EVALUATION OF ISO :1996 AND THE SHADOW SHIELD TECHNIQUE FOR THE MEASUREMENT OF SCATTERED RADIATION A THESIS

Transcription

1 EVALUATION OF ISO :1996 AND THE SHADOW SHIELD TECHNIQUE FOR THE MEASUREMENT OF SCATTERED RADIATION A THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Daniel R. Steele, B.S. Nuclear Engineering Graduate Program The Ohio State University 2010 Thesis Committee Dr. Thomas E. Blue, Advisor Dr. Richard Denning

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3 ABSTRACT ISO specifies the characteristics and production methods of X-Ray and Gamma-Ray reference radiation for calibrating protection level dosimeters and rate dosimeters. The standard limits scattered radiation to 5% and provides a test that one may use to check conformity with the scatter specification. Through the use of an MCNP model built to mimic the OEMA Calibration facility, it was shown that, using the definition of scatter provided by the standard, it is possible to pass the test provided, yet fail the scatter limit criterion. The shadow shield technique is another widely accepted test used to determine the scatter contribution. Using the same MCNP model, an analysis of the shadow shield technique was performed. It was found that, using the definition of scatter found in ISO 4037, the shadow shield technique dramatically under predicts scatter. ii

4 Dedicated to my family iii

5 ACKNOWLEDGEMENTS I would like to thank my thesis advisor, Dr. Thomas E. Blue, along with my co-advisor, Kevin Herminghuysen, for the guidance and support they provided to me throughout the duration of my project. I would also like to thank the Faculty, Staff, and students of the Ohio State Nuclear Engineering Program. Special thanks go toward Kyle Metzroth for aiding me in the use of the computer cluster and also Jon Kulisek for sharing extensive knowledge on MCNP5/MCNPX and Linux. I would also like to thank Carol Smidts for appointing me as her Graduate Teaching Assistant, which helped to fund my education. This work was supported by Dr. Thomas Blue via the Ohio Emergency Management Agency (OEMA). iv

6 VITA July, Born, Mansfield Ohio, USA B.S. of Physics John Carroll University 2007-Present. Graduate Teaching/Research Associate The Ohio State University MAJOR FIELD: Nuclear Engineering FIELDS OF STUDY v

7 TABLE OF CONTENTS PAGE ABSTRACT... ii ACKNOWLEDGEMENTS... iv CHAPTER 1: INTRODUCTION Objectives Motivation...1 CHAPTER 2: BACKGROUND OEMA Calibration Lab ISO Standard : Shadow Shield Technique Range Calibration...8 CHAPTER 3: MATERIALS AND METHODS JL Shepherd Range Irradiator Range Irradiator Room Specifications MCNP5/MCNPX Discussion of Scatter Calculation Shadow Shield Variation of Building Materials Irradiator...23 vi

8 3.5 Error Analysis...24 CHAPTER 4: MODELS Simplified Shadow Shield Model Complex Shadow Shield Model...27 CHAPTER 5: RESULTS Validation Calculation of Γ Scatter Results Scatter from the Walls and Irradiator Simplified Shadow Shield Model Complex Shadow Shield Conformity to Inverse Square Law (ISO Specified Test)...49 CHAPTER 6: CONCLUSIONS MCNP Model Shadow Shield Technique ISO CHAPTER 7: FUTURE WORK...53 BIBLIOGRAPHY...55 APPENDIX A...56 MCNP Input Files...56 vii

9 LIST OF FIGURES Figure 1. Simplified Shadow Shield Model at 1 meter on the Y=0 plane...6 Figure 2. Simplified Shadow Shield Model at 1 meter on the Z=0 plane...7 Figure 3. Measured Scatter Percentage Versus Distance for Nominal 400 Ci Source...9 Figure 4. Exposure Rate Versus Distance for 400 Ci Calibration...10 Figure 5. Test to Check if Air-Kerma Rates Conform to the Inverse Square Law...11 Figure 6. Cross Sectional View of the Cs-137 Source in the X-Z Plane at Y= Figure 7. Cross Sectional View of the Cs-137 Source on the X-Y Plane at Z= Figure 8. Decay Schematic of 137 Cs...15 Figure 9. Vised 3d model of Irradiator Room...19 Figure 10. Percent of Exposure Due to Scatter from Walls and Floor Versus Distance...23 Figure 11. Photograph of Experimental Setup without Lead Shadow Shield...28 viii

10 Figure 12. 3D Vised Rendition of Experimental Setup Including the Wood Plank, Steel Carts, and Concrete Blocks Figure 13. Specific Gamma Ray Constant vs. Distance of Bare Source Particles in Void...33 Figure 14. Specific Gamma Ray Constant vs. Distance of Source Capsule in Air...34 Figure 15. Exposure Rate vs. Distance for Base Case of the Simplified Shadow Shield, Calculated Using Eqn. 2, for the Nominal 400 Ci Source...35 Figure 16. Measured and Calculated Γ e Vs. Distance for Base Case of Complex Shadow Shield...37 Figure 17. Track Length Estimation of Flux on the Z=0 Plane for Base Case of the Simplified Shadow Shield...40 Figure 18. Scatter Percentage Due to Irradiator, Walls and Floor vs. Distance for the Wall, Floor and Irradiator Importance Set to Zero without the Shield Support Structure, Calculated Using Eqn Figure 19. S % Due to Walls and Floor vs. Distance for the Wall and Floor Importance Set to Zero without the Shield Support Structure, Calculated Using Eqn Figure 20. S % Due to Irradiator and S % Due to Walls and Floor vs. Distance for the Wall, Floor and Irradiator Importance Set to Zero ix

11 without the Shield Support Structure, Calculated using Eqns. 12 and Figure 21. Calculated S % for the Simplified Shadow Shield Model Calculated Using Eqn. 1 vs. Distance and S % Due to the Walls and Floor without the Shielding Support Structure vs. Distance, Calculated using Eqn. 5 Compared with the Measured S %...47 Figure 22. Comparison of S % with Simplified Shadow Shield, Complex Shadow Shield, and Measured Values, Calculated using Eqn Figure 23. Ẋ*r 2 for the Base Case of the Complex Shadow Shield...49 x

12 LIST OF TABLES Table 1. Photon Energies and Corresponding Probability of Emission for 137 Cs in Secular Equilibrium with its Daughter 137 Ba...15 Table 2. Tabulated photon emission probability and activity for Cs Table 3. Exposure Rates vs. Distance for Measured and Calculated Base Cases for the Base Case of the Simplified Shadow Shield using Eqn Table 4. Measured and Calculated values of Γ e with distance...38 Table 5. Processed Data for Scatter due to Irradiator, Walls and Floor for the Wall, Floor and Irradiator Importance Set to Zero without the Shield Support Structure, Calculated Using Eqn Table 6. Processed Data for Scatter Due to the Walls and Floor for the Wall and Floor Importance Set to Zero without the Shield Support Structure, Calculated Using Eqn xi

13 CHAPTER 1 INTRODUCTION 1.1 Objectives The objectives of this thesis were: To construct an MCNP model that accurately simulates the OEMA Range Irradiator. To use the MCNP model to determine the scatter contribution as a function of distance from the source. To evaluate the ISO-specified test and shadow shield technique. 1.2 Motivation A common technique for measuring the contribution of scattered radiation to measured dose is the shadow shield technique. The Ohio Emergency Management Agency (OEMA) uses this technique in conjunction with their gamma calibration range, which is 10 meters in length. There is a requirement, 1

14 outlined in ISO standard :1996, which limits the percentage of scattered exposure that factors into their calibration. The range has been calibrated and scatter contributions have been measured using the shadow shield technique. The results of the calibration showed that, in order to satisfy the requirements of ISO :1996, use of the range should be restricted to distances less than 7 meters. An MCNP model was constructed to identify the origins of the scattered component of the exposure in the hope that this component can be reduced if its origins are known. 2

15 CHAPTER 2 BACKGROUND 2.1 OEMA Calibration Lab The Ohio Emergency Management Agency (OEMA) performs calibrations of devices which are used for gamma radiation detection. The calibration facility is located on the west campus of The Ohio State University, and is one of many similar instrument calibration facilities. The OEMA s facility includes a range irradiator and a track which allow the user to select a source-todetector distance between 46 cm and 997 cm in 1 mm increments. The gamma irradiator is a JL Shepherd 81-14T which utilizes three sources with activities at manufacture of 0.04, 4, and 400 Ci. The room in which this system lies has dimensions of (11.7 x 3.37 x 5.48) meters. 2.2 ISO Standard :1996 3

16 The International Organization for Standardization (ISO) published a standard (ISO :1996) which gives a general outline for gamma-ray calibration requirements. The scope of the standard is quoted as, specification of the characteristics and production methods of X and gamma reference radiation for calibrating protection level dosimeters and rate dosimeters at air kerma rates from 10 µgy*h -1 to 10 Gy*h -1 and for determining their response as a function of photon energy. Section 6.3 of the standard (henceforth called ISO Section 6.3) outlines the requirement for scattered radiation as follows: The air-kerma rate due to radiation scattered by the environment shall not exceed 5% of that due to direct radiation. The most interesting section of the standard, pertaining to this thesis, is Section 6.4 (henceforth called ISO Section 6.4). The section is entitled checking installation conformity and gives the limit of scattered radiation stating, The following test shall be carried out in order to check that, at the various experimental distances, the contribution due to scattered radiation extraneous to that of the source capsule does not exceed 5% of the total air-kerma rate. The test used to check this condition is given in the next paragraph of the standard and is that, The air-kerma rates shall be measured on the axis of the beam at the various points of test. After correcting for air attenuation, the air-kerma rates shall be proportional within 5% to the inverse square of the distance from the source centre to the detector centre. [1] What can be concluded from these two 4

17 statements is that if one can meet the criteria of the Section ISO 6.4 test, one can assume that scatter is less than 5% for all points which pass the test. 2.3 Shadow Shield Technique The shadow shield technique is a method for experimentally determining the exposure due to scattered radiation. The method is conceptually very simple. First, the exposure rate is measured at the desired distance using a secondary standard ionization chamber. The detector is then shielded from the central beam using lead bricks configured into a block large enough to obscure the chamber and with a thickness of 10 cm. The measurement is then repeated with the shadow shield in place [3]. The scatter percentage is then found by, (1) where is the ion chamber exposure rate when the ion chamber is shielded, and is the ion chamber exposure rate when the ion chamber is unshielded. A 3 dimensional representation of the shadow shielded model without the apparatus required to support the shield can be seen via Vised in Figs. 1 and 2. The figures depict the cylindrical irradiator with the lead shadow shield clearly defined in front of the detector. 5

18 Figure 1. Simplified Shadow Shield Model at 1 meter on the Y=0 plane 6

19 Figure 2. Simplified Shadow Shield Model at 1 meter on the Z=0 plane Ideally, the lead blocks would be in front of the detector without any other material. To enable the block to exist in front of the detector without the aid of other scatter-increasing components would require an apparatus which is economically unrealistic. Therefore, two carts and a wood plank were used to suspend the shield in front of the detector. In order to keep the shielded and unshielded runs similar, the unshielded measurement (and simulation) was performed with all necessary apparatus to support the lead bricks. When it came time to block the beam, the only addition was the shield. 7

20 2.4 Range Calibration In order to benchmark the MCNP model, the results were compared to those of a calibration of the JL Shepherd Range Irradiator, performed by Kevin Herminghuysen in March The calibration, which included a measurement of the scatter component using the shadow shield technique, concluded that the scatter component exceeded 5% at distances greater than 7 meters [2]. The results of the scatter measurement are shown in Fig. 3. The scatter measurement used a 1L ion chamber (Far West Technology Model IC-1000) as the detector for distances greater than 2 meters. For distances closer than 2 meters, an 80cc chamber was used (Far West Technology Model IC- 80). Different chambers were used to ensure that the full beam completely covered the detector and to limit the unshielded detector current. This detector change causes a discontinuity within the graph of scatter percentage vs. distance at 2 meters. 8

21 Scatter (%) Distance (meters) Figure 3. Measured Scatter Percentage versus Distance for Nominal 400 Ci Source source. Fig. 4 shows the measured exposure rate as a function of distance from the 9

22 1000 Exposure Rate (R/h) Distance (m) Figure 4. Exposure Rate versus Distance for 400 Ci Calibration The calibration report also addressed the test given in ISO Standard 6.4. Fig. 5 shows the product of the exposure rate and the square of the distance. It is important to note that while the standard specifies air-kerma rates, this study is done using exposure rates which are interchangeable. If the data were perfectly fit to, then r 2 would be a constant. Since the standard is vague in the description of the test (i.e. one doesn t know whether the 5% should be compared with the mean value or the extremes) it will be assumed that the test is measured with respect to 5% of the mean value. It can then be concluded that the results of the calibration, as well as the shadow shield data, pass the test outlined in ISO Section

23 Ẋr 2 (m 2 *R/h) Distance (m) Figure 5. Test to Check if Air-Kerma Rates Conform to the Inverse Square Law Note that the results shown in Figs. 3, 4, and 5 are for the nominal 400Ci source only. It is acceptable to use only one of the sources, since it is expected that will have a similar behavior for the others. 11

24 CHAPTER 3 MATERIALS AND METHODS 3.1 JL Shepherd Range Irradiator The irradiator being used by the OEMA is the JL Shepherd 81-14T Range Irradiator. The 81-14T is a three source irradiator with each source being 137 Cs. The nominal activities at the time of manufacturing were 0.04, 4 and 400 Ci. However, measurements done by Herminghuysen [2] show that the strength of the source, which should have been 400 Ci, was much closer to 330 Ci when new. It is believed that the current source strength is 250 Ci based on measured exposure rates and assumed gamma constants. The source sits cm above the floor and its center of mass is placed at the origin of the MCNP model. The source capsule is made up of the source material itself, which is cesium chloride, and a series of shielding materials. The source material is cylindrical in shape with a radius of cm and a height of 5.08 cm. The nominal 400 Ci source is surrounded by two stainless steel shells, 12

25 each of thickness cm, and an outer layer of aluminum with thickness cm. A pictorial depiction of the source can be seen in Figs. 6 and 7. Figure 6. Cross Sectional View of the Cs-137 Source in the X-Z Plane at Y=0 13

26 Figure 7. Cross Sectional View of the Cs-137 Source on the X-Y Plane at Z=0 The irradiator itself is cm tall with a radius of cm. It is constructed of lead with a cm layer of stainless steel on exterior surfaces. The beam port is a truncated rectangular pyramid with angles of 7.5 degrees with respect to the horizontal and vertical and an area of ( x ) cm at the inner radius of the irradiator. The photon emission probabilities per decay for 137 Cs, in secular equilibrium with its daughter 137m Ba (from Grove Engineering Libraries [4]) are presented in Table 1. A schematic of the gamma decay scheme is presented in Fig

27 Table 1. Photon Energies and Corresponding photon emission probabilities per decay for 137 Cs in Secular Equilibrium with its Daughter 137 Ba Photon Energy Figure 8. Decay Schematic of 137 Cs The fractional yield, calculated from Eqn. 2 and Table 2, is input for MCNP. (2) 15

28 The fractional yield when summed over all photon energies sums to one. It was found with the aid of Mircoshield, a tool developed by Grove Software [4]. Table 2 gives the energy, emission rate, emission probability per decay and fractional yield, for each energy photon for the decay of Cs-137 (including the decay of Ba-137m, which is assumed to be in secular equilibrium with Cs-137). The tabulated emission rate assumes a Cs-137 activity of 250 Ci. Table 2. Tabulated photon emission probability and activity for Cs 137 Group # E g A g Y(E g ) * * * * * The number of photons that are emitted per decay when summed over all photon energies is the total photon yield per decay (Y), henceforth called simply the total photon yield. 16

29 The total photon yield is used in post-processing the results of MCNP calculations. The exposure rate was calculated from the exposure per source particle from MCNP, using Eqn. 3 (3) 3.2 Range Irradiator Room Specifications The JL Shepherd Range Irradiator is located in a (11.7 x 3.37 x 5.48) meter room which is surrounded by thick concrete walls. It is centered between the side walls with cm on either side with respect to the centerline. The wall behind the irradiator is at a distance of cm while the target wall lies cm from the center of the source. The ceiling in the facility is a dropped panel ceiling whose material has little to no effect on the scatter. The structural ceiling is much higher than the maximum height of the beam; therefore, the ceiling was not included in the model. The room in which the irradiator is located is constructed of various materials. The walls to the left, right and behind the 17

30 irradiator are constructed of concrete block and filled with mortar. In order to simplify the model, Portland concrete was used with the density adjusted downward to The material data for concrete was found using the Compendium of Material Composition Data for Radiation Transport Modeling from Pacific Northwest National Laboratory (PNNL) [5]. The far wall and the floor are both poured Portland concrete with a density of 2.3 and a thickness of cm. There are also lead sheets on the far wall which are meant to decrease scatter from the back wall. A three dimensional representation of the modeled room is depicted in Fig. 9, where the irradiator is depicted as the cylinder and the spheres on the centerline are the detectors used. Also shown in the figure is the coordinate system for the model which is: the positive x axis co-linear with the centerline of the irradiator beam, the z-axis lying perpendicular to the floor and directed upward, and the y-axis lying perpendicular to the left and right walls and directed to the left when viewed from the perspective of beam photons. 18

31 Figure 9. Vised 3D model of Irradiator Room 3.3 MCNP5/MCNPX Developed by Los Alamos National Lab, Monte Carlo N-Particle Transport Code or MCNP, is a computer code used for the modeling and understanding of nuclear processes. The model was created to represent the actual room which contains the irradiator. The room was modeled to be the same dimensions and material as described in Section 3.2. The f4 tally was used along with the de and df cards, which alter the output of the f4 tally. The de card gives values for the photon energy, while the df gives corresponding values of the 19

32 response function. The response function (Eqn. 5.30, Shultis and Faw) is given by (4) where E is the photon energy in MeV, and is given in Table C.5 of Shultis and Faw [7]. The response function above assumes that the energy will be deposited by photons attenuated in air. Using these response functions in MCNP allows one to attain an f4 tally output in units of. 3.4 Discussion of Scatter Calculation A variety of model configurations were used in MCNP to compare with the shadow shield technique. Configurations included two different models of the shadow shield of different complexity. The walls, floor and irradiator were also altered by setting their importances to zero Shadow Shield 20

33 The simplified model of the shadow shield does not include the shield supports. The shield is modeled as a single piece of lead, but in reality, there were four bricks which measure (5.08 x x 20.32) cm configured to be (20.32 x x 10.16) cm. The more complex model of the shadow shield includes the shield support. For both the simple and the complex models, the percent of exposure due to scatter is found using the same method. Exposure rates were first calculated without the shadow shield, yielding calculations were repeated with the shield in place, yielding. Then. The scatter percentage is then found using Eqn Variation of Building Materials To find the exposure rate due to uncollided particles, runs were done with the wall and floor importance equal to zero. To find the scattered percentage of photons scattered from the walls and the floor and the irradiator for the normal unshielded run (corresponding to in Eqn. 1), we use the equation: (5) 21

34 where is the unshielded exposure rate for the base model and is the exposure rate for the modified case, with the components indicated by the subscript m given zero importance (as indicated by the superscript 0). Results are presented with the walls and floor importance set to zero, since any particle which enters a wall and floor cell will be killed and will not have the ability to scatter back to the detector. Calculations of the scatter percentage based on Eqn. 5 are presented for this configuration, in which case the subscript m refers to the wall and floor. Scatter that is calculated with m denoting the wall and floor is henceforth referred to as scatter due to walls and floor, and can be seen in Fig

35 Percent of Exposure Rate Due to Scatter (%) Distance (cm) Figure 10. Percent of Exposure Due to Scatter from Walls and Floor versus Distance Irradiator If one considers the definition of scatter given in ISO Section 6.4, then particles which scatter off the walls, floor, and irradiator alike are considered to contribute to scatter. In order to find the scatter due to the irradiator, walls, and floor a method similar to that described in the previous section was used. The importance of the walls, floor, and irradiator were set to zero, so that any particle that came in contact with those structures would be killed. Exposure rates were 23

36 then recorded at the same distances as for the unshielded base case for which in Eqn. 5 was calculated. Using these simulations, the scatter percentage due to the walls, floor, and irradiator were found using Eqn. 5. The scatter percentage solely due to the irradiator was easily found by subtracting the scatter from the walls and floor from the scatter from walls, floor, and irradiator. 3.5 Error Analysis In MCNP s output file, the values for exposure rates are assigned calculated fractional errors. MCNP provides an estimate of the relative error for each tally. It is important to note that the method for calculating the error in the shielded MCNP runs and the method of computing error in the cases in which the importances of the walls and floor or walls, floor, and irradiator were set equal to zero are very different. The simpler case is when the shadow shield is used in the model, where the scatter percentage is calculated according to Eqn. 1, as the ratio of the exposure rate for the shielded run to the exposure rate for the corresponding unshielded run. The absolute error in the scatter percentage for this case is computed as (6) 24

37 where ε s is the MCNP calculated fractional error in and ε u is the MCNP calculated fractional error in. When looking at the cases where the wall and floor or wall and floor and irradiator importances are set equal to zero, the scatter percentage is calculated according to Eqn. 5 and the calculation of the errors is more complicated. The amount of scattered radiation is calculated from (7) where S % is the percentage of scattered radiation, is the exposure rate for the base (normal) unshielded case and is the exposure rate for an unshielded run with the importances of components m of the geometry set equal to zero. Defining (8) results in the following expression for scattered radiation (9) Since and are very close in magnitude, Eqn. 8 results in errors larger than for the previous case, since the difference of two similar numbers is 25

38 being factored into the equation. The expression for the absolute error ( ) in the scatter percentage for the cases with modified walls is (10) where ε u is the MCNP calculated fractional error in and ε m is the MCNP calculated fractional error in. 26

39 CHAPTER 4 MODELS 4.1 Simplified Shadow Shield Model The simplified MCNP model with the shadow shield includes the room, the irradiator, the detector and the shadow shield placed in front of the detector. The shield support is neglected 4.2 Complex Shadow Shield Model The shadow shield was supported with a wood plank, two steel carts and concrete blocks, as shown in Fig

40 Figure 11. Photograph of Experimental Setup without Lead Shadow Shield The wood plank has dimensions of (18 x x 3.81) cm and is modeled as pine wood. The steel carts were modeled as hollow rectangular prisms, with the front and back sides removed. The carts were modeled with the sides formed of the same steel used to shield the irradiator, and were modeled to have a thickness of cm, as measured. The carts had a depth of cm, height of cm, and a width of cm. The MCNP model of the experimental setup, including the shield support but without the shield itself, is shown in Fig

41 Figure 12. 3D Vised Rendition of Experimental Setup Including the Wood Plank, Steel Carts, and Concrete Blocks. 29

42 CHAPTER 5 RESULTS 5.1 Validation In order to ensure that the model reflected the actual characteristics of the room, a number of checks were conducted. These checks included: 1) comparing the specific gamma constant (Γ) for the source calculated with MCNP with accepted values, and 2) comparing the exposure rates calculated with MCNP with measurements of the exposure rate for points on the centerline of the beam. Regarding check (1), validation for the source modeling in the MCNP calculations was performed by comparing to the accepted value of Γ (0.33) to values of Γ that were calculated using MCNP. The MCNP model was of Cs-137 source photons born in void with a source spatial distribution that is consistent with the spatial distribution of the irradiator source. Regarding check (2) the comparison of MCNP predictions of exposure rates to measured values is not 30

43 absolute, since the results derived from MCNP are given on a per source particle basis. In order to derive exposure rate from the results of MCNP calculations, one must know the source activity. For the irradiator being used, only the nominal activity at the time of manufacture is known, as stated in Section 3.1. To find the current source activity, a Γ of 0.33 was assumed in processing the experimental data, and this assumption led to a calculated source activity of 250 Ci, as stated in Section 3.1. This source activity was then used to calculate an effective specific gamma constant ( Γ e ) from the experimental data that includes the effects of scatter. The source activity does not have to be known to determine Γ or Γ e using MCNP. Specifically, Γ and Γ e were calculated directly from the MCNP results using the equation (11) where is the calculated exposure per source particle and r is the source to detector distance. MCNP calculations yield the exposure per source particle as a result. In order for one to correctly calculate Γ from Eqn. 11, one must know the relationship between decay and source particle production, which is just the total photon yield (Y) which was previously calculated in Section

44 5.1.1 Calculation of Γ Values of Γ obtained using Eqn. 11 with MCNP results were compared to the accepted value of the gamma ray constant for Cs-137 namely (0.33 )). For the calculation of Γ the exposure per source particle was calculated for a source in an infinite void. The resulting calculated Γ versus distance is given in Fig 13. The value of Γ is slightly higher than expected, about to One reason for this may be that the accepted value for the specific gamma constant does not include contributions from x-rays, and the x-rays were included in the MCNP model [6]. 32

45 Gamma (R*m^2*Ci^- 1*h^-1) Distance (m) Figure 13. Specific Gamma Ray Constant vs. Distance of Bare Source Particles in Void Fig. 14 shows the gamma constant of the bare source capsule in air, which includes the stainless steel and aluminum shells, as well as the cesium chloride. The inclusion of the materials within the source capsule dropped Γ from ~0.334 to ~ This shows that there is some absorption in the source capsule. 33

46 Gamma (R-m^2/hr-Ci) Distance (m) Figure 14. Specific Gamma Ray Constant vs. Distance of Source Capsule in Air Comparison of Calculated Base Case and Measured Exposure Rates The comparison of calculated exposure rate for the base case versus distance compared with that of the calibration data for the same geometry can be seen in Fig. 15 and in the data in Table 3. Note that in Table 3 measurements were taken in logarithmic increments from 0.46 to 7 meters, while calculated values are for 1 meter increments from 1 to 10 meters. Since there is not a one to one correspondence between measured and calculated data the agreement between the measurements and the calculations may be difficult to discern. However, the agreement between the measurements and the calculations can be 34

47 readily seen in Fig. 15. When performing a power regression, it was found that the calculated data agreed with the measured data with a correlation coefficient, R 2, value of In conclusion, the model of the JL Shepherd Range Irradiator created in MCNP seems to give a very good prediction of exposure rate versus distance when compared to the measured values of exposure rate versus distance for the nominal 400 Ci source, when the assumed source strength in the post-processing of MCNP data is 250 Ci. Base Case Exposure Rate Measured Exposure Rate 1000 Exposure Rate (R/h) Distance (m) Figure 15. Exposure Rate vs. Distance for Base Case of the Simplified Shadow Shield, Calculated Using Eqn. 3, for the Nominal 400 Ci Source 35

48 Table 3. Exposure Rates vs. Distance for Measured and Calculated Base Cases for the Base Case of the Simplified Shadow Shield using Eqn. 3 Distance (m) Calculated Exposure (R/h) Distance (m) Measured Exposure (R/h) Comparison of Calculated Γ e for Base Case of Complex Shadow Shield with Measured Γ e To be able to see variations from the dependence of, calculated and measured values of Γ e were compared. When the activity was calculated by Herminghuysen, it was assumed that the specific gamma constant was A comparison was done, seen in Fig. 16, contrasting Γ e predicted by MCNP with an encapsulated source for the base case of the complex shadow shield (includes shield support structure in place but no lead brick) with values of Γ computed 36

49 from the measured exposure rates for the same geometry as was modeled. Table 4 displays the values and associated errors for the measured and calculated Γ e. Since the measured values assumed a Γ of 0.33 for the calculation of source activity, it is not surprising that the measured values of Γ e are very close to The calculated values of Γ e vary from to Gamma (R-m^2/ hr-ci) Measured Gamma Modeled Gamma Distance (m) Figure 16. Measured and Calculated Γ e vs. Distance for Base Case of Complex Shadow Shield 37

50 Table 4. Measured and Calculated values of Γ e with distance Calculated Measured Distance Γ e Absolute error in Γ e Distance Γ e Figure 16 shows that for both cases, the calculated and measured values of Γ e fall within 5% of Γ e =0.33, where the ±5% limits are denoted by the dashed lines. 38

51 The increase in the MCNP calculated Γ e at far distances is due to scatter from the walls and floor. The increase in the MCNP calculated Γ e with decreasing distance from the irradiator is due to a cloud of scattered particles due to scatter from the irradiator, which can be seen when a mesh tally is performed. This mesh tally, calculated using the TMESH card, allows the user to physically see the flux density as a 2-D meshed plot. The plot is from the origin of the model (i.e. the center of the source) to 700cm in the x direction, while spanning the width of the room, in the Z=0 plane. The meshed plot shows that at distances greater than approximately 4 to 5 meters the beam conforms to a constant broadening with increasing distance, since the beam edge then becomes a straight line. The cloud of scatter originating from the beam port shield and the irradiator is visible for close distances via Fig

52 Figure 17. Track Length Estimation of Flux on the Z=0 Plane for Base Case of the Simplified Shadow Shield 5.2 Scatter Results With the MCNP model validated, simulations were run for four cases: the simplified shadow shield model, the complex shadow shield model, the case where only scatter from the walls was included, and another case which uses the definition of scatter from ISO Section 6.4 which includes scatter from everything 40

53 except the source capsule. In all cases, S % was calculated using Eqn. 5. The S % calculated using Eqn. 5 were compared to measured data for S % Scatter from the Walls and Irradiator In the model which sets the floor, walls, and irradiator importance equal to zero, the only particles which reach the detector are those solely from the source capsule. The corresponding S % is consistent with the definition of scatter found in ISO 4037 Section 6.4. With the definition of scatter found in ISO Section 6.4, as shown in Fig. 18, it can be seen that the scatter due to the irradiator, walls and floor is much larger than the anticipated value of <5%. The data that was used to calculate S % plotted in Fig. 18 is shown in Table 5. The S % in Fig. 18 may be interpreted as the total scatter extraneous to the source capsule. The S % for the case in which the importance of the walls and floor was set to zero is shown in Fig. 19. The data that was used to calculate S % plotted in Fig. 19 can be seen in Table 6. In this case, is calculated with the wall and floor importance set equal to zero, which means that particles which pass through the walls and floor do not contribute to the exposure and are therefore do not contribute to the S %. It is interesting to notice the large increase of the S % with increasing distance for distances greater than 7 meters. 41

54 25 20 Scatter (%) Distance (m) Figure 18. Scatter Percentage Due to Irradiator, Walls and Floor vs. Distance for the Wall, Floor and Irradiator Importance Set to Zero without the Shield Support Structure, Calculated Using Eqn. 5 42

55 Scatter (%) Distance (m) Figure 19. S % Due to Walls and Floor vs. Distance for the Wall and Floor Importance Set to Zero without the Shield Support Structure, Calculated Using Eqn. 5 43

56 Table 5. Processed Data for S % due to Irradiator, Walls and Floor for the Wall, Floor and Irradiator Importance Set to Zero without the Shield Support Structure, Calculated Using Eqn. 5 Distance (m) Exposure Rate (R/h) Base Case Fractional Error in Exposure Rate Exposure Rate (R/h) Walls/Floor/Irradiator imp:=0 Fractional Error in Scatter Exposure % Rate Absolute Error in Scatter %

57 Table 6. Processed Data for S % Due to the Walls and Floor for the Wall and Floor Importance set to Zero without the Shield Support Structure, Calculated Using Eqn. 5 Distance (m) Exposure Rate (R/h) Base Case Fractional Error in Exposure Rate Exposure Rate (R/h) Walls/Floor imp:=0 Fractional Error in Scatter Exposure % Rate Absolute Error in Scatter % by The scatter due to the irradiator was determined by subtraction as shown (12) It was found that most of the scatter in the room was due to scatter from the irradiator itself. The irradiator was found to have a large contribution to scatter at close distances and when farther out the scatter due to the irradiator still contributed to roughly 9 % of the exposure. The evolution of the scatter due to 45

58 the irradiator as a function of distance can be seen in Fig. 20 along with the evolution of scatter due to the walls and floor. Scatter from Walls and Floor Scatter from Irradiator Scatter (%) Distance (m) Figure 20. S % Due to Irradiator and S % Due to Walls and Floor vs. Distance for the Wall, Floor and Irradiator Importance Set to Zero without the Shield Support Structure, Calculated using Eqns. 12 and Simplified Shadow Shield Model As previously mentioned, the simplified shadow shield model is the model containing the walls, floor, irradiator, source, detector and shadow shield, but not the support structure for the shadow shield. The detectors in the MCNP model 46

59 were placed at 1 meter increments from 1 meter to 10 meters. S % was calculated using Eqn. 1. The scatter results for this case can be seen in Fig. 21, along with the data for the case with scatter due to the walls and floor. Simplified Shadow Shield Measured Values Scatter (%) Distance (m) Figure 21. Calculated S % for the Simplified Shadow Shield Model Calculated Using Eqn. 1 vs. Distance and S % Due to the Walls and Floor without the Shielding Support Structure vs. Distance, Calculated using Eqn. 5 Compared with the Measured S % It is interesting to note that the calculated S % for the simplified shadow shield agrees very well with the corresponding results for the S % due to the walls and floor for distances less than 6 meters. There is less good agreement for distances greater than 6 meters, due to the fact that the shadow shielded 47

60 technique, at large distances, blocks some of the upstream scattered radiation from the walls and floor along with the radiation coming directly from the source that it is intended to block Complex Shadow Shield One would expect the S % to be larger for the model in which the shield support is included than for the simplified shadow shield method. The additional scatter from the wood plank and steel cart should move the scatter curve closer to measured results found in the calibration. When looking at Fig. 22, it can be seen that the addition of the shield support increases the S % by roughly 1 unit for all distances. Again, S % was calculated using Eqn Simplified Shadow Shield Complex Shadow Shield Measured Values Scatter (%) Distance (m) Figure 22. Comparison of S % with Simplified Shadow Shield, Complex Shadow Shield, and Measured Values, Calculated using Eqn. 1 48

61 5.2.4 Conformity to Inverse Square Law (ISO Specified Test) It is also important to check the results of the model to ensure that the S % passes the test provided in ISO Section 6.4, namely that the product of falls within 5% of the inverse square law. Finding the exposure rates require one to assume an activity, which in this case was assumed to be 250 Ci. The exposure rate times the square of the distance for the modeled base case is shown in Fig. 23. Figure 23 shows that every point falls within 5% of the solid line, which represents a perfect inverse square. Note the ±5% limits are denoted by the dashed lines. Ẋr 2 (m2*r/h) Distance (m) Figure 23. Ẋ*r 2 for the Base Case of the Complex Shadow Shield 49

62 CHAPTER 6 CONCLUSIONS 6.1 MCNP Model Considering the results from the MCNP model, and how they compare to the measured values, it can be said that the model is accurate. The beam is symmetric, the exposure rates agree with measured values within 5%, and bare source gamma constant agrees with the tabulated value, 0.33 within 3%. The comparison of the S % extracted from MCNP with the measured S % indicate that the shadow shield technique slightly overestimates the scatter due to the walls and floor for distances less than 7.5 meters, and underestimates the scatter due to the walls and floor for distances greater than 7.5 meters. This is previously presented in Fig

63 6.2 Shadow Shield Technique From the data extracted from the MCNP model that takes into account only the scatter due to the walls, it was found that the shadow shield technique slightly overestimates the scatter due to the walls and floor up to distances of about 7.5 meters as seen previously in Fig 21. Past this distance, the shield itself blocks a portion of upstream scatter, which is why the technique under predicts the scatter at far distances. Fig. 22 shows that the inclusion of the shielding support structure increases the scatter component, which shifts the scatter percentages higher than the scatter percentages found from the simplified shadow shield model. Overall, Fig. 21 shows that the shadow shield technique is a good method for the estimation of S % up to 7.5 meters when scatter is limited to the walls and floor. 6.3 ISO Given the definition of scatter in ISO Section 6.4, Fig. 18 in Section shows that the shadow shield method is not a sufficient method for finding the scatter contribution. The amount of scatter from the irradiator, walls and floor through the entire data range is more than double the acceptable value for scatter, 5 %. 51

64 Recall that both the measured exposure rate data and the calculated gamma rate data passed the test outlined in section 6.4 of ISO One might erroneously conclude that because this test is passed, that the scatter profile should be uniformly below 5%. However, it is shown in Fig. 20 that this is not the case when using the definition of scatter in ISO Section

65 CHAPTER 7 FUTURE WORK Given the results of the study, future work may include designing a process to measure scatter that does not inherently increase scatter due to its implementation. The method currently used at the OEMA includes components such as a wooden plank, additional concrete blocks, and steel carts in the setup of the measurement. One possible solution to the problem is to have the lead shields suspended by a wire, which would eliminate scatter from the wood plank and steel carts. Inherent problems with this system include stabilization of the lead shield, difficulty moving the shield with the detector, and reproducing the same distance between the lead shield and the detector. Perhaps the best solution, though most work intensive, would be to change the test within the ISO standard. As of now, it can be shown that passing the test of having data fall within 5% of a perfect inverse square does not mean that 53

66 one meets the criteria for having S % due to everything except the source capsule be less than 5%. Perhaps a new test can be developed which would be less contradictory. 54

67 BIBLIOGRAPHY 1. ISO, International Organization for Standardization. X and Gamma Reference Radiation for Calibrating Dosimeters and Doserate meters and for Determining Their Response as a Function of Photon Energy Part 1. ISO Herminghuysen, Kevin. Range Calibration Report. Ohio Emergency Management Agency, Radiological Instrument Maintenance & Calibration Facility. March Minnitti, R. Calibration of a 137 Cs γ-ray Beam Irradiator Using Large Size Chambers. National Institute of Standards and Technology, Ionizing Radiation Division Grove Software, Inc. Microshield User s Manual Williams III, R.G, Gesh, C.J., Pagh, R.T. Compendium of Material Composition Data for Radiation Transport Modeling Pacific Northwest National Lab. April Attix, F.H. Introduction to Radiological Physics and Radiation Dosimetry. Wiley-Interscience. New York Shultis, J. Kenneth / Faw, Richard E. Radiation Shielding. The American Nuclear Society. March

68 APPENDIX A MCNP Input Files 56

69 JL Shepherd Irradiator Bare Room c c c **BLOCK 1 ROOM/IRRADIATOR CELLS ** c concrete floor imp:p=1 c right concrete wall imp:p=1 c left concrete wall imp:p=1 c far concrete wall imp:p=1 c back concrete wall imp:p=1 c air filled room #14 #15 #9 #12 #13 #20 #21 #22 #23 #24 #25 #26 #27 #28 #29 #1 #2 #3 #4 #5 #10 #11imp:p=1 c do not track outside of sphere imp:p=0 c irradiator #12 #13 imp:p=1 c far left lead shield imp:p=1 c far right lead shield imp:p=1 c beam port imp:p=1 c SS shield in beam port #12 #20 imp:p=1 c inner lead shield top imp:p=1 c inner lead shield bottom imp:p=1 c target sphere imp:p= imp:p= imp:p= imp:p= imp:p= imp:p= imp:p= imp:p=1 57

70 imp:p= imp:p=1 c ****** BLOCK 2 SURFACES ****** 1 pz c floor surface 8000 pz c floor bottom 8000 px c far concrete wall inner 4 px c far concrete wall outer 5 py c left wall inner 6 py c right wall inner 7 py c left wall outer 8 py c right wall outer 9 pz c top of concrete walls 10 px c back concrete wall inner 11 px c back concrete wall 58uter 20 c/z c irradiator cylinder 58uter 21 c/z c irradiator cylinder inner 22 pz c top of irradiator 23 pz pz 3 50 RPP c lead shield at far wall 51 py py P c Angled Beam Port 61 P c 62 P c 58

71 63 P c 64 px 0 c origin of beam port 65 px 25 c end of beam port 70 sx c outside universe 80 p c SS shield on beam port 81 p c 82 p c 83 p c 90 sx c target 91 sx sx sx sx sx sx sx sx sx c 101 RPP mode p ctme 1200 c nps $nps at 1 meter is LOST c *********SOURCE********* sdef ERG=d1 POS= AXS=0 0 1 RAD=d2 EXT= d3 PAR=2 c photon energies si1 L c photon probabilities sp si sp si sp

72 c ****TALLIES******* fc14 Track Length Estimation of Flux 1 f14:p 20 fc24 Track Length Estimation of Flux 2 f24:p 21 fc34 Track Length Estimation of Flux 3 f34:p 22 fc44 Track Length Estimation of Flux 4 f44:p 23 fc54 Track Length Estimation of Flux 5 f54:p 24 fc64 Track Length Estimation of Flux 6 f64:p 25 fc74 Track Length Estimation of Flux 7 f74:p 26 fc84 Track Length Estimation of Flux 8 f84:p 27 fc94 Track Length Estimation of Flux 9 f94:p 28 fc104 Track Length Estimation of Flux 10 f104:p 29 c de c df 8.70E E E E E E E E E E E E E E E E E-10 c ***********materials cards************** c c m $Averaged Portland Concrete

73 c m $steel c m $lead c m $ air c print -160 prdmp 2j 1 61