Absolute Neutron Fluence Determination

Z. Chowdhuri, K. Coakley, M.S. Dewey, J.S. Nico, and A.K. Thompson - NIST
G. Hansen and W.M. Snow - Indiana University
C. Keith - Jefferson National Accelerator Facility

Figure 1

While the motivation for the neutron flux determination program has its origin in the neutron lifetime project, it should be noted that there are a number of other applications for our improved techniques. These include the improved calibration of primary dosimetry standards, the recalibration of primary standards for neutron activity, the determination of important cross-sections, the improved assay of nuclear targets, and the development of improved primary standards for nuclear methods in analytical chemistry. Along with our efforts aimed at the determination of _n, we are pursuing these avenues of research.

The neutron monitor employed in the neutron decay measurement is designed to measure the neutron density in the beam. The capture flux has an efficiency which is weighted by the reciprocal of the neutron velocity (at least within the cold to thermal range). As is typical of such 1/v detectors, the ¹⁰B and ⁶Li detectors used in the lifetime measurement are transmission devices which are nearly transparent to neutrons. By contrast, the "black" detectors have an efficiency which is essentially independent of incident neutron velocity. As illustrated in Figure 1, an intercalibration between the black and capture flux detectors in a monochromatic beam is required. We are developing two distinct types of "black" detector, one using radiometric measurements of power deposition and the other using a prompt-gamma technique.

The Lithium-Target Radiometer

Figure 2

⁶Li is an excellent choice for such a device since (1) it has a large (941 b) absorption cross section for thermal neutrons, (2) there are few processes that compete with the reaction mentioned above, and (3) the Q-value is quite large (4.7821 MeV) and known with high accuracy. Since lithium metal has a complicated phase structure at low temperatures, it was necessary to find a ⁶Li-rich compound for use as a target. The compound ⁶LiPb has been used in tests to date, and a ⁶Li_0.75Mg_0.25 alloy has also been prepared as a target. Operating at a temperature of 1.8 K, the radiometer with the lithium-based target possesses a noise floor of 100 pW in a 20 minute measurement cycle (10 minutes with power off followed by 10 minutes with power on). This corresponds to a sensitivity of 130 neutrons per second. Figure 3 shows a plot of the radiometer response to the monochromatic neutron beam.

Figure 3

Note that the calorimetric technique requires that all of the energy from the neutron reaction is converted to heat in the target. Some energy will not appear as heat: the decelerating tritons and alphas create bremsstrahlung radiation which can escape the target, and crystal vacancies and other defects which do not anneal at the low target temperature are energy traps. Although estimates indicate that such losses should be below the 0.1 % level, an independent measurement of this fraction is required. For this reason, we are preparing to perform a measurement with liquid ³He as the absorbing target. Like ⁶Li, ³He has a large absorption cross section for thermal neutrons with negligible production of gammas. The smaller Q value of the reaction relative to ⁶Li makes such a target less sensitive, but it has the advantage that there is no radiation damage in the liquid. Thus the primary motivation is to perform a measurement in which the (unknown) fraction of energy lost to radiation damage of the solid targets is essentially zero.

We have conducted an extensive series of measurements using several techniques (prompt-gamma activation, PIXE, analysis of neutron-activated image plates) to determine the systematic corrections in this measurement technique for the calorimeter targets. There are corrections due to secondary nuclear reactions, gamma and x-ray production, neutron backscattering, neutron attenuation in the cryostat radiation windows, and radiation damage. With the sole exception of the radiation damage contribution, which will be constrained by the liquid ³He measurements, all of the systematic corrections are small and either measured or measurable. The measurement using the lithium targets has been finished, and the final results are in preparation.

The Prompt-Gamma Black Detector

The prompt-gamma ray black counter and its calibration by both the coincidence method and alpha standards have undergone extensive testing at the NG-6 end station at the NIST Center for Neutron Research; and calibrations of both boron and lithium capture flux detectors have been carried out at the monochromatic beam at the NG-7 Neutron Interferometer and Optics Facility.

The two independent measurements of the boron capture gamma detection efficiency were found to agree within ± 0.15 %. The overall uncertainty of the efficiency determination by standard alpha source was about ± 0.14 %, while the uncertainty of the alpha-gamma coincidence method was about ± 0.30 %. A demonstration of the accuracy of the standard alpha sources employed in this work has been completed by carrying out a blind comparison of alpha counting standards with the Radioactivity Group at the Institute for Reference Materials and Measurements in Belgium. Recently resolved problems with the alignment of the solid-angle defining apertures may further reduce some of the systematic errors for this device. We plan a new run of the upgraded prompt-gamma detector in the near future.