Monte Carlo Simulations of a Neutron Detector

1. Current work on dark matter points toward Weakly Interacting Massive Particles (WIMPs) as the leading candidate. As suggested by their name, these particles do not interact electrically with matter but rather through weak forces and gravitational fields. WIMPs are detected when an incident particle deposits ~10keV on the detector. Spallation neutrons produce a background in the same region. These spallation neutrons are created when a high-energy neutron (60MeV) collides with the Pb shielding around the WIMP detector. Although multiple neutrons are created, not all are detected in the WIMP detector making it impossible to distinguish between the two cases. To measure the background density of spallation neutrons a second detector is being designed and built. It is a gadolinium-loaded water detector and will be able to detect these low energy background neutrons. The signature created by spallation neutrons are multiple captures in the detector in a small, approximately 25-microsecond time frame. Results from Monte Carlo simulations will be used to diagnose the new detector design and show that it performs as intended. Monte Carlo Simulations of a Neutron Detector Laura Boon Case Western Reserve University, Department of Physics Advisor: Daniel Akerib, Department of Physics Michael Dragowsky, Department of Physics Abstract Introduction Methods The Detector The Wilkinson Microwave Anisotropy Probe (WMAP) collaboration calculated that 23% of the universe is dark matter, 4% baryonic matter and 73% dark energy (8). This means that we only understand (at least partly) 4% of the matter in the universe. There are many theories as to what this other matter could be. Current cosmological models predict that dark matter is “cold”, or non-relativisitc. Currently the most compelling theory is that dark matter is WIMPs (Weakly Interacting Massive Particles). This work has been done with Monte Carlo N-Particle Transport Code (MCNP) designed at Los Alamos National Laboratory for simulations of neutrons, photons and electrons (5). More specifically I will use MCNP-PoliMi, a specialized version of MCNP that also records the time of neutron-nuclei and other particle interactions (8). This program is perfect for this project because of this addition. I will simulate the capture of spallation neutrons in a small time frame and need to be able to measure the difference between unrelated and simultaneous events, on the order of 25 µseconds. The source used will be 252Cf, which has a half-life of 2.6 years, multiplicity of 4 and released neutrons around 2 MeV (3). *Please note that these are the specifics of the actual source that I then programmed into MCNP* Side view Front view Figure 6: Simple diagram of the detector. The blue is the water tank with Gd salt dissolved in it, and the yellow is the lead around the source to reduce the gammas emitted into the detector. The detector is 40x40x80cm and the lead is a 10cm cube. Figure 1: A pie chart showing a break down of baryonic, dark matter and dark energy in the universe. Courtesy: www.nrao.edu Results The purpose of this detector is to get a benchmark measurement of spallation neutrons for CDMS. The current prototype under testing is a 40x40x80 cm box filled with water and dissolved Gd salt with a 252Cf source providing the neutrons we are measuring. The source fissions with a multiplicity of 4 neutrons and multiple gammas. These gammas are absorbed in the lead around it while the neutrons are able to reach the detector. MCNP is able to produce similar situations so that the properties of the detector can be better studied. • Reducing background events is the biggest challenge for dark matter searches such as the Cryogenic Dark Matter Search (CDMS). The main source of background are neutrons with recoil energies in the WIMP signal region. As the group expands its search, neutrons create a greater problem, which creates the need for a better detection model. Each layer of shielding eliminates a different source of background events. Refer to Figure 2. • Earth’s crust. • Reduces cosmic rays • Location of interactions that produce low and high-energy neutrons. • Polyethylene. • Shields low energy neutrons, • Not possible to make the shielding thick enough to stop high energy neutrons. • Lead. • Shields against high-energy -rays. • Location of high-energy neutron interactions that create spallations neutrons. • These spallation neutrons are what create WIMP like signals in the detectors. • This project will aid in the work being done to understand the neutron background that mimic WIMP signals. Figure 3: A log plot of the time between captures. As expected all of the captures happened within 25 microseconds of each other. Figure 7: Here is a picture of the actual detector that is currently being tested in Soudan. The one simulated did not have the PMT’s in the center but rather assumed that when a neutron was captured was when it was “seen” by the PMT readouts. Figure 3 Figure 4: This is a graph of the simulated 252Cf multiplicity. This is used to check that the simulation matches the known properties of the 252Cf source, so we know our results can be trusted. The red is the capture multiplicity. Acknowledgements Figure 4 I would like to thank the following people for their support and help on this project: Daniel Akerib, Michael Dragowsky, Ryan Sacks and Joel Sander. Figure 5: A log plot of the energy deposited by each neutron. This kind of information cannot be calculated with the physical detector, but is good to better our understanding of what is happening through the process. References [1] Akerib, Dan; Harry Nelson. “Multiplicity Meter for Benchmarking Cosmogenic Neutron Backgrounds for Underground Experiments 2008 Annual Report”. (Private Communication) [2] CDMS collaboration. “Learn about the CDMS Experiment”http://cdms.case.edu/learn/experiment.html15 October 2008 [3] Cushman, P. Californium 252 Source for calibration of CDMS detectors in the Soudan Underground Laboratory. 15 March 2003. [4] Hennings- Yeomans, Raul. “First 5 Tower WIMP-Search Results from the Cryogenic Dark Matter Search with Improved Understanding of Neutron Backgrounds and Benchmarking” Thesis, January 2009. [5] MCNP Users. Los Alamos National Laboratory <MCNP-green.lanl.gov> 10 October 2008. [6] Miller, Chris. Cosmic Hide and Seek: the Search for the Missing Mass. 1995 [7] Our Universe. NASA. http://map.gsfc.nasa.gov/universe/uni_matter.html 14 October 2008. [8] Pozzi, Sara A. “MCNP-PoliMi: a Monte Carlo code for correlations measurements.”Nuclear instruments and Methods in Physics Research. 11 November 2003. Figure 5 Conclusions WIMP Detectors Lead Polyethylene • Multiple neutrons captured from the same fission are captured within the expected 25 microsecond range. • Based on the solid angle subtended by the detector we have a reasonable capture rate (1/6th of the released neutrons). • The lead around the source is a good shield of the gamma rays produced in the 252Cf fission. • Now that we have a working model of the detector we will able to better understand future results. Rock Figure 2: Drawing of the shielding for CDMS. Courtesy: Raul’s thesis