Franz X. Gallmeier

CEM03.03 and LAQGSM03.03 Event Generators for the MCNP6, MCNPX, and MARS15 Transport Codes

A calorimetric-time-of-flight technique was used for real-time, high-precision measurement of neutron spectra at an angle of 175 from the initial proton beam direction, which hits a face plane of a cylindrical lead target of 20 cm in diameter and 25 cm thick. A comparison was performed between the neutron spectra predicted by the MARS, RTS&T, MCNP6, and the MCNPX 2.6.0 transport codes and that measured for 200, 400, 600, 800, and 1000 MeV protons. Neutron spectra were measured within the energy range from 0.7 to 250 MeV almost continuously. The transport codes tested here describe with different success the measured spectra, depending on the energy of the detected neutrons and on the incident proton energy, but all the models agree reasonably well with our data. 2010 Elsevier B.V. All rights reserved.

TORT solutions for the 3D radiation transport benchmarks for simple geometries with void region

Abstract We present the solutions for the set of three-dimensional radiation transport Benchmark problems obtained with the TORT transport code using its three optional methods: Theta Weighted (θW), Linear Nodal (LN), and Linear Characteristic (LC). Only the cases with 50% scattering are presented in this paper since the nonscattering cases are bound to suffer severe ray effects. By solving the problems on a sequence of refine meshes we illustrate that for some points defined in the benchmarks the solution converges with mesh refinement. However, the solution at most points does not converge with mesh refinement, and we illustrate that this is a consequence of ray effects in the void region. Also, we compare TORTs solutions to the Monte Carlo reference solution and observe that even when TORTs solution converges with mesh refinement, it usually does not converge to the Monte Carlo reference. This behavior also results from ray effects, and therefore we conjecture it will appear in varying degrees in all discrete ordinates accurate solutions because ray effects exist in the exact solution of the discrete ordinates equations. While this result is disappointing from the benchmarking point of view, it bodes well for TORTs ability to produce highly accurate solutions to the discrete ordinates approximation. Eliminating ray effects requires extensions of the solution algorithm, e.g. via a first collision source, while preserving the desirable features of the discrete ordinates methodology.

Optimizing moderator dimensions for neutron scattering at the spallation neutron source

In this work, we investigate the effect of neutron moderator dimensions on the performance of neutron scattering instruments at the Spallation Neutron Source (SNS). In a recent study of the planned second target station at the SNS facility, we have found that the dimensions of a moderator play a significant role in determining its surface brightness. A smaller moderator may be significantly brighter over a smaller viewing area. One of the immediate implications of this finding is that for modern neutron scattering instrument designs, moderator dimensions and brightness have to be incorporated as an integrated optimization parameter. Here, we establish a strategy of matching neutron scattering instruments with moderators using analytical and Monte Carlo techniques. In order to simplify our treatment, we group the instruments into two broad categories: those with natural collimation and those that use neutron guide systems. For instruments using natural collimation, the optimal moderator selection depends on the size of the moderator, the sample, and the moderator brightness. The desired beam divergence only plays a role in determining the distance between sample and moderator. For instruments using neutron optical systems, the smallest moderator available that is larger than the entrance dimension of the closest optical element will perform the best (assuming, as is the case here that smaller moderators are brighter).

Characterization of the radiation background at the Spallation Neutron Source

We present a survey of the radiation background at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, TN, USA during routine daily operation. A broad range of detectors was used to characterize primarily the neutron and photon fields throughout the facility. These include a WENDI-2 extended range dosimeter, a thermoscientific NRD, an Arktis 4He detector, and a standard NaI photon detector. The information gathered from the detectors was used to map out the neutron dose rates throughout the facility and also the neutron dose rate and flux profiles of several different beamlines. The survey provides detailed information useful for developing future shielding concepts at spallation neutron sources, such as the European Spallation Source (ESS), currently under construction in Lund, Sweden.

The Second Target Station at the ORNL Spallation Neutron Source

After the Spallation Neutron Source (SNS) construction project was completed in June, 2006, development of plans to construct a second target station (STS) at SNS began. These plans have evolved to the establishment of a reference concept for a STS and associated neutron beam instruments, and the evaluation of the expected performance for this station, all of which have been documented in a White Paper. Based on this White Paper, the Department of Energy has approved development of a detailed conceptual design leading to a construction project for the STS. The STS reference design is based on pulse stealing from the 60 Hz SNS accelerator system, with one pulse of every three going to the STS and the other two going to the first target station. The STS would operate in long-proton-pulse mode with no pulse compression in the accumulator ring, and would be optimized for production of intense beams of cold neutrons. The reference concept for the STS and the estimated performance for this concept will be discussed

ACTIVATION OF TRACE ELEMENTS AND IMPURITIES - A NEW ANSATZ FOR MONTE CARLO CALCULATIONS

Abstract In the framework of activation calculations of accelerator components with Monte Carlo methods, an unsolved problem is to take into account the spallation products of trace elements and impurities in a bulk material. Because of the low probability of spallation reactions with these elements, a large number of primary particles are necessary to obtain some information about their spallation products. A new algorithm for treating high-energy reactions has been implemented into MCNPX 2.5.0 to overcome these deficiencies. With this algorithm, spallation reactions of all constituents of a material will be performed at each high-energy interaction. This leads to the production of spallation products from all elements in a material. We will present examples of how this new methodology influences the outcome of activation calculations.

Implementation of Neutron Mirror Modeling Capability into MCNPX and Its Demonstration in First Applications

Abstract Reflection of thermal and cold neutrons by polished surfaces and so-called supermirrors effect radiation fields in and around neutron beamlines. To allow the prediction of these radiation fields with MCNPX 2.5.0, two new input cards were implemented for defining mirror properties of surfaces. Mirror properties can be linked to any type of surface, in contrast to other neutron optics codes, where the mirror properties are part of component descriptions, allowing the simulation of very complex neutron optical devices. First calculations are under way to verify the new capability against combinations of MCNPX and MCSTAS (neutron optics code) simulations. Also, simulations are under way to compare the predicted neutron beam characteristics against measurements conducted at Paul Scherrer Institut.

Comparison of Selected Codes for Calculating Induced Radioactivity at Accelerator Facilities

Abstract Component radioactivation is an important problem in accelerator facilities, impacting operations, maintenance, decommissioning, and disposal. Radionuclide inventories are calculated for an 8-cm-diam, 30.9-cm-long lead target irradiated by 660-MeV protons using the particle transport code MCNPX and the transmutation codes CINDER’90, ORIHET-3, and SP-FISPACT. The results using the various codes and data libraries are compared with experimental measurements. Comparisons are also made between the outputs of the three codes for nuclides not represented in the measurements. For more than half the nuclides studied, the codes agree with the measurements within a factor of 2, and nearly all agree within a factor of 10. The present set of codes and nuclear data files are largely adequate for calculating radioactivation in accelerator facilities, but there is room for substantial improvement for selected radionuclides.

A sample holder for simultaneous Raman and neutron vibrational spectroscopy

We have built a sample holder (called a center stick or sample stick) for performing simultaneous Raman and neutron vibrational spectroscopy on samples of material at the VISION neutron vibrational spectrometer of the Spallation Neutron Source at Oak Ridge National Laboratory. This equipment holds material samples in the neutron beam within the cryogenic environment of the VISION spectrometer, allowing for samples to be studied at temperatures as low as 5 K. It also provides the capability for gas to be loaded to or evacuated from the sample while it is loaded at VISION. The optical components for directing and filtering light are located within the cryogenic volume, in physical proximity to the sample. We describe the construction of this sample holder and discuss our first measurements of simultaneous Raman and neutron vibrational spectra. The samples that we report on were of 4-nitrophenol at a temperature of 20 K and of cryogenic hydrogen of a number of different orthohydrogen fractions.

Single event effects test facility options at the Oak Ridge National Laboratory

This article consists of a collection of slides from the authors conference presentation.