Search

Molecular scattering differential cross sections of 59.5 keV γ-rays have been measured for some compounds of Na, Mg, Al, Ca, and Fe elements at 90°. The γ-rays of Am-241 were counted using a Si(Li) detector of EDXRF (energy dispersive X-ray fluorescence) system. The experimental results have been compared with non-relativistic and relativistic theoretical values. Key words: essential elements, molecular scattering differential cross sections, energy dispersive X-ray fluorescence system. Nous mesurons les sections efficaces differentielles pour des molecules bombardees par rayons γ de 59.5 keV, pour des composes de Na, Mg, Al, Ca et Fe a 90°. Les rayons γ emanant de 241Am sont comptes par un detecteur Si(Li) d'un systeme EDXRF (analyse dispersive en energie en fluorescence X). Nos resultats experimentaux sont compares a des valeurs theoriques non relativistes et relativistes. [Traduit par la Redaction] Mots-cles: element essentiel, section efficace differentielle moleculaire, analyse dispersive en energie en fluorescence X. PACS Nos.: 32.90.+a, 32.30.-r, 32.10.-f, 33.20.-t, 34.50.-s, 33.15.-e., 1. Introduction The coherent and incoherent scattering interaction of photons with matter is crucial for obtaining information about the characteristics and structure of samples. The scattering of γ-rays by atoms [...]

A technique for validating electron energy resolution [[eta].sub.e] results measured using the modified Compton coincidence technique (MCCT) has been developed. This technique relies on comparing measured gamma-ray energy resolution [[eta].sub.[gamma]] with calculated values that were determined using the measured [[eta].sub.e] results. These [[eta].sub.[gamma]] calculations were based on Monte Carlo photon transport simulations, the measured NaI(Tl) electron response, a simplified cascade sequence, and the measured [[eta].sub.e] results. To demonstrate this technique, MCCT-measured NaI(Tl) [[eta].sub.e] results were used along with measured [[eta].sub.[gamma]] results from the same NaI(Tl) crystal. Agreement to within 5% was observed for all energies considered between the calculated and measured [[eta].sub.[gamma]] results for the NaI(Tl) crystal characterized. The calculated [[eta].sub.[gamma]] results were also compared with previously published [[eta].sub.[gamma]] measurements with good agreement (< 10%). In addition to describing the validation technique that was developed in this study and the results, a brief review of the [[eta].sub.e] measurements made using the MCCT is provided. Based on the results of this study, it is believed that the MCCT-measured [[eta].sub.e] results are reliable. Thus, the MCCT and this validation technique can be used in the future to characterize the [[eta].sub.e] of other scintillators and to determine NaI(Tl) intrinsic energy resolution. Index Terms--Electron energy resolution, electron response, light yield nonproportionality.

Gamma dose-rate profiles have been measured by means of a self-powered gamma detector (SPGD) in a gamma irradiation facility using [sup.60]Co sources, situated in the side pool of the BR2 materials testing reactor. Because of its small size (diameter of only 4 mm), the SPGD is ideally suited to perform very localized gamma dose-rate measurements. It has been shown that the SPGD can provide precise relative measurements of the local gamma dose rate in the range 0.5-20 kGy/h. A very detailed Monte Carlo code has been developed to relate the gamma dose rate (for a given gamma spectrum) to the current induced by the SPGD. The obtained conversion factor could thus be used to determine the absolute gamma dose rate. Index Terms--Dosimetry, gamma ray detectors, nuclear measurements, self-powered detectors.

Currently at the Idaho National Engineering and Environmental Laboratory, a handheld device is being developed to measure fast neutrons and gamma rays using a single detector. The handheld detection system presented here uses a single 12.7 mm (diameter) by 12.7 mm (length) liquid scintillator detector (BC501). The detection system can be made small and light. A small and light device can be used in several applications such as customs inspection, border security, and environmental radiation monitoring. The use of only one detector requires that the neutrons and gamma rays be distinguished by the shape of their pulses in the detector. Two methods of pulse shape discrimination (PSD) are presented here: charge integration and zero crossing. Figures of merit were calculated for both methods for a threshold energy range of 50-600 keVee. Results show that the zero crossing method gives much better PSD for 100 keVee and lower, whereas the charge integration method leads to better separation above 100 keVee. However, the neutrons and gamma rays are totally separated for energies of 100 keVee and above in both techniques. We are currently designing a miniaturized electronic system to be incorporated into the handheld device. Index Terms--Pulse shape discrimination (PSD).

We describe a coded-aperture-based gamma-ray imager that uses a unique hybrid germanium detector system. A planar, germanium strip detector, 11 mm thick, is followed by a coaxial detector. The 19 x 19 strip detector (2-mm pitch) is used to determine the location and energy of low-energy events. The location of high-energy events is determined from the location of the Compton scatter in the planar detector. The energy is determined from the sum of the coaxial and planar energies. With this geometry, we obtain useful photopeak quantum efficiency in a position-sensitive mode out to 500 keV. The detector is used with a 19 x 17 URA coded aperture to obtain spectrally resolved images in the gamma-ray band. We discuss the performance of the planar detector, the hybrid system, and present images taken of laboratory sources. Index Terms--Coded aperture, gamma-ray detectors, germanium radiation detectors, nuclear imaging, URA.

We describe a coded aperture gamma-ray imager that uses a CsI(Na) scintillator coupled to a Hamamatsu R3292 position-sensitive photomultiplier tube (PSPMT) as the position-sensitive detector. We have modified the normal resistor divider readout of the PSPMT to allow use of nearly the full 10-cm diameter active area of the PSPMT with a single scintillator crystal 1 cm thick. This is a significant performance improvement over that obtained with the standard readout technique where the linearity and position resolution start to degrade at radii as small as 3.0 cm with a crystal 0.75 cm thick. This represents a recovery of over 60% of the PSPMT active area. The performance increase allows the construction of an imager with a field of view 20 resolution elements in diameter with useful quantum efficiency from 60-700 keV. In this paper, we describe the readout technique, its implementation in a coded aperture imager, and the performance of that imager. Index Terms--Arms-control, coded aperture imager, gamma-ray detectors, nuclear imaging, position-sensitive photomultiplier tube (PSPMT).

A new radiation monitoring system, which has the capability of both gamma imaging and estimation of real-time exposure dose rate, was developed by an integration of a visual image and a dose image. It is composed of a portable gamma imaging system for visualizing gamma-source distribution and a dose conversion unit (DCU) for real-time dose conversion from gamma spectrum. Using Monte Carlo simulation, gamma-ray spectra for energies from 10 keV to 5 MeV with energy intervals of 10 keV were calculated for a cylindrical NaI (Tl) detector used in our developed system. These calculated spectra were compared with the measured spectra for several monoenergetic gamma rays using a multichannel analyzer. Based on the simulation data and the dose conversion coefficients obtained from ICRU 47, the dose conversion factors were also calculated by matrix calculation. The feasibility of the DCU using the dose conversion factors was evaluated by measurement of exposure dose rates as a function of distance between the gamma ray and detector for several standard sources. The exposure dose rates measured by the DCU are within an 11.5% deviation with analytical values based on gamma factor. They are also similar to the dose rates measured by a high-pressure ionization chamber with maximum difference of six. We finally visualized two-dimensional distributions of gamma source and measured the exposure dose rate simultaneously.

We describe a stacked detector made of thin cadmium telluride (CdTe) diode detectors. By using a thin CdTe device, we can overcome the charge loss problem due to the small mobility and short lifetime of holes in CdTe or cadmium zinc telluride (CdZnTe) detectors. However, a CdTe detector with a thickness of more than 5 mm is needed for adequate detection efficiency for gamma-rays of several hundred keV. Good energy resolution and good peak detection efficiency are difficult to obtain using such a thick CdTe detector. The stacked detector enabled us to realize a detector with both high-energy resolution and good efficiencies for gamma rays up to several hundred keV. In order to verify this concept, we constructed a prototype made of ten layers of a 0.5-mm-thick CdTe diode detectors with a surface area of 21.5 mm x 21.5 mm. With this, we have achieved 5.3-keV and 7.9-keV energy resolution [full width at half maximum (FWHM)] at 356 keV and 662 keV, respectively, at the temperature of -20 [degrees]C. Index Terms--Cadmium telluride (CdTe), CdTe diode detector, cadmium zinc telluride (CdZnTe), stacked detector.

In order to characterize CdTe/CdZnTe detectors in a planar configuration, we have developed a new method to extract [mu][tau] products. In this method, we prepare an analytic spectral model based on the charge transport properties in the device, which is intended to be used in fitting calculation. The low mobility-lifetime ([mu][tau]) products of carriers in CdTe/CdZnTe detectors produce a position dependency in the charge induction efficiency. The model takes the induction efficiency and interaction positions of photons into account. Since the model is parameterized by [mu][tau] products, it can extract [mu][tau] products. Here, we demonstrate how the model works based on the results from 2-mm-thick HPB CdZnTe and THM CdTe detectors. Index Terms--CdTe, CdZnTe, gamma-ray detector, mobility-lifetime products, spectral modeling.

A gamma-ray tracking algorithm has been implemented and tested, using simulated data, for gamma rays with energies between 0.1 and 2 MeV, and its performance evaluated for a 90-mm-long, 60-mm-diameter, cylindrical, 36 (6 x 6) segment detector. The performance of the algorithm in two areas was determined: Compton suppression and Doppler shift correction. It was found that for gamma rays of energies around 1 MeV, a ratio of photopeak counts to total counts of 2:3 could be obtained using the tracking algorithm, with only a 2% reduction in detection efficiency, compared to the untracked data. Approximately 80% of first interaction points could be correctly identified, enabling a good Doppler shift correction. A detector of the type simulated has recently been delivered, together with a compactPCI digital data acquisition system comprising 36 12-bit, 40-MHz flash ADCs, and 6 200-MHz DSPs. Some initial data has been recorded using this system, and the performance of the tracking algorithm on this real data is comparable to its performance on simulated data.

Thallium bromide (TlBr) is a wide-bandgap compound semiconductor characterized with high photon stopping power. In this study, large-volume [gamma]-ray detectors (approximately 3.8 x 3.8 x 3.8 [mm.sup.3]) have been fabricated from TlBr crystals grown by the horizontal travelling molten zone (TMZ) method using zone-purified material. In order to extract the energy information of the incident radiation from the thick detectors, the short charge-collection time technique has been applied to the detectors. By this technique, the induced charge on the electrode is measured for a period of time that is short compared to the carrier transit time in order to minimize the deviation of the measured induced charge. The large-volume TlBr detectors irradiated with [sup.22]Na and [sup.137]Cs [gamma]-ray sources at room temperature have exhibited energy resolutions of 18.7% and 17.4% (FWHM) for the 511- and 662-keV peaks, respectively. To our knowledge, this is the first time that full-energy peaks have been obtained from TlBr detectors several millimeters thick. The time stability of the detector operation is also studied. Index Terms--[gamma]-ray spectroscopy, semiconductor detectors, thallium bromide (TlBr).

The multi-channel photomultiplier (Philips XP1702) can be utilized as a high resolution area detector for thermal neutrons and high energy gamma rays. This detector type is needed e.g. for special neutron scattering experiments and for the PET method applied to small animals. In various test measurements this will be demonstrated by coupling suitable scintillators (NaI, BGO, and 6LiI single crystals and 6Li-glass) and dispersers to the photocathode and by feeding the 64 output signals of the PM into a resistor network via voltage-to-current converters. The final interpolated position per event is determined by pulse height division of the four amplified signals taken from the comers of the network. Experimental results concerning the achievable spatial resolution, the homogeneity and the linearity of the detector response and of the quantum efficiencies will be presented. Light experiments with well collimated pulsed rays and with beams of different spot sizes at the photocathode are helpful to explain the measured properties of this new neutron and gamma ray detector.

The ability of a silicon carbide radiation detector to measure neutron and gamma radiation levels in a TRIGA reactor's mixed neutron/gamma field was demonstrated. Linear responses to epicadmium neutron fluence rate (up to 3 x [10.sup.7] [cm.sup.-2] [s.sup.-1]) and to gamma dose rate (0.6 - 234 krad-Si [h.sup.-1]) were obtained with the detector. Axial profiles of the reactor core's neutron and gamma-ray radiation levels were successfully generated through sequential measurements along the length of the core. The SiC detector shows a high level of precision for both neutrons and gamma rays in high-intensity radiation environments - 1.9% for neutrons and better than 0.6% for gamma rays. These results indicate that SiC detectors are well suited for applications such as spent fuel monitoring where measurements in mixed neutron/gamma fields are desired.

The spectrum of a Co-60 facility includes more than the two photopeaks of gamma ray emission. If there is a large low energy contribution from scattering, dose enhancement might be a problem. It is important to know the spectrum of a Co-60 facility and understand how experimental modifications can change that spectrum. The AFRL Co-60 cell spectrum is found to be a clean spectrum with small low energy contributions and dominant Co-60 photopeaks. Experimental modifications to reduce dose enhancement such as the use of a Pb/Al box and even better a Pb/Sn/Cu/Al box are found to decrease the low energy contributions. Experimental modifications to reduce dose rate such as using lead attenuators in front of the experiment and/or raising the source partially are found to significantly alter the spectrum, sometimes creating large low energy contributions.

Astronomers have puzzled about the origin of gamma-ray bursts (GRB) for 25 years. New events have been discovered since 1991, at the rate of around one a day, although objects responsible for the outbursts could not be identified. GRBs can now be rapidly and accurately located since the launch of the BeppoSAX satellite in 1996. This has been followed by discoveries of x-ray and optical counterparts.

The NaI(Tl) scintillation response has been studied for over four decades. Two of the significant conclusions that have been made from these studies (with respect to using NaI(Tl) to detect gamma rays and X rays) are: 1) the NaI(Tl) scintillation response is nearly linear (especially over limited energy ranges) and 2) the NaI(Tl) scintillation response has significant nonproportionalities. While these conclusions may at first appear to be inconsistent with each other, upon further consideration these conclusions are not mutually exclusive. In fact, these conclusions are fully consistent with each other. Published results typically confirm only one of these conclusions (as it was likely the intention to study either linearity or proportionality, but not both) and are not always presented such that direct comparisons can be made with other results. Consequently, it is critical that both the intent of the study and the means with which the results are presented be appropriately considered. Otherwise, it is possible to make inaccurate conclusions about both linearity and proportionality. To facilitate accurate conclusions and meaningful comparisons, this study defines a fully consistent nomenclature, examines some of the subtleties which must be considered when comparing scintillation response results, and uses a review of the literature to demonstrate both the definitions and the subtleties.

An optical transient within the error box of the gamma ray burst GRB 970508 was imaged 4 hours after the event. It displayed a strong ultraviolet excess, and reached maximum brightness 2 days later. The optical spectra did not show any emission lines, and no variations on time scales of minutes were observed for 1 hour during the decline phase. According to the fireball and afterglow models, the intensity should rise monotonically before the observed optical maximum, but the data indicate that another physical mechanism may be responsible for the constant phase seen during the first hours after the burst., Gamma ray bursts (GRBs), brief flashes of cosmic high-energy photons, remain one of the most elusive mysteries for high-energy astrophysicists (1), mainly because we do not know how far away [...]

We describe an innovative detector designed primarily for the study of cosmic gamma-ray bursts aboard interplanetary spacecraft. The experiment is a fully functional, low cost, low mass, low-power instrument, which, when combined with other similar instruments, has the capability of localizing bursts to arcminute precision. The burst monitor is based on a photomultiplier tube and a plastic/CsI(Tl) phoswich detector and records photons in the 20-150-keV energy range with time resolutions up to several milliseconds. In addition, it records photon energy spectra. Events in the plastic scintillator are distinguished by a pulse shape discrimination circuit and are used to count electrons and protons. In a planetary orbiter or fly-by mission, this capability can be used to detect the presence of a magnetosphere. We describe in detail a version of this instrument which was built for the Russian Mars 1996 mission. Index Terms - Bursts, gamma-ray.

Nuclear instrumentation has been designed and tested for a large field of view soft x-ray sensor utilizing Si Avalanche Photodiodes (APDs). The instrument will be launched on a low cost satellite mission in mid 1998. The mission's primary scientific objective is to determine the distance scale to the sources of cosmic gamma ray bursts by utilizing interstellar absorption effects observable in their x-ray emission at energies below 2 keV. The soft x-ray (SXR) sensor incorporates 7 identical panels, each with 16 APDs collectively collimated to a 1 steradian field of view. The SXR panels are radiatively cooled to -35 degrees C, and taken together, view a full hemisphere. The instrumentation for each of the sensors in the SXR detector includes a low noise, charge sensitive preamplifier/shaping amplifier and an Analog Electronics Unit (AEU) containing a voltage amplifier, peak track and hold circuit, and threshold-timing logic. Each panel has an Analog to Digital Interface (ADI) based on a field programmable gate array (FPGA), which controls the digitization of the event amplitudes, implements the In-Flight Calibration (IFC) for each APD, and coordinates the data output to and command input from the instrument CPU.

The stacked GSO(Ce) spectrometer consists of three plastic scintillation detectors, three rectangular GSO(Ce) scintillators (43 x 43 x 43 [mm.sup.3]) and a large cylindrical GSO(Ce) ([Phi] 62 x 120 [mm.sup.3]). It has been developed to measure proton emission spectra up to 450 MeV. As a preliminary test, [Gamma]-rays from 137Cs and cosmic-rays were measured. We compare the results of the stacked GSO(Ce) spectrometer and those of the stacked NaI(Tl) spectrometer. The performances of the stacked GSO(Ce) spectrometer in intermediate energy region are discussed.

A new type of 3He-filtered GM counter has been used to measure the dose of [Gamma]-rays existing in a thermal neutron field. The detector assembly consists of a commercially available GM counter surrounded by a cylindrical aluminum vessel filled with 3He gas at a pressure up to 6 atmospheres. The contribution of secondary [Gamma]-rays produced by thermal neutron capture in the GM counter itself can be excluded by increasing the 3He pressure, because the thermal neutron capture reaction of 3He(n,p)T results in the emission of charged particles of a proton and a triton. In order to determine the response of the GM counter, the energy spectrum deposited in the counter gas to monoenergetic [Gamma]-rays was calculated by EGS4 Monte-Carlo code for a [Gamma]-ray energy range from 60 keV to 10 MeV and then the response was derived through calibration of the counter at [Gamma]-ray energies of 60 keV, 662 keV and 1.17 MeV +1.33 MeV. The [Gamma]-ray energy distribution in the thermal neutron field was obtained by an MCNP Monte-Carlo calculation, assuming a three dimensional experimental assembly of a graphite pile, which also included the concrete floor in the experimental room. The [Gamma]-ray fluence and the effective dose equivalent in the thermal neutron field have been determined by using the GM counter response, the [Gamma]-ray energy distribution in the field and the conversion factor from fluence to effective dose equivalent.

Gallium arsenide resistive photoconductors are widely used for the characterisation of picosecond radiation pulses. They are used to measure both the intensity and the temporal shape of the pulse. We have investigated the influence of the physical and electrical properties of high resistivity GaAs crystals (LEC, VGF) supplied by various manufacturers on the detector response to fast visible and gamma-ray pulses. The detectors characteristics: response time and sensitivity, were tested both before and after pre-irradiation with fission neutron at integrated doses in the range 5 x [10.sup.14] to 1 x [10.sup.16] neutrons/[cm.sup.2]. The original GaAs material properties were found to have a significant influence on the neutron pre-irradiated photoconductor response times and sensitivities for integrated doses up to 1 x [10.sup.15] neutrons/[cm.sup.2].

A new scintillator material, lutetium orthoaluminate (LuAl[O.sub.3]:Ce or LuAP), has been characterized by measuring its light output, scintillation decay time and energy resolution. Successful detection of 511 keV [Gamma]-rays has been accomplished using LuAP coupled to semiconductor photosensors (silicon avalanche photodiode and silicon p-i-n photodiode). The background spectrum due to the radioactive isotope 176Lu in LuAP has been measured and characterized, and potential applications of LuAP are addressed.

Accurate knowledge of the sources of detector background is essential in [Gamma]-ray astronomy for correct data interpretation, and for optimising detector design and operation. In the low-to medium-energy range, a major source of background arises from radioactivity that is induced in the detector medium itself by nuclear spallation and neutron capture. Results are presented from the Shuttle Activation Monitor flown in the Space Shuttle middeck area on two missions during August 1989 and September 1991. This experiment has been valuable in the study of induced radioactivity in BGO scintillators resulting from bombardment by cosmic-rays, trapped protons, and their secondaries created by interactions in a heavy spacecraft. Data from mono-energetic proton irradiation experiments have allowed investigation of activation effects in BGO for less complicated irradiation conditions, and the results from these experiments are also presented. Detailed simulations have been performed to determine BGO detector activation and response, and these take into consideration the effects of shielding by the spacecraft structure. The results compare very well to the experimental data, allowing features in the measured spectra to be identified. They also highlight the importance of shielding both in reducing activation by trapped protons, and enhancing cosmic-ray induced activation due to secondaries.

The Tracking and Imaging Gamma-Ray Experiment (TIGRE) uses multilayers of silicon strip detectors both as a gamma-ray converter and to track Compton recoil electrons and positron-electron pairs. The direction and energy of the Compton scattered gamma ray and pair particles are measured with arrays of small CsI(Tl)-photodiode detectors. With a large [Pi]-steradian field-of-view, the prototype instrument described here is sensitive to gamma rays from 0.3 to 100 MeV with a typical energy resolution of 3% (FWHM) and a 1-[Sigma] angular resolution of 120 arc min. at 2 MeV. It has a high absolute detection efficiency of 8% over the full energy range. The telescope will be described and Monte Carlo calculations will be presented. Measurements with a small desktop instrument are presented elsewhere in this volume.

A small desktop prototype of the Tracking and Imaging Gamma-Ray Experiment (TIGRE) has been assembled and tested at 511 keV and 900 keV. TIGRE was designed to observe cosmic gamma ray sources at energies of 0.3 to 100 MeV. Its major feature is its use of multi-layer silicon strip detectors to track Compton recoil electrons and positron-electron pairs. Our small prototype consists of 7 double sided silicon strip detectors 3.2 cm x 3.2 cm x 300 micron with 1 mm pitch in both the x and y directions. The direction and energy of the Compton scattered gamma ray is measured with small CsI(Tl) photodiode detectors. Knowing the energy and momentum of the scattered electron and scattered photon allows us to determine the incident direction uniquely. In the small prototype 36 CsI(Tl) crystals of 1 cm x 1 cm x 1.7 cm were used. Non-tracked events, those interacting in only a single silicon plane, can only be determined to within the Compton scatter ring. The silicon strips were calibrated using the 60 keV photons from [Am.sup.241] and the Landau peak obtained from a [Sr.sup.90] beta source. The energy resolution of the silicon was measured to be 8 keV (1 [Sigma]) at 60 keV and 7.8% FWHM for CsI at 900 keV. Total energy resolutions at 511 and 900 keV were measured to be 11% and 8.9% FWHM respectively. An important requirement of TIGRE will be its ability to separate the upward moving gamma rays produced by cosmic ray interactions in the atmosphere from the downward moving gamma rays. For tracked events this is done by defining a Direction of Motion (DOM) parameter for the electron by its energy deposition and multiple scattering in the silicon layers. Measurements at 511 and 900 kev show that the DOM parameter is correctly predicted at 70% and 75% for tracked events which constitute 9% and 20% of the data. Monte Carlo simulations show similar results and show the percentage increasing to 98% at 6 MeV in which nearly all of the events are tracked.

The high density, high light output, and short decay time of LSO (lutetium orthosilicate, [Lu.sub.2]Si[O.sub.5]:Ce) make it an attractive scintillator for gamma ray spectroscopy. The low cost, small size, high quantum efficiency, and ruggedness of silicon photodiodes make them attractive photodetectors for this same application, although their high noise (compared to a photomultiplier tube) reduces their appeal. In this work we measure the gamma ray energy resolution, timing accuracy, and conversion factor from gamma energy to number of electron-hole pairs produced with a 3x3x22 [mm.sup.3] LSO scintillator crystal read out with a 3x3 [mm.sup.2] silicon PIN photodiode. When the detector is excited with 511 keV photons, a photopeak centered at 1940 [e.sup.-] with 149 keV fwhm is observed and a timing signal with 35 ns fwhm jitter is produced. When the detector is excited with 1275 keV photons, a photopeak centered at 4910 [e.sup.-] with 149 keV fwhm is observed and a timing signal with 25 ns fwhm jitter is produced. While these performance measures are inferior to those obtained with photomultiplier tubes, they are acceptable for some applications.

A combined weight window and biasing approach has been developed to optimize the specific purpose McLDL code for simulating the spectral response of gamma-gamma lithodensity logging tools. The method is based on the use of photon importance maps which are strongly dependent upon the geometric configuration of the logging tool. A subspace in the formation is taken as an importance region and a one-group importance generator is used in conjunction with the current McLDL code. The splitting of a photon only occurs within the important subspace, and direction biasing and the exponential transform are applied to drive photons into that region or the detector. The approach has been tested on a computational benchmark problem and the Atlas Wireline Services' commercial gamma-gamma lithodensity logging tool. Compared to the results from the previous McLDL code, this new approach has eliminated the unacceptably large weight fluctuations and substantially improved the code's performance in computational accuracy and efficiency by decreasing the relative standard deviation in total detection yield by an order of magnitude and increasing the figure of merit by more than 70 times. Also, the revised McLDL code is more efficient than the general purpose Monte Carlo code MCNP in the simulation of gamma-gamma lithodensity logging tool responses.