Spectral Distribution of the ANITA Neutron Beam

At the Gustav Werner Cyclotron of The Svedberg Laboratory (TSL) in Uppsala, the high-intensity neutron beam ANITA (Atmospheric-like Neutrons from thIck TArget) is generated by bombarding a 25 mm thick tungsten target with 178 MeV protons. In the lethargy representation, the energy-weighted spectral yield E•Y_E, shows a so-called spallation peak at approx. 100 MeV and an evaporation peak at approx. 1 MeV. This spectral distribution is, thus, similar to that of the neutron components of the ambient background radiation induced by the primary cosmic radiation. The ANITA beam is therefore used especially for the investigation of radiation-induced failures in semi-conductor data memories (SRAM). For these experiments, especially the high-energy share of the spectral neutron distribution above approx. 10 MeV is important.

The characterisation of the spectral distribution by means of the time-of-flight method is made difficult by the unfavourable time structure of the proton beam. The time interval between the proton pulses is only 44 ns. In contrast, the duration of a pulse is, with 5 ns, relatively large. Therefore, it is necessary to use a relatively long flight path of approx., 15 m in order to obtain a sufficient energy resolution. Thereby, however, only neutrons with energies higher than 70 MeV have flight paths short enough to be clearly attributed to one proton pulse. For low-energy neutrons, this attribution is ambiguous and leads to a so-called "frame overlap background" in the experimental time-of-flight spectra.

Within the scope of a measurement campaign supported by the EFNUDAT Project [1], time-of-flight measurements were therefore performed with a ²³⁸U fission ionisation chamber on the ANITA beam, and a procedure was developed by which the "frame-overlap background" can be reliably determined. By combining measurements at different distances, it was possible to experimentally determine the spectral distribution above approx. 13 MeV. For this purpose, the shape of the spectral distribution to be expected for ANITA was described by a model function. This model is based on spectral distributions which were calculated with the neutron transport code MCNPX [2] using the nuclear data library TENDL [3]. The model parameters describing the high-energy spallation peak were determined with the WinBUGS [4] software from the experimental data and the already existing information using a Bayesian procedure for the estimation of parameters. Thereby, all the available experimental data were used in a consistent and equal way. Besides the parameter values, this procedure also provides the uncertainties and the correlations of the estimated parameter values. The parameters for the low-energy evaporation peak were taken from the transport computations.

Figure 1 shows four experimental time-of-flight spectra for various flight paths, as well as the time-of-flight distribution computed on the basis of the model of the spectral distribution, and the "frame-overlap" background. In Figure 2, the model function for the spectral distribution, as well as the spectral distribution determined on the basis of the time-of-flight spectra and computed with MCNPX are shown. All distributions are normalized to the electrical charge of the protons hitting the tungsten target. The deviations between the model function and the experimental results which are noticeable at high neutron energies can partly be attributed to the relatively poor time resolution of the ²³⁸U fission ionisation chamber and of the proton beam. Under such conditions, the procedure of parameter estimation is no longer sensitive for the precise shape of the spectral distribution close to the maximum energy. The lower energy limit of the spectral distribution determined by means of the above-described method is basically due to the shortest flight path which is possible owing to the set-up of the ANITA facility.

Figure 1 : Neutron time-of-flight distributions for different flight paths. The histogram renders the experimental distributions determined on the ANITA beam by means of a ²³⁸U fission ionisation chamber. The blue line shows the frame overlap background, while the red line represents the total time-of-flight distribution computed on the basis of the model function for the spectral distribution. The TOF parameter corresponds to the difference between a time zero point, which is different for each partial image, and the arrival time of the neutrons at the fission ionisation chamber.

Figure 2 : Spectral yield Y_E per target charge Q of the ANITA beam. The black histogram shows the spectral yield computed from the time-of-flight measurements after subtraction of the frame overlap underground, and the blue line represents the model function. The parameters of the function were determined using WinBUGS from the time-of-flight spectra. The red histogram renders the results of the simulation with MCNPX.

Literature

1. European Facilities for Nuclear Data Measurements (EFNUDAT), website www.efnudat.eu
2. D.B. Pelowitz, Laboratory Report LA CP 050369, Los Alamos National Laboratory (2005)
3. A.J. Koning and D. Rochman, Report JEFF-DOC 1262, November 17, 2008
4. D.J. Spiegelhalter, A. Thomas and N.G. Best, WinBUGS Version 1.4, MRC Biostatistics Unit (2003), the WinBUGS software is available on the Internet at http://www.mrc-bsu.cam.uk/bugs