 39,0 44, ,0 220,0 155,0 43,0 27,0 33,0 41, ,0 185,0 142,0 32,0 44,0 51,0 31, ,0 199,0 151,0 53,0 32,0 32,0 42, ,0 193,0 172,0 44,0 29,0 35,0 47, ,0 174,0 150,0 40,0 44,0 36,0 31, ,0 202,0 131,0 45,0 30,0 34,0 41, ,0 209,0 128,0 37,0 33,0 33,0 40, ,0 215,0 159,0 34,0 37,0 43,0 39, ,0 204,0 149,0 49,0 39,0 44,0 49, ,0 201,0 133,0 34,0 33,0 26,0 30, ,0 212,0 132,0 31,0 36,0 29,0 27, ,0 210,0 142,0 39,0 42,0 35,0 39, ,0 211,0 139,0 49,0 40,0 31,0 42, ,0 194,0 133,0 41,0 23,0 43,0 39, ,0 237,0 134,0 42,0 43,0 43,0 43, ,0 193,0 155,0 37,0 39,0 37,0 34, ,0 214,0 143,0 47,0 35,0 26,0 47, ,0 210,0 149,0 46,0 39,0 30,0 40, ,0 191,0 164,0 32,0 38,0 31,0 43, ,0 194,0 135,0 40,0 27,0 38,0 39, ,0 195,0 160,0 43,0 29,0 34,0 48, ,0 222,0 126,0 37,0 41,0 31,0 37, ,0 205,0 148,0 50,0 36,0 44,0 34, ,0 194,0 122,0 40,0 44,0 33,0 40,0 sum 7879,0 6114,0 4319,0 1225,0 1055,0 1088,0 1191,0 std 88,8 78,2 65,7 35,0 32,5 33,0 34,5 mean 262,6 203,8 144,0 40,8 35,2 36,3 39,7 std 17,3 12,4 12,3 5,8 6,1 6,8 5,4 35

36 57 Co A= 15 kbq Angle ,0 213,0 112,0 70,0 38,0 30, ,0 217,0 93,0 51,0 40,0 31, ,0 228,0 87,0 57,0 26,0 33, ,0 211,0 93,0 55,0 25,0 31, ,0 208,0 100,0 61,0 34,0 27, ,0 214,0 100,0 44,0 31,0 24, ,0 178,0 85,0 45,0 31,0 32, ,0 151,0 101,0 79,0 27,0 27, ,0 177,0 97,0 58,0 30,0 26, ,0 166,0 89,0 48,0 41,0 27, ,0 175,0 92,0 53,0 37,0 24, ,0 167,0 95,0 55,0 28,0 40, ,0 172,0 90,0 64,0 27,0 19, ,0 175,0 87,0 72,0 29,0 21, ,0 175,0 101,0 59,0 33,0 15, ,0 166,0 92,0 53,0 38,0 28, ,0 151,0 108,0 65,0 43,0 30, ,0 165,0 105,0 64,0 37,0 27, ,0 192,0 109,0 57,0 36,0 24, ,0 155,0 94,0 64,0 33,0 29, ,0 175,0 88,0 53,0 30,0 27, ,0 173,0 97,0 46,0 33,0 28, ,0 166,0 86,0 63,0 31,0 20, ,0 144,0 71,0 52,0 41,0 18, ,0 187,0 97,0 48,0 33,0 40, ,0 149,0 99,0 77,0 33,0 20, ,0 168,0 104,0 55,0 36,0 28, ,0 171,0 102,0 51,0 29,0 29, ,0 183,0 95,0 59,0 30,0 26, ,0 156,0 99,0 66,0 35,0 21,0 sum 7759,0 5328,0 2868,0 1744,0 995,0 802,0 std 88,1 73,0 53,6 41,8 31,5 28,3 mean 258,6 177,6 95,6 58,1 33,2 26,7 std 17,3 22,2 8,4 9,0 4,8 5,7 36

37 60 Co A= 32,5 kbq Angle ,0 216,0 219,0 198,0 175,0 177,0 182, ,0 221,0 202,0 222,0 196,0 180,0 163, ,0 219,0 198,0 209,0 196,0 153,0 189, ,0 203,0 212,0 211,0 189,0 191,0 219, ,0 222,0 209,0 183,0 184,0 172,0 166, ,0 204,0 197,0 191,0 192,0 155,0 181, ,0 220,0 209,0 149,0 185,0 183,0 171, ,0 213,0 224,0 177,0 198,0 168,0 182, ,0 202,0 221,0 187,0 192,0 167,0 176, ,0 239,0 193,0 193,0 176,0 167,0 189, ,0 232,0 222,0 188,0 202,0 177,0 183, ,0 244,0 222,0 204,0 197,0 174,0 197, ,0 241,0 203,0 214,0 198,0 187,0 176, ,0 229,0 214,0 179,0 205,0 176,0 186, ,0 219,0 202,0 169,0 178,0 157,0 168, ,0 215,0 219,0 177,0 156,0 181,0 167, ,0 220,0 210,0 192,0 180,0 173,0 199, ,0 219,0 200,0 196,0 212,0 160,0 187, ,0 221,0 198,0 208,0 211,0 187,0 178, ,0 215,0 217,0 205,0 199,0 163,0 173, ,0 215,0 235,0 189,0 193,0 160,0 158, ,0 238,0 226,0 186,0 174,0 191,0 166, ,0 232,0 228,0 192,0 191,0 178,0 165, ,0 207,0 222,0 185,0 199,0 184,0 179, ,0 221,0 231,0 180,0 192,0 188,0 168, ,0 212,0 191,0 196,0 194,0 171,0 169, ,0 257,0 184,0 174,0 202,0 197,0 184, ,0 215,0 192,0 185,0 194,0 178,0 165, ,0 223,0 215,0 202,0 184,0 162,0 177, ,0 221,0 201,0 179,0 195,0 179,0 166,0 sum 6721,0 6655,0 6316,0 5720,0 5739,0 5236,0 5329,0 std 82,0 81,6 79,5 75,6 75,8 72,4 73,0 mean 224,0 221,8 210,5 190,7 191,3 174,5 177,6 std 18,8 12,6 13,3 15,1 11,8 11,6 12,9 37

38 60 Co A= 32,5 kbq Angle ,0 173,0 177,0 191,0 170,0 163, ,0 209,0 178,0 165,0 163,0 171, ,0 198,0 183,0 170,0 161,0 149, ,0 194,0 197,0 156,0 141,0 141, ,0 190,0 169,0 177,0 171,0 177, ,0 215,0 173,0 167,0 184,0 148, ,0 225,0 188,0 154,0 140,0 158, ,0 196,0 181,0 160,0 166,0 203, ,0 196,0 152,0 176,0 161,0 156, ,0 199,0 185,0 149,0 173,0 179, ,0 209,0 160,0 138,0 156,0 164, ,0 220,0 182,0 159,0 175,0 134, ,0 209,0 169,0 164,0 163,0 163, ,0 186,0 157,0 163,0 173,0 172, ,0 202,0 173,0 181,0 189,0 174, ,0 187,0 174,0 160,0 170,0 150, ,0 230,0 203,0 156,0 149,0 164, ,0 218,0 182,0 181,0 161,0 183, ,0 173,0 195,0 161,0 151,0 147, ,0 190,0 166,0 158,0 161,0 168, ,0 199,0 178,0 189,0 138,0 201, ,0 188,0 200,0 184,0 186,0 165, ,0 198,0 169,0 167,0 156,0 168, ,0 179,0 178,0 172,0 169,0 160, ,0 203,0 199,0 153,0 148,0 178, ,0 185,0 168,0 160,0 168,0 166, ,0 202,0 180,0 177,0 174,0 184, ,0 190,0 186,0 176,0 176,0 169, ,0 181,0 169,0 170,0 175,0 149, ,0 204,0 171,0 138,0 156,0 159,0 sum 6202,0 5948,0 5342,0 4972,0 4924,0 4963,0 std 78,8 77,1 73,1 70,5 70,2 70,4 mean 206,7 198,3 178,1 165,7 164,1 165,4 std 16,4 14,4 12,6 13,2 13,1 15,8 38

39 137 Cs A= 169,4 kbq Angle Sum Std Mean std Ba A= 32,65 kbq Angle Sum Std Mean std