Physics:

 

 

Hot Particle Dosimetry, Part-I:

Enhanced EGSnrcMP 

 

Dose Estimates Over EGS4 For A 60Co Hot Particle, Compared To  Imaging Thermoluminescence Detector Measurements

 

by *Ibrahim E. Othman, **Mohamed Amer and  ***Abobaker M. Elhouni

*Physics Department, Faculty of Science, Misurata University
**Biomedical Engineering Department, Libyan Academy

*** Electric and Electronic Engineering Department , Libyan Academy

 

Abstract— Hot particles are ubiquitous on all nuclear power reactor systems and in the environment. They can give rise to highly non-uniform spatial dose distributions and present a potential radiological hazard particularly to the skin and lungs. The development of dose limits for hot particles, and their implementation, depends on the availability of validated dose measurement methods and calculation techniques. Preliminary investigations of dose distributions around 60Co hot particle using the Imaging Photon Detector (IPD) and Electron Gamma Shower code (EGS4) showed some inconsistencies for the dose comparisons at some various areas and depths. In the present work, the comparisons between measurements and calculations have been extended to include further depths and very much enhanced simulations. An nIntel quad-processor linux workstation running the new EGSnrcMP was extensively used to simulate the dose estimates using both LiF ad CaSO4:Dy dosemeters. EGSnrcMP code system with the new implementations of the underlying physics and coding based on PRESTA II showed great enhancements over the old EGS4 code for the doses in three dimensions (3D-Dose distributions). Using PRESTA-I corrections and the ICRU 37 stopping power data, the EGS4 integrated beta spectrum is approximately the same for two different PRESTA-I inputs, which in both cases represents an underestimate of about 11% compared with the measured value. In the case of EGSnrc, this 11% reduced to ±5%. This assured the existence of some shortcomings for the old EGS4 even with the careful usage of PRESTA-I routine. For the dose distribution measurements LiF gives good agreement with calculations at small areas for both shallow and deep depths for both EGS4 and EGSnrc. However, for large areas the EGS4 calculated doses are less than the measured doses and the agreement gets worse with increasing depth while the EGSnrc results are much better. For CaSO4:Dy, the results are different from those for LiF. Because CaSO4:Dy dosemeters are thicker (6.1 mg cm-2) and  more sensitive than LiF (3.6 mg cm-2), the effect of poor count rate statistics at large areas is not the same as for LiF dosemeters.

 

INTRODUCTION

The evaluation of the hazard posed by very small (dimension < 1 mm diameter) radioactive particles has become known as the "hot particle" problem(1,2). Hot particles present a potential radiological hazard in all nuclear reactor systems and in the environments(3,4). Extensive studies of hot particles and their distribution in nuclear power plants around the world made in the USA and Bulgaria reported hot particles composed of both irradiated nuclear fuel and neutron activation metallic hot particles(5,6). Hot particles similar to 60Co particles of concern within reactor facility were found on islands and along the shoreline of the Hanford Reach of the Colombia River, USA(7). As an environmental impact, Mixed fission product hot particles of diameters up to  several mm have also been observed in the fallout from the Chernobyl nuclear power plant accident(8-13). In the UK, fuel fragments composed mainly of mixed fission products have been found in the environs of the prototype fast reactor at Dounreay in Scotland.  The activities of these particles ( up to ~ 108 Bq) has led to concern regarding possible severe biological effects following skin contact or ingestion.  Fragments of irradiated nuclear fuel in the Dounreay local environment have been shown by the Scottish Environment Protection Agency, SEPA, 1998. These and many similar studies have highlighted the importance of hot particles as a routine radiological hazard which has been recognized in European and American power reactors. The basis of future suitable dose limits for hot particle exposures will be the revised analysis of results from past and ongoing radiological studies(14,15). The interpretation of dose response data for various biological effects observed in animals or cell systems requires validated measurements and calculations of the dose distributions around hot particles(16-19). For model 60Co, 170Tm and 106Ru/Rh  hot particle sources (Beta particle energy (Emax=0.318, 0.986 and 3.54 MeV, respectively), extensive dose measurements were made using extrapolation chamber (ECH) and radiochromic dye film (RDF) and dose calculations using the Monte Carlo code MCNP were achieved(2, 19, 20). The two experimental techniques ECH and RDF agree well within ±20% at the surface but exhibit an increasing deviation (RDF/ECH > 1) with depth where the gamma component of the dose is dominant. Predictions from the Monte Carlo MCNP code generally agree with the measurements within ±1.25% at the surface but, except in the case of ECH estimates using a 3mm diameter electrode, differences of ±40% are seen at greater depth. Therefore, alternative techniques had to be developed. Using novel thin clear fused quartz (CFQ) chips, we described the applicability of a thermoluminescence TL imaging photon detector (IPD) for measuring the spatial and depth dose distributions around a model 60Co hot particle(21). The CFQ chips presented a good homogeneity for the spatial dose distribution measurements. The only limitation is that the thickness and density preclude their use for low energy beta particles emitted from 60Co because the mass thickness is similar to the beta particle range in this case. Othman, I E(22) extended the use of TL imaging using ultra thin LiF and CaSO4:Dy powder deposited on Kapton films. The use of LiF and CaSO4:Dy TL dosemeters for 60Co will be described here. Comparisons for EGS4 and enhanced calculations using EGSnrcMP will be presented.

 

Materials and Methods

Irradiation with 60Co model hot particle

The model hot particles used in the present study is a standardized model hot particle sources of a common design shown schematically in Fig. (1a). Detailed information about the source was given in reference (18). It was designated as ECO (245 ± 5 µm diameter). The main elemental composition is, Co (%64.47), Cr (%28.4), Mo(%5.72), Si (%0.71), Fe (%0.21) and C (%0.3). It has a density of 8.4  ± 0.2  gm cm-3. The aluminum window  (purity= 99.99%, density=2.7 gm cm-3) used for both sources  was 15  µm thick. The mount recess depth for ECO is 250 µm and the recess diameter is 1.5 cm. The main components of this Al mount are Al (93.6%), Cu (5%) and Fe (1.4%). For irradiating the ultra thin LiF and CaSOo4:Dy, groups of stacks of 10 dosemeters were put in contact with the model hot particle source for different exposure time. For the source-stack alignment, a small jig made of perspex has been used to encompass both the source and the stack. For the Monte Carlo simulation using different versions (EGS4 and EGSnrcMP), concerning the absorbed dose, central cylindrical elements were targeted for the absorbed dose calculations. To calculate the spectrum, the experimental  set-up shown in Fig. (1b) was simulated and the detector response was calculated in 10 KeV bins.

 

 

 

(a)

(b)

Figure 1: (a) Schematic diagram of irradiation setup for LiF and CaSO4:Dy dosemeters using a 60Co model hot particle inside a perspex jig. (b) Cross section view of the experimental setup of the 60Co model hot particle used for electron spectra measurements. The Si(Li) detector diameter is 1.8 cm.

 

Thermoluminescence Imaging Photon Detector (IPD)

The IPD system at the University of Oxford allows the measurement of the thermoluminescence (TL) emitted by phosphors to be imaged with a spatial resolution of ~33µm. The IPD was first used by Smith et al.(21) and Wheeler (22). Detailed information about the operation of the IPD can be found in these two sources. The system consists of a micro-channel plate detector device (MCP), optics necessary to focus the image on the photo-cathode, a TL heater glow oven and a computer for image acquisition. The construction of the MCP enables the determination of the coordinates of the position where a photon hits the photo-cathode. In practice the dynamic range on the Oxford reader is presently limited by a 106 total count restriction for the whole image. Therefore, if the sample is very bright, then neutral density filters must be used to attenuate the emitted light. In the present work, the heating rate of the heating strip (1°C s-1) and the read-out temperature range were set manually on the temperature controlling unit.

 

For the dose measurements, two types of novel ultra thin TL phosphors were used,  LiF and CaSO4:Dy powder deposited on Kapton film. The films were manufactured by and obtained from CORAD (Montpellier, France) using ultra fine powder which we separated from bulk using a sedimentation technique. The sensitive phosphor layer thickness is 3.6 mg cm-2 for LiF and 6.1 mg cm-2 for CaSO4:Dy dosemeters. The backing thickness for both dosemeters is 7.6 mg cm-2 of Kapton tape. The heating and the acquisition of the image are operated independently so the acquisition of the image can be started at any chosen temperature. This means that the image from any given temperature range (e.g. a specific TL glow curve peak region) can be acquired. The acquired image can be displayed on the computer screen and quantitative measurements can be obtained by integrating chosen regions of interest. The glow curve for any region of interest can be examined and the image for a restricted temperature region can be viewed. The data can also be exported in different formats, thus allowing automated analysis of the image.  For the IPD data acquisition, we have used a software package (RADDS) for the evaluation of radial dose distributions and for the calculation of doses averaged over specific areas around the hot particle source. Radial Dose Distribution Software (RADDS) used was developed originally for the analysis of hot particle images in radiochromic dye film using a scanning microdensitometer. In the IPD system the calculated resolution of the image is ~33µm/pixel. This was determined by imaging a novel thin (100 µm thick) clear fused quartz (CFQ) dosemeter of a known length and measuring the corresponding number of pixels on the image. Two groups, 30 dosemeters each, of LiF and CaSO4:Dy were used to study the reproducibility, glow curve shape, fading and annealing behaviour.

 

The Monte Carlo EGSnrc Code System

The EGS4, Electron Gamma Shower code (version 4), code is a general purpose Monte Carlo (MC) code system that simulate coupled transport of electrons and photons with energies of several TeV down to several KeV(24). The Parameter Reduced Electron Step Transport Algorithm, PRESTA-I, was used in the simulations to avoid step size artefacts(25) of low energy electrons. In EGSnrc a completely new electron transport algorithm is used which removes all known shortcomings of the EGS4/PRESTA-I algorithm(28) .

 

In the present work, the experimental set-up of the source and target details (Fig 1) has been exactly simulated. The spherical 60Co hot particle source was modeled as a surface loop and the positions and directions of created particles were randomized and normalized isotropically in all directions. A direct method was used for the energy sampling in which the particle energy is represented as a function of the normalized cumulative probability distribution of the 60Co theoretical spectrum. Two different user codes were written to simulate the experimental set-up, one for beta spectrua and the other for the absorbed dose.  The default $CALL_HOWNEAR macro required by PRESTA-I was overwritten to match our complicated geometry of the actual experiments. Step size ESTEPE of 1% with the default PRESTA-I were used. Each electron was followed down to a cut-off energy (0.512 MeV) which includes both the electron rest mass energy (0.511 MeV) and its kinetic energy. Data for stopping power were created using ICRU 37(26). The calculations of beta spectra were carried out for the sake of comparison with the beta spectra from the  60Co hot particle measured at the Nuclear Physics Department of The University of Oxford and for testing the sampling technique used inside the SHOWER loop. The beta  and gamma radiation components of the absorbed dose were simulated separately and then added to obtain the total absorbed dose for comparison with the measured one.

 

RESULTS AND DISCUSSIONS

IPD Measurements

The sensitivity of CaSO4:Dy among different dosemeters within the batch is good (< ±4%). The individual dosemeter cannot however be used for multiple successive dose measurements because the necessary high annealing temperature (420 oC) damages these dosemeters. In contrast, the sensitivity of each LiF dosemeter vary  within a factor of three. However, individual LiF dosemeters could be annealed at only 300 oC to give a good individual reproducibility (~ ±3%) for successive use. Therefore, in the present work, it was decided to use CaSO4:Dy dosemeters only once and to use them in batches of 12.  LiF dosemeters were used individually, with individual calibration. All the measurements were carried out at 6 hours after irradiation to allow for the rapid fading of low temperature TL peaks. The read-out temperature ranges were chosen to be 150-250 oC for LiF and 170-300 oC for CaSO4:Dy to encompass the main glow peaks. Fig 2a shows a typical IPD image for clear fused quartz CFQ sample only for the sake of demonstration. Fig 2b is the radial dose distribution across the whole image. For the calibration, groups of 25 dosemeters from each batch were irradiated, in terms of absorbed dose in tissue, with a teletherapy 60Co gamma radiation source at the Research Institute, Churchill Hospital, Oxford. The dose rate of the source, as measured ionometrically using a 0.6 cm-3 graphite ionisation chamber calibrated at the National Physical Laboratory (NPL), was 1.056 Gy/min. Figs 2c and 2d shows the calibration curves of LiF and CaSO4:Dy dosemeters, respectively. The minimum detectable dose (corresponding to 1 count per pixel) for the IPD system in the current configuration is about 1.0 Gy for LiF and 0.04 Gy for CaSO4:Dy. A stack of dosemeters was irradiated using the 60Co hot particle  in the perspex jig as shown in Fig 1.

 

OLE-object

 

 

 

 

 

 

 

(c)

OLE-object

 

 

 

 

 

 

 

(d)

Figure 2: (a) A typical image obtained after a 15-minute exposure of a CFQ sample where the dimensions are represented in pixels (1 mm = 30 pixels). The bright area produced by the irradiation with hot particle is clearly visible. (b) dose distributions across the image. (c) calibration curve of LiF. (d) calibration curve of CaSO4:Dy. TL intensity was integrated over the temperature range 150-250 oC.

 

Fitting results from these calibration curves along with the IPD images were analysed using a specially developed Radius Dose Distribution Software (RADDS) to obtain the dose distributions around the hot particle.

 

EGS Calculations

Table (1) shows the calculated integrated beta spectra for 60Co by using MCNP, EGS4, EGSnrc and the Si(Li) measured spectrum. This represent the electron flux emitted from the surface of the hot particle. The standard errors are for 20 million histories for MCNP, 30 million for EGS4 and 150 million for EGSnrc. The measurements of the spectra have a systematic uncertainty in the source activity (4%) and in the source-detector solid angle (2%). Other possible source of systematic errors arise from the correction for Compton electrons produced in the 5mm aluminum absorber and from the possibility of small deviations in the location of the source from run to run. Such effects should be no greater than the statistical errors. The EGS4 results seem to agree well with the MCNP emulated version of the Integrated Tiger Series (ITS) where both codes are originally targeting the electron simulation. The results  from both codes underestimate the spectra for about 11% compared to the measurements. On the other hand, the results using the default version of MCNP(4B) agree consistently with the measurements, while that using the earlier verison MCNP(4A) overestimate the spectrum by about 10%. Overall, both the EGS4 and MCNP Monte Carlo codes give a good agreement (±10%) with the measurements. Therefore, this benchmark study provides a good basis for applying the EGS4 PRESTA-I and apparently better EGSnrc  for further investigation of 60Co model hot particle regarding the estimation of spatial and depth dose distributions.

 

Table 1: Measured and calculated integrated beta spectrum for 60Co model hot particle source (COVI) using the experimental setup shown in Fig 1. Values given are number of electrons per disintegration.

MeasuredValue

MCNP

EGS4

EGSnrc

4A

(Default)

4B

(Default)

ITS

(Default)

ESTEPE

(Default)

ESTEPE

(0.001)

Default PRESTA

3.31E-04

(±5%)

3.59E-04

(±1.2%)

3.36E-04

(±1.1%)

2.93E-04

(±0.95%)

3.00E-04

(±0.92%)

2.99E-04

(±0.91%)

3.20-04

(±0.61%)

 

Fig 3a shows the beta spectrum emitted from 60Co hot particles of different diameters using EGS4 and EGSnrc. The figure includes the cases of the model hot particle and point source emulation that gives a good agreement with the theoretical 60Co Spectrum. This good agreement gives an indication that the sampling technique in particular, and that the EGS4 and EGSnrc Codes in general, are working efficiently. Fig 3b shows a comparison between the measured and calculated spectrum on the basis of experimental set-up.

 

(a)

(b)

Figure 3: (a) calculated beta spectra emitted from a 60Co source sphere with different sizes using EGS4. (b) comparison between measured and calculated (ESTEPE=0.001 and PRESTA-I=Default) spectra for 60Co model hot particle. The figure also shows the new EGSnrc calculations.

 

There is some fluctuation in the energy bins for the calculated spectrum but in terms of overall integration there was a ~11% under estimation was found for the EGS4 calculation compared with both the  measurements and EGSnrc which is more efficient with under estimation of only 5%. ( Villarreal et al found 10% under estimation with MCNP4B+ITS, 11% over estimation with MCNP4A and 1% over estimation with MCNP4B. 

 

Figs 4a and 4b show the spatially calculated dose distributions at different depths for LiF and CaSO4:Dy dosemeters, respectively. The doses are the summation of beta and gamma components emitted from the model 60Co hot particle.

Figs 4c and 4d show the calculated beta/gamma ratio of spatial and depth dose distributions around 60Co model hot particle for LiF and CaSO4:Dy dosemeters, respectively.

 

(a)

(b)

(c)

(d)

Figure 4: Calculated spatial dose distributions around 60Co model hot particle at different depths depths using (a)  LiF and  (b) CaSO4:Dy. Numbers in the legend are depths in mg cm-2. The calculation were made using EGSnrc. Figs (c) and (d) shows the calculated beta/gamma ratio of spatial and depth dose distributions around 60Co model hot particle for LiF and CaSO4:Dy dosemeters, respectively.

 

Dose Calculations Against Measurements

Direct comparisons between EGS4/EGSnrc calculations and IPD measurements of the dose distributions around 60Co model hot particle were done for LiF and CaSO4:Dy dosemeters. These comparisons are shown in Fig 5 for the two different dosemeters.

 

 

 

 

 

 

 

(a)

(b)

(c)

(d)

Figure 5: Comparisons between dose calculations [(a) EGS4 and (b) EGSnrc] and measurements [IPD] for LiF while (c) and (d) are for CaSO4:Dy. Numbers in the legend are depths in mg cm-2.

 

For LiF the agreement is good for small areas for all investigated depths. The LiF dosemeters (3.6 mg cm-2) are thinner and ~26 times less sensitive than CaSO4:Dy (6.1 mg cm-2) so poor count rate statistics at large area and higher depths are to be expected. The poor count rate statistics due to beta particles is compensated by a significant gamma component in the case of 60Co. This gamma component represents more than 50% of the total dose at large areas and higher depths. An independent investigation of the optical system, using a uniform irradiation from a large area 90Sr beta source, showed a 35% gradual decrease, in the radial direction across the field of view, in the intensity of the detected light by the imaging detector. Correction for this factor was not possible with the current setup of the IPD system since it is not possible to reproducibly position the sample at the center of the field of view. For CaSO4:Dy, the results are different from that of LiF. Because CaSO4:Dy dosemeters are thicker and more sensitive, the effect of poor count rate statistics at large area is not observed. The trend in the dose curves represents an overestimation of EGS4 calculated doses at small areas and underestimation of dose at large areas. This is similar to that previously obtained when the Monte Carlo MCNP was used to estimate the dose distributions around the same 60Co model hot particle. Furthermore, the homogeneity of CaSO4:Dy powder is not as good as for LiF and this is probably the cause of the dose overestimation at small areas. Overall, for LiF at shallow depths (1.8 and 13 mg cm-2), the ratio EGS4/IPD varies between 1.05 and 0.85 for small (<10-6 cm2) and large (1.0 cm2) areas, respectively. For measurements at greater depths (24.2 & 35.4 mg cm-2) the ratio EGS4/IPD is between 1.1 and 0.7. For CaSO4:Dy, there are similar trends in the ratio EGS/IPD but with slightly larger differences. For example, near the surface (3.05 mg cm-2) the ratio EGS/IPD varies from 1.0 to 0.8. At a depth of 44.15 mg cm-2 the ratio varies between 1.55 and 0.6. In the case of routine skin dosimetry in radiological protection, the relevant dose evaluation is over an area of 1 cm2 at a depth of 7 mg cm-2. For this particular case, the ratio EGS/IPD is ~ 1.0 for LiF and ~0.8 for CaSO4:Dy. This figure is similar to the MCNP/GAF ratio(2).

 

CONCLUSION

Spatial and depth dose distributions around a cobalt  60Co model hot particle were measured using the imaging photon detector (IPD) and ultra thin LiF and CaSO4:Dy TL dosemeters. Dose distributions due to gamma radiation and beta particles  emitted by a 60Co model hot particle and its beta particle spectra were calculated using the Monte Carlo electron gamma shower EGS4 and EGSnrc codes. The integrated beta spectrum for the two different PRESTA-I inputs is approximately the same, which in both cases represents an underestimate of about 11% compared with the measured value. The new EGSnrc underestimate the spectrum with only ~6%.

 

Ultra thin LiF (3.6 mg cm-2) and CaSO4:Dy (6.1 mg cm-2) compared to RDF have greater sensitivity, shorter exposure times, lower minimum detectable dose. For practical routine radiological protection it is necessary to evaluate the skin dose as an average over an area of 1 cm2 at a depth of 70 μm.  In this case EGS and IPD measurements using LiF dosemeters agree to within ~ ± 5%.  IPD measurements using CaSO4 are about 20% less than calculations. There is no absolute standard for hot particle dose measurements or calculations. The extrapolation chamber, which is used as a standard measurement techniques for large area beta source dosimetry, cannot be used as a definitive primary standard for hot particles because of the highly non-uniform exposure, which precludes the use of the Bragg-Gray relationship.  Measurements have now been carried out on 60Co test hot particle sources using extrapolation chamber (with corrections for non-linearity) radiochromic dye film, and LiF and CaSO4 thermoluminescence dosimetry (IPD). Calculations have been made using EGS (PRESTA) MCNP4A, MCNP4B, and MCNP4A+ITS. The default setting for each Monte carlo code appears to give the best results, when evaluated in terms of calculated and measured beta spectra emitted from the hot particle. MCNP4B predicts the beta spectrum to within 1%. The other codes to within 10%.  Agreement decreases with depth for all codes.  Comparison between measurements and calculations over the whole range of areas and depths measured in various studies do not fit any systematic pattern.  In broad terms the agreement is within ~ ± 25% over a wide range of areas and depths. Agreement decreases with depth for all codes.  Comparison between measurements and calculations over the whole range of areas and depths measured in various studies do not fit any systematic pattern.

 

Using EGS4, there was a trend in the curves describing this agreement represented by an over estimation of calculated dose compared with measured dose at small area and an under estimation of calculated compared to measured dose at large areas. This behaviour is similar to that previously obtained when the Monte Carlo MCNP was used to estimate the dose distributions around the same 60Co model hot particle. However, with better platform, fast computing, we could run much more histories and the trend seems to disappear. So, the trend was obviously due to a lack of enough histories. In conclusion, both techniques have been successfully applied to evaluate dose distributions around hot particles. For the EGSnrcMP simulation,  the results are much better than the EGS4 ones. For the ICRP/ICRU recommended skin dose, at a depth of 0.07 mm (7 mg cm-2) averaged over an area of 1 cm2, the calculated to measured integrated dose ratio using EGSnrcMP is about 1.0±0.03 for both CaSO4 and LiF, respectively.

 

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