Comparison of measured and calculated spatial dose distributions for a bench-mark 106Ru/106Rh hot particle source.

This study is a part of a programme of research to provide validated dose measurement and calculation techniques for beta emitting hot particles by the construction of well-defined model hot particle sources. This enables parallel measurements and calculations to be critically compared. This particular study concentrates on the high-energy beta emitter, (106)Ru/(106)Rh (Emax = 3.54 MeV). This source is a common constituent of failed nuclear fuel, particularly in accident situations. The depth dose distributions were measured using radiochromic dye film (RDF); an imaging photon detector coupled to an LiF thermoluminescent dosemeter (LiF-IPD) and an extrapolation ionisation chamber (ECH). Dose calculations were performed using the Monte Carlo radiation transport code MCNP4C. Doses were measured and calculated as average values over various areas and depths. Of particular interest are the doses at depths of 7 and 30-50 mg cm(-2), and averaged over an area of 1 cm2, as recommended by the International Commission on Radiological Protection for use in routine and accidental over-exposures of the skin. In this case, the average ratios (MCNP/measurement) for RDF, ECH and LiF-IPD were 1.07 +/- 0.02, 1.02 +/- 0.01 and 0.83 +/- 0.16, respectively. There are significantly greater discrepancies between the ECH and LiF-IPD measurement techniques and calculations-particularly for shallow depths and small averaging areas.

Dose distribution measurements and calculations for Dounreay hot particles.

Discrete fragments of irradiated nuclear fuel have been discovered on the foreshore at the Dounreay nuclear site in Scotland, offshore on the seabed and at nearby beaches which have public access. The fragments contain mainly (137)Cs and (90)Sr/(90)Y and for particles recovered to date, (137)Cs activities are within the range of 10(3) to 10(8) Bq. The most active particles found at Sandside Beach contain approximately 3 x 10(5)Bq (137)Cs. Direct measurements of the spatial dose distributions from 37 fuel fragments were measured in detail for the first time using radiochromic dye film as part of a national evaluation of the associated potential radiological hazard. Monte Carlo code calculations of the doses are in good agreement with measurements, taking into account variations to be expected due to differences in shape and the increasing importance of self-absorption for the larger, more active fragments. Dose measurements provide little evidence for wide variations in the (137)Cs:(90)Sr/(90)Y ratio between fragments. Specific attention is given to the evaluation of skin dose, averaged over an area of 1 cm(2) at a depth of 0.07 mm, since this is of major radiological concern. There is no obvious dependence of skin dose on the site of origin of the fragments (foreshore, seabed or beaches) for a given (137)Cs activity level. A dose rate survey instrument (SmartION) was shown to provide a rapid and convenient method for skin dose assessment from fuel fragments in the (137)Cs activity range measured (2 x 10(5) to 2 x 10(7) Bq). A conversion factor multiplier of 240 can be applied to the open window SmartION scale reading to estimate the skin dose rate within +/-25%.

Enhanced biological effectiveness of low energy X-rays and implications for the UK breast screening programme.

Recent radiobiological studies have provided compelling evidence that the low energy X-rays as used in mammography are approximately four times--but possibly as much as six times--more effective in causing mutational damage than higher energy X-rays. Since current radiation risk estimates are based on the effects of high energy gamma radiation, this implies that the risks of radiation-induced breast cancers for mammography X-rays are underestimated by the same factor. The balance of risk and benefit for breast screening have been re-analysed for relative biological effectiveness (RBE) values between 1 and 6 for mammography X-rays. Also considered in the analysis is a change in the dose and dose-rate effectiveness factor (DDREF) from 2 to 1, women with larger than average breasts and implications for women with a family history of breast cancer. A potential increase in RBE to 6 and the adoption of a DDREF of unity does not have any impact on the breast screening programme for women aged 50-70 years screened on a 3 yearly basis. Situations for which breast screening is not justified due to the potential cancers induced relative to those detected (the detection-to-induction ratio (DIR)) are given for a range of RBE and DDREF values. It is concluded that great caution is needed if a programme of early regular screening with X-rays is to be used for women with a family history of breast cancer since DIR values are below 10 (the lowest value considered acceptable for women below 40 years) even for modest increases in the RBE for mammography X-rays.

Nonstochastic effects of different energy beta emitters on the mouse skin.

The effect of irradiating varying areas of mouse skin from 860 down to 0.8 mm2 with different energy beta emitters was studied to clarify protection problems of localized doses to the skin. Both 90Sr and 170Tm sources show area effects for dose-response curves. The 90Sr doses that produced moist desquamation in 50% of irradiated fields (MD-50 doses) were 22, 42, 70, and 1000 Gy for 400-, 95-, 20-, and 0.8-mm2 sources. The MD-50 doses for 170Tm were 50, 54, 90, and 170 Gy for 860-, 64-, 20-, and 3.1-mm2 sources. Thus for the larger 170Tm sources there is much less area effect. There was no significant difference in effect between the different energy 90Sr and 170Tm sources for moist desquamation. A simple hypothesis based upon the repopulation of epithelial cells from the edges of the irradiated field and/or from surviving follicle basal cells can explain these area and energy effects in the mouse and in parallel pig skin experiments. The doses needed for 50% of the mice to show ulceration after 64-, 20-, and 3.1-mm2 170Tm sources were 260, 550, and 8300 Gy, respectively, while those for 90-, 20-, and 0.8-mm2 90Sr sources were 150, 210, and 3100 Gy. Thus there is a definite area and energy effect for these sources for this deep dermal damage. The steep rise in dose needed to produce given skin reactions for the smallest area (0.8 mm2) should reassure those faced with assessing the hazard of sub-millimeter-sized particles in/on human skin.

Nonstochastic effects of different energy beta emitters on pig skin.

Circular areas of pig skin from 1- to 40-mm diameter were irradiated with beta emitters of high, medium, and low energies, 90Sr, 170Tm, and 147Pm, respectively. The study provides information for radiological protection problems of localized skin exposures. During the first 16 weeks after irradiation 90Sr produced a first reaction due to epithelial cell death followed by a second reaction attributable to damage to the dermal blood vessels. 170Tm and 147Pm produced the epithelial reaction only. The epithelial dose response varied as a function of beta energy. The doses required to produce moist desquamation in 50% of 15- to 22.5-mm fields (ED50) were 30-45 Gy from 90Sr, approximately 80 Gy from 170Tm, and approximately 500 Gy from 147Pm. A model involving different methods of epithelial repopulation is proposed to explain this finding. An area effect was observed in the epithelial response to 90Sr irradiation. The ED50 for moist desquamation ranged from approximately 25 Gy for a 40-mm source to approximately 450 Gy for a 1-mm source. The 5-, 9-, and 19-mm 170Tm sources all produced an ED50 of approximately 80 Gy, while the value for the 2-mm source was approximately 250 Gy. It is also suggested that the area effects could be explained by different modes of epithelial repopulation after irradiation. After high energy beta irradiation repopulation would be mainly from the field periphery, while after lower energy irradiation repopulation from hair follicle epithelium would predominate.

Late nonstochastic changes in pig skin after beta irradiation.

Late radiation-induced changes in pig skin have been assessed following irradiation with beta-rays from a 22.5- or 15-mm-diameter 90Sr/90Y source and a 19- or 9-mm-diameter 170Tm source. Late damage, in terms of dermal atrophy, was assessed 2 years after irradiation from measurements of dermal thickness in irradiated and normal skin. After 90Sr irradiation maximum atrophy, a dermal thickness of 40-50% of the control value, occurred at a dose of approximately 40 Gy from the 22.5-mm source and approximately 75 Gy from the 15-mm source. In the case of 170Tm the 19- and 9-mm sources produced similar degrees of atrophy at equal doses. Maximum atrophy occurred at approximately 70 Gy, when the dermis was approximately 70% of the thickness of normal skin. Significant late tissue atrophy was seen at doses, from both types of radiation, which only produced minimal erythema in the early reaction. Such late reactions need to be taken into account when revised radiological protection criteria are proposed for skin.

Time- and dose-related changes in the thickness of skin in the pig after irradiation with single doses of thulium-170 beta particles.

Time-related changes in skin thickness have been evaluated in the pig using a noninvasive ultrasound technique after exposure to a range of single doses of 0.97 MeV beta particles from (170)Tm plaques. The reduction in relative skin thickness developed in two phases; the separation into two phases was statistically justified only after 120 Gy (P = 0.04). The first phase was between 12 weeks and 24 weeks after irradiation. No further changes were seen until 48-60 weeks after irradiation, when a second phase of skin thinning was observed. No further changes in relative skin thickness were seen in the follow-up period of 104 weeks. The timing of these phases of relative skin thinning was totally independent of the radiation dose; however, the severity of each phase of radiation-induced skin thinning was related to the dose. The pattern of changes was similar to that reported previously after irradiation with 2.27 MeV beta particles from (90)Sr/(90)Y, but the degree of dermal thinning was less for a similar skin surface dose. From a comparison of the depth-dose distribution of the beta particles from the two radionuclides, it was concluded that the target cell population responsible for both the first and second phase of skin thinning in pig skin after irradiation may be located at approximately 800 microm depth. This corresponds to an area in the reticular dermis in pig skin and may be the appropriate site at which to measure the average dose to the dermal tissue.

Fitting the Armitage-Doll model to radiation-exposed cohorts and implications for population cancer risks.

The Armitage-Doll model of carcinogenesis is fitted to Japanese bomb survivors with the DS86 dosimetry and to three other radiation-exposed cohorts. The model is found to provide an adequate description of solid cancer incidence and also, to a lesser extent, of that of leukemia as a function of radiation dose when up to two radiation-affected stages are assumed. For non-leukemias the optimal model is one in which there are two radiation-affected stages separated by two additional stages. In the case of leukemia one radiation-affected stage or two adjacent stages provide suitable fits. There appear to be significant differences between the optimal models fitted to each cohort, although there is no heterogeneity within the Japanese data set by sex, by cancer type, or by age at exposure. Low-dose and low-dose-rate population risks for a population having the cancer and overall mortality rates of the current UK population are calculated on the basis of the optimal models fitted to the Japanese data to be about 8.3 x 10(-2) excess cancer deaths person-1 Sv-1, 10.1 x 10(-2) radiation-induced cancer deaths person-1 Sv-1, or 1.40 years of life lost person-1 Sv-1. Risks for a population having the mortality rates of the current Japanese population are about 6.5 x 10(-2) excess cancer deaths person-1 Sv-1, 7.8 x 10(-2) radiation-induced cancer deaths person-1 Sv-1, or 0.89 years of life lost person-1 Sv-1. It is a feature of the Armitage-Doll model, and other multistage models of carcinogenesis, that if radiation acts at more than one stage then (inverse) dose-rate effects may arise as a result of interactions between the effects of a protracted dose at the various radiation-affected stages. However, it is shown in this paper that these three measures of cancer risk in general display fairly slight dependence on administered dose in the range 0.001 to 1.0 Sv and on the length of the time over which the dose is administered in the range 1 to 100 years. Dose-rate effects resulting from the protraction of a radiation exposure over many years acting on (the same) cells at various stages of a multistep process of carcinogenesis are therefore expected to be slight. Dose-rate effects which have been observed in epidemiological studies and cellular radiobiology may thus find their explanation in other phenomena such as short-term intracellular repair.

Time variations in the risk of cancer following irradiation in childhood.

The Japanese atomic bomb survivors and three other cohorts of children exposed to radiation are analyzed, and evidence is found for a reduction in the radiation-induced relative risk of cancers other than leukemia with time following exposure. Multiplicative adjustments to the excess risk either of the form exp[-delta.(time since exposure)] or of the form [time since exposure] gamma give equivalent goodness of fit. Using the former type of adjustment an annual overall reduction of 6.9-8.6% in excess relative risk is indicated (depending on the year after which this reduction might take effect). Using the second type of multiplier an adjustment to the excess relative risk varying between [time after exposure]-2.0 and [time after exposure]-3.2 fits best overall. All these reductions are statistically significant at the 5% level. There is no significant variation by cohort, by sex, by cancer type, or by age at exposure group in the degree of annual reduction in excess relative risk. Although time-adjusted relative and absolute risk models give equivalently good fits within each cohort, there is significant variation between cohorts in the degree of increase of risk with time in the absolute risk formulation, in contrast to the lack of such heterogeneity for the relative risk formulation. It is shown that if the range of observed reductions in relative risk is assumed to operate 40 or more years after exposure in the youngest age groups, the calculated UK population risks would be reduced by 30-45% compared to those based on a constant relative risk model.

Dose- and source-size-related changes in the late response of pig skin to irradiation with single doses of beta radiation from sources of differing energy.

Late radiation damage to pig skin has been assessed at 104 weeks after exposure to sources of 90Sr/90Y (Emax 2.2 MeV) and 170Tm (Emax 0.9 MeV). Damage was assessed from measurements of dermal thickness in histological sections of irradiated skin and was compared with that of unirradiated skin to establish the relative reduction in dermal thickness. The size of the source varied from 0.1 to 40.0 mm in diameter; this covered the range of source sizes designed to simulate exposure to radioactive "hot" particles (< or = 1.0 mm diameter) up to the lower range of field size that patients may be exposed to in radiotherapy treatments. Radiation doses were measured using an extrapolation chamber with a collecting electrode of 1.1 mm2, and thus the quoted doses represent an average dose over this area. For the larger 90Sr/90Y sources of > or = 5 mm diameter and the larger 170Tm sources of > or = 2 mm diameter there was no evidence, based on levels of damage consistent with a > or = 10, > or = 20, > or = 30, and > or = 40% reduction in relative dermal thickness, for any effect of source size on the ED50 value for each of these specified levels of effect. However, there was a marked effect of beta-particle energy; the skin surface doses associated with the ED50 values (+/- SE) for a > or = 20% reduction in relative dermal thickness were approximately 12 and approximately 40 Gy for 90Sr/90Y and 170Tm, respectively. This difference in skin surface dose for an equivalent level of dermal injury reflects the variation in the depth dose from these two beta-particle emitters. These skin surface doses, for what was considered to be a clinically detectable dermal effect, were below the ED10 for the early skin response of moist desquamation. This supports the selection of late dermal thinning as the effect on which to base dose limits in radiation protection for more generalized larger area skin exposures. In comparison, single exposures to a small area, from sources designed to simulate those from hot particles, reinforced the view that acute ulceration should be the effect on which dose limitation is based. Either the isoeffect doses for a clinically detectable reduction in relative dermal thickness of > or = 20%, following a single exposure to a small area, were higher than the ED10 for acute ulceration or the area of skin showing dermal thinning was so small that it was not considered to be detrimental.

General considerations of the choice of dose limits, averaging areas and weighting factors for the skin in the light of revised skin cancer risk figures and experimental data on non-stochastic effects.

In recent years considerable clinical and experimental data have become available which can form the basis of a better understanding of the response of skin to radiation exposure and should lead to improved radiological protection criteria. Recent biological data from man and pig on the non-stochastic effects following exposure with a range of beta-emitters are combined with recent epidemiological analyses of skin cancer risks in man to form a basis for suggested improved protection criteria for the skin following whole- or partial-body skin exposures. Specific consideration is given to the choice of an organ weighting factor for the evaluation of effective dose-equivalent. Since the stochastic and non-stochastic end-points involve different cell types, which reside at different depths in the skin, the design of an ideal physical dosemeter may depend on the proportion of the body skin that is exposed and the penetrating power of the radiation. Possible choices of design parameters for skin dosemeters are discussed. The limitation of skin exposure from small radioactive sources ('hot particles') is an important practical problem, which is addressed for the first time using animal data. Dose limitation on the basis of an average dose to an area of skin in the vicinity of the particle or on the basis of number of beta-particles emitted could be used. Animal data, for several energies and sizes of beta-emitting sources, indicate that limitation of the average dose to 1 Gy (average dose to 1 cm2 of skin at a depth between 100 and 150 microns) should prevent the occurrence of transient acute ulceration from small sources with dimensions of less than about 1 mm.

The risk of non-melanoma skin cancer incidence in the Japanese atomic bomb survivors.

The latest Japanese atomic bomb survivor non-melanoma skin cancer incidence dataset is analysed and indicates substantial curvilinearity in the dose-response curve, consistent with a possible dose threshold of about 1 Sv, or with a dose-response in which the excess relative risk is proportional to the fourth power of dose, with a turning-over in the dose-response at high doses (> 3 Sv). The time distribution of the radiation-induced excess risk is best described by a model in which the relative excess risk is proportional to a product of powers of time since exposure and attained age. The fits of generalized relative risk models with exponential functions of time and age at exposure (and in particular of attained age) to adjust the relative risk are less satisfactory, as also are the fits of other models in which products of powers of time since exposure, age at exposure and attained age adjust the excess absolute risk. Sensitivity analyses indicate the importance of likely adjustments to the Hiroshima neutron doses for the optimal model parameters, particularly if values of the neutron relative biological effectiveness (RBE) of more than 5 are assumed. If adjustments recently proposed are made to the Hiroshima neutron doses, then using the optimal model (in which excess risk is proportional to the fourth power of dose) the best estimate of the neutron RBE is 1.3 (95% CI < 07.1). However, uncertainties in skin dose estimates for the atomic bomb survivors means that the findings with respect to the neutron RBE and the non-linearity in the dose-response curve should be treated with caution.

Dimensions of the human eye relevant to radiation protection.

An attempt has been made to clarify the two most important issues relevant to personal eye dosimetry. This involves the identification of the cells which are most at risk from radiation and the specification of their position in the eye. A survey of the radiobiological literature concerning animals and humans shows that the epithelial cells in the equatorial region of the lens are those which are involved in radiation cataract induction. The depth of these cells has been evaluated in the human eye by means of geometrical construction. The relevant dimensions have been determined from a survey of published anatomical data and supplemented by new data obtained by slit-image photography. In a normal adult population (20-65 years) the minimum depth of the incriminated cells is 2.3 plus or minus 0.4 mm; the upper and lower values are associated with young and old subjects respectively. Approximate calculations for isotropic 90Sr/90Y and 106Rh beta-radiation fields indicate that a planar dosemeter, which integrates the tissue dose between depths of 2.5-3.5 mm, should give a reasonable measure of the mean equatorial dose for the variety of eye and irradiation geometries likely to be met during a life-time exposure. The long established, but tentative, value of 3 mm for the effective depth of the lens is thus confirmed.

The measurement of thermal neutron flux depression for determining the concentration of boron in blood.

Boron neutron capture therapy (BNCT) is a form of targeted radiotherapy that relies on the uptake of the capture element boron by the volume to be treated. The treatment procedure requires the measurement of boron in the patient's blood. The investigation of a simple and inexpensive method for determining the concentration of the capture element 10B in blood is described here. This method, neutron flux depression measurement, involves the determination of the flux depression of thermal neutrons as they pass through a boron-containing sample. It is shown via Monte Carlo calculations and experimental verification that, for a maximum count rate of 1 x 10(4) counts/s measured by the detector, a 10 ppm 10B sample of volume 20 ml can be measured with a statistical precision of 10% in 32 +/- 2 min. For a source activity of less than 1.11 x 10(11) Bq and a maximum count rate of less than 1 x 10(4) counts/s, a 10 ppm 10B sample of volume 20 ml can be measured with a statistical precision of 10% in 58 +/- 3 min. It has also been shown that this technique can be applied to the measurement of the concentration of any element with a high thermal neutron cross section such as 157Gd.

A wide dynamic range, high-spatial-resolution scanning system for radiochromic dye films.

This paper describes the evaluation of an inexpensive, commercially available 35 mm transparency slide scanner as a potential alternative scanning device for GafChromic HD-810 radiochromic dye film. Besides its low cost, the principal advantages of this type of scanner are high spatial resolution and high speed (a typical scan taking less than 1 min). With broad-band illumination the useful dose range using grey-scale imaging of GafChromic HD-810 is limited to about 50-800 Gy. By using the colour-scale imaging capability of the scanner we have been able to achieve a significant extension covering a similar range (15-2000 Gy) to that attainable using monochromatic illumination. The short-term reproducibility of the system is good, with a coefficient of variation of doses estimated from repeat scanning of uniformly exposed calibration films of less than 2%. Long-term stability is ensured by the scanning of a manufacturer-supplied test slide. The slide scanner system has been used in the determination of depth dose distributions from a model 'hot particle' source containing 106Ru/Rh. GafChromic dye film stacks irradiated by the source were read out on both the slide scanner and a conventional Joyce Loebl MDM6 scanning stage microdensitometer. The overall agreement between the dose estimates provided by the two systems was within 10%.

Bomb survivor selection and consequences for estimates of population cancer risks.

Health records of the Japanese bomb survivor population [with the 1965 (T65D) and 1986 (DS86) dosimetry systems] have been analyzed and some evidence found for the selection effect hypothesized by Stewart and Kneale. This is found to be significant in only the first of the periods examined (1950-1958), and the effect diminishes in magnitude thereafter. There are indications that the effect might be an artifact of the T65D dosimetry, in which it is observed more strongly than in the DS86 data. There is no evidence to suggest that selection on this basis might confer correspondingly reduced susceptibility to radiation-induced cancer. If, however, one makes this assumption, as suggested by Stewart and Kneale, then current estimates of population cancer risks might need to be inflated by between 5% and 35% (for excess cancer deaths, Gy-1) or between 8% and 40% (for years of life lost, Gy-1) to account for this. It is likely that these figures, even assuming them not to be simply an artifact of the T65D dosimetry, overestimate the degree of adjustment required to the risk estimates.

A review of the risks of leukemia in relation to parental pre-conception exposure to radiation.

The apparent risk of childhood leukemia resulting from paternal pre-conception radiation exposure found among children of the Sellafield (West Cumbria, UK) workforce is compared with the apparent risk in a number of other epidemiological studies. In particular, the extent of the incompatibility of the leukemia pre-conception exposure risks in the offspring of the Sellafield workforce born in the village of Seascale with the risks for those born in the rest of west Cumbria, and with the risks in the offspring of the Japanese bomb survivors, the Ontario radiation workers, and the Scottish radiation workers is discussed. A variety of animal data relating to the possibility of leukemia arising as a result of parental pre-conception exposure is also considered. It is concluded that the extent of the inconsistency of the leukemia risks in the Seascale data with this body of epidemiological and experimental data makes it highly unlikely that the association observed in the West Cumbria dataset represents a causal relationship.

Skin carcinogenesis following uniform and nonuniform beta irradiation.

In practical situations where workers or the general public may be exposed to ionizing radiation, the resulting irradiation is rarely uniform. The risk figures and dose limits recommended by the International Commission on Radiological Protection (ICRP) are based largely on clinical and epidemiological studies of reasonably uniform irradiated organs. The paucity of clinical or experimental data for highly nonuniform exposures has prevented the ICRP from providing adequate recommendations applicable to this practical situation. This weakness has led on a number of occasions to the postulate that highly nonuniform exposures of organs, such as the lung or the skin, could be 100,000 times more carcinogenic than ICRP risk figures would predict. This so-called "hot-particle hypothesis" found little support among reputable radiobiologists, but could not be clearly and definitively refuted on the basis of experiment. An experiment design, based on skin tumour induction in mouse skin, is described which was developed to test the "hot-particle hypothesis." In collaboration with the Radiobiology Department of St. Bartholomew's Hospital Medical College, London, the skin of 1200 SAS/4 male mice has been exposed to a range of uniform and nonuniform sources of the beta emitter 170Th (Emax 1 MeV). Nonuniform exposures were produced using arrays of 32 or 8 2-mm-diameter sources distributed over the same 8-cm2 area as a uniform control source. Average skin doses varied from 2-100 Gy. The results for the nonuniform sources show a 30% reduction in tumour incidence by the 32-point array at the lower mean doses compared with the response from uniform sources. The eight-point array showed an order-of-magnitude reduction in tumour incidence compared to uniform irradiation at low doses. These results, in direct contradiction to the "hot-particle hypothesis," indicate that nonuniform exposures produce significantly fewer tumours than uniform exposures.

The skin in radiological protection--recent advances and residual unresolved issues.

Exposure of the skin is important in radiological protection because, as the most superficial organ of the body, it can often receive the highest absorbed dose from an external exposure. It also has the highest radiation-induced cancer incidence risk factor of any organ (although mortality is very low). The ICRP and NCRP have, particularly over the past 15 y, been able to set dose limits for the exposure of the skin on the basis of an extensive body of radiobiological, clinical and epidemiological data. Some of the main advances in skin dose limitation in radiological protection and some of the remaining unresolved issues are reviewed.

Electron spectra measurements and Monte Carlo calculations for a 60Co standardised hot particle source.

A benchmark set of measured beta particle spectra for a standardised 60Co hot particle source is presented. The spectra were obtained for conditions similar to those encountered in practical dosimetric applications. The measured spectra were compared with Monte Carlo calculations using the MCNP code. These comparisons provided information to guide the selection of the optimal set-up parameters of the code. Important differences were observed in the MCNP calculated spectra when ITS and the default indexing style algorithm were used. Overall the calculations using the default mode of MCNP version 4B provide the best agreement with the measured electron spectra.