Neutron spallation measurements and impacts on low-background experiments

Ultralow-background experiments, such as neutrinoless double-β decay, solar neutrino, and dark-matter searches, are carried out deep underground to escape background events created by cosmic-ray muons passing through the detector volumes. However, such experiments may nevertheless be limited in sensitivity by cosmogenically induced backgrounds. This limit can be attributable to cosmogenically created radioactive isotopes produced either in situ during operation or prior to construction when the detector construction materials are above ground. An accurate knowledge of the production of the latter source of background is of paramount importance to be able to interpret the results of low-background experiments. One way to deal with the characterization of cosmogenic background production is to use Monte Carlo simulations to model the spallation reactions arising from cosmic-ray neutrons, protons, and muons. The objective of this work was to evaluate the degree of accuracy that such simulations could provide by comparing measurements for various materials to results from two standard Monte Carlo codes using the same physics model for generating intranuclear cascades. The simulated results from both codes provide the correct trends of neutron production with increasing material density. However, there was substantial disagreement between the models and experimental results for lower-density materials of Al, Fe, and Cu. The model values, when normalized to the Pb experimental results, show disagreement with experiment by a factor of about two for Fe and Cu and significantly greater for Al. It is concluded that additional neutron-induced spallation measurements are required to refine models routinely employed in underground physics research. Further data collection against the above materials is an initial list for benchmarking.

Figure 1

Neutron counter system during data taking (a) and top view with HDPE in the target position (b).

Figure 2

Distribution of neutron multiplicity in a 100-μs time window caused by cosmic neutrons in Fe.

Figure 3

Scaling of neutron rate with target volume of Pb.

Figure 4

Measured multiplicities for the different materials studied.

Figure 5

geant4-simulated neutron multiplicity from sea-level cosmic-ray showers in the different materials studied.

Figure 6

Simulated neutrons out from a cube of material as a function of neutron density. The square symbols are for materials that were measured, and the trend line is for these measured materials.

Figure 7

Spectra computed with geant4 for neutrons exiting each of the materials.

Figure 8

mcnpx and geant4 results for total neutrons out for different materials.

Figure 9

mcnpx results versus atomic mass $\left(A\right)$ for neutrons out and neutrons detected with or without the detector in place, normalized to the number of incident neutrons.

Figure 10

Comparison of simulation to measurement for counts greater than Poisson in a 1-ms window.

Figure 11

Comparison of R + A analysis of measurements to simulation of counts greater than Poisson in a 1-ms window.