Limitations in the PHOTON Monte Carlo gamma transport code

Three Monte Carlo gamma transport codes, MCNP, EGS, PHOTON, differ in the degree of difficulty in implementing them for calculation and in the requirements for the input file. Differences in the results were discovered when evaluating the same case using these three transport codes. These differences that are energy dependent are presented here. r 2002 Published by Elsevier Science B.V. PHOTON is a newly developed, user-friendly, Monte Carlo gamma transport code that was published recently [1]. PHOTON uses a simplified input-file structure wherein the system and the source are described using combinatorial geometry; it also contains its own build-in cross-sectional libraries. However, at low photon energies (o100 keV), care must be exercised in using it since Rayleigh scattering, fluorescence yields, form factors, and scattering factors are not included in the cross-section libraries. Since they cannot presently be included in the libraries, unless the code is modified, at low energies this might limit the code’s usefulness. At energies above 1MeV, we detected systematic discrepancies between the PHOTON code and two well-established Monte Carlo photon-transport codes, MCNP4B [2] and EGS4 [3], when calculating energy deposition spectra in a NaI detector. These discrepancies, that were found to be energy-dependent, are discussed in this communication. We calculated energy-deposition spectra in NaI (Tl) detectors varying in size using PHOTON code at incident photon energies in the range from 1 to 10MeV. An input file for the PHOTON code consisted of a point source on the central axis of a cylindrical detector placed 10 cm away from its front face. The energy-deposition spectra (not the pulse height distributions) were calculated for an isotropic point source in which the emitted radiation was limited by the solid angle subtended by the detector, and the space outside the detector was assumed to be in a vacuum [4]. The energy spectra were calculated using 512 channels 20 keV wide. Since no experiments were planned, these calculations were verified with independent calculations using the MCNP4B and EGS4 codes. For this purpose, identical input files were prepared for the other two codes and the calculations were carried out for 10 histories. It was noticed that the execution times, for 4.4MeV gamma rays on a Pentium P166 PC, using PHOTON, MCNP4B, *Corresponding author. Tel.: +1-631-344-3656; fax: +1631-344-7244. E-mail address: [email protected] (L. Wielopolski). 0168-9002/02/$ see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 9 0 0 2 ( 0 1 ) 0 1 2 2 6 8 and EGS4 codes were 35min, 6min, and about 4 h, respectively. The long computing time for the EGS4 code is due to its treatment of electrontransport using an analog Monte Carlo scheme, not present in the other two codes. The results reported here represent calculations only for a single size (600 600) detector. Fig. 1 shows the calculated energy spectra obtained from all three codes at 0.662MeV. In general, these spectra are in a good agreement except in the valley region on the low-energy side of the photopeak. This discrepancy is partially attributed to the differences in the energy cut-off used in each one of the codes to terminate the particle history, which in the PHOTON code is higher, and, in part, to the different step-sizes used in each of the transport codes. Changing the cut-off energy affected the degree of the discrepancy in this region of the spectrum. However, the discrepancy reported here occurs at higher incident energies. At energies above 0.662MeV, energy spectra were calculated at 2, 4.4, 6, 8, and 10MeV. Only two spectra, corresponding to carbon photopeak, at 4.4MeV, and in the vicinity of the 9.17MeV nitrogen peak, at 10MeV, are shown in Figs. 2a and 3a, respectively. While there is a general agreement between the MCNP4B and EGS4 results, there is a clear discrepancy between these two codes and the PHOTON calculations. This discrepancy increases systematically with the increase in the energy of the incident gamma radiation (Figs. 2a and 3a). Similar discrepancies were apparent in the energy spectra calculated for different detector sizes, not shown here. The missing energy deposition in the PHOTON code, and at the same time higher photopeak yields (see Table 1), have been attributed to the omission of the contribution to the energy spectra from bremsstrahlung radiation, due to electrons and positrons, that results from photoelectric-, Compton-, and pair-production interactions in the detector. To confirm this hypothesis an MCNP code was modified to turn off the production of bremsstrahlung radiation in the detector. These results showed a significant improvement in the agreement between the PHOTON code and the modified MCNP4B code at all incident photon energies; these improvements at 4.4 and 10MeV are shown in Figs. 2b and 3b, respectively. No change in the spectrum was observed at 0.662MeV at which the contribution of bremsstrahlung is negligible. Table 1 summarizes yields at the Fig. 1. Energy deposition into a 600 600 NaI detector from a 0.662MeV point source calculated by the PHOTON, MCNP4B2, and EGS4 Monte Carlo transport codes. I. Orion, L. Wielopolski / Nuclear Instruments and Methods in Physics Research A 480 (2002) 729–733 730