Experiments towards a neutron target for measurements in inverse kinematics

Abstract

Neutron-induced reactions play an important role in fundamental nuclear physics, nuclear astrophysics, and applications. In the case of reactions on rare isotopes, there are limited options for direct experimental measurements. The Neutron Target Demonstrator project at Los Alamos National Laboratory seeks to test the feasibility of moderating spallation neutrons within a 1~m$^3$ graphite cube to create a standing neutron target for neutron-induced reaction measurements in inverse kinematics. This paper presents the results of experimental neutron flux distribution tests using neutron sources (ranging from 1~keV to 50~MeV) created by accelerators at the University of Notre Dame and Texas A\&M University. Measurements were made with both the full graphite cube as well as a ''half cube'' setup in which half of the graphite cube was removed. The measured distributions agree with simulated distributions in the case of the full cube moderator, although there remain discrepancies in certain cases for the half cube moderator. The results shown here will provide useful information for an upcoming experimental campaign to test the neutron target proof-of-principle.

Figures (17)

• Figure 1: Energy evolution of neutrons crossing the region of the ion beam pipe. The neutron source was a 800-MeV spallation source in the middle of the proton beam pipe surrounded by a graphite cube of 300 cm length. High energy neutrons reach the ion pipe only at very early times. The later, the lower the neutron energy when crossing the ion pipe. Between 0.1 ms and 0.1 s, only thermal neutrons are present in the ion beam pipe. Both axes have a logarithmic binning with 10 bins/decade.
• Figure 2: Black: A projection of Figure \ref{['fig:hockey_run_0171']} onto the energy axis. This spectrum corresponds to netrons passing the ion pipe. Red: As black, but time-weighted. The time which in the neutrons spend in the ion pipe is used for this time-weighted neutron spectrum. Since slower neutron need much more time crossing the ion pipe, much more of them are present at any given time. This spectrum corresponds therefore to the neutrons actually present in the ion pipe. The x-axis has a logarithmic binning with 20 bins/decade.
• Figure 3: Simulated setup: Full cube on the left and half cube on the right. The red pipe indicates the position of the ion beam pipe, which corresponds to the position of the gold wires. The full cube consists of 54 large graphite blocks of type A (blue) and 2 smaller blocks of type B to cover the end of the proton beam line (pink). The half cube consists of only 27 pieces of type A.
• Figure 4: Gamma spectrum of 4 activated gold foils, positioned in the middle of the half-moderator at 45 MeV, at Texas A$\&$M (run IVb)).
• Figure 5: Source energy distribution of neutrons produced inside the moderator. The effect of the moderator on the neutron distribution is not included here. All spectra are normalized to maximum 1. The binning of the x-axis is logarithmic. The neutron source reaction was $^{7}$Li(p,n) using a 0.1 mm Li layer. The spectra are based on the PINO code RHK09pino_online.