Report on The Study of (α,n) Neutron Yield and Energy Spectrum Dongming Mei for the AARM collaboration 1

Motivation (α,n) neutrons produced in the materials, which will be used to build the detector components for low background experiments, are important backgrounds Both neutron yield and energy are important Cross sections in particular resonances should be validated The calculation done for alphas from 238 U and 232 Th are not sufficient – 228 Th and 226 Ra needs to be taken into account The yield and energy spectrum in different materials are only calculated by a few groups The validation of the calculations are needed – Start with independent comparisons – Measurements and benchmarks are also on the way 2

Calculations by the SNO collaborators R. Heaton et al., Nucl. Geophys. V 4, 499 (1990). R. Heaton et al., Nucl. Instrum. Methods Phys. Res. A 276, 529 (1989). 3

USD Calculations D.M.Mei, C.Zhang, A.Hime, NIMA606, (2009) 4

Calculations using SOURCES pdf pdf Radial Prot Dosimetry, 2005: 155 (1-4)

Comparison (USD vs SOURCES)Implemented By European Scientists Marco Selvi (one of several) The overall agreement in the neutron yield is anyhow quite good,I found everything within a factor of 2, which is not so bad I would say. 6

Comparison Accomplished by US Scientists K.Palladino (MIT), H.Qiu (SMU), S.Scorza (SMU) 7

Modus Operandi TENDL vs SOURCES4 lib cross sections TENDL 2011 and 2012 have been considered as the USD website inputs. TENDL is a nuclear data library (validated) which provides the output of the TALYS nuclear model code system SOURCES4 cross section input libraries come from EMPIRE calculations and for some isotopes a combination of measurements and EMPIRE calculations USD website vs SOURCES4 calculations: compare the radiogenic neutron spectra coming from both codes and some simulation quick checks for Cu. 8

X section comparison ✔ Good agreement in the (alpha,n) ROI (0-10MeV) for most of the isotope considered – details ✖ C13, O17, … : SOURCES4 inputs match TENDL 2011 at low energy and then match TENDL 2012 at high energy SOURCES4 input – TENDL 2011 –TENDL 2012 Cu65 -> ok! C13 9

X section input libraries For many isotopes SOURCES4 cross section input libraries consist in a combination of measurements and EMPIRE calculations -> we believe SOURCES4 cross section libraries the right choice. - EMPIRE is the code recommended by IAEA. - Neither EMPIRE nor TALYS can calculate properly resonance behavior which has been experimentally observed (if we trust the data) 10

Copper check - Inputs SOURCES4 calculation considers – 63 Cu = 70%, 65 Cu = 30% –1ppb 232 Th in Cu (100% 232 Th) –1ppb U in Cu (99.28% 238 U % 235 U) USD website considers –Nat Cu –1ppb 232 Th in Cu (100% 232 Th) –1ppb U in Cu (100% 238 U) 11

Copper Check - Neutron Spectrum Comparison 12

Spectra Integration (n/s/cm3) SOURCES4 Th: 9.49 E-12 n/s/cm 3 U: 2.90 E-12 n/s/cm 3 USD website Th: 1.11 E-11 n/s/cm 3 U: 3.46 E-12 n/s/cm 3 SOURCES4/USD Discrepancies Thorium ~15% Uranium ~13% 13

Simulation check We have performed some quick simulations: propagate both USD and SOURCES4 radiogenic neutron spectra in the same experimental geometry to check the background neutron rate 1 Million neutrons from U and Th decay chains each in a simple geometry: 1 Cu can ( cm 3 ) around 100kg of germanium detector. The same Cu can has been contaminated with both USD and SOURCES spectra Cu contamination level: Th: 0.02mBq/kg U: 0.1 mBq/kg Neutron rate from (alpha,n) reactions and has been reported 14

SOURCES4/USD ~20% discrepancy in the rate found in Ge detectors USD rate > SOURCES4 rate 15

Glass composition from Hamamatsu, more elements and higher neutron yields than just silicon, oxygen and boron due to inclusion of 4% each by mass fraction inclusion of sodium, aluminum and barium Borosilicate glass check 16

Borosilicate Spectra Integration (n/s/cm3) SOURCES4 Th: 1.27 E-10 n/s/cm 3 U: 3.63 E-10 n/s/cm 3 USD website Th: 6.98 E-11 n/s/cm 3 U: 2.45 E-10 n/s/cm 3 SOURCES4/USD Discrepancies Thorium ~82% Uranium ~48% 17

Borosilicate Simulations Simulations done within RAT, utilizing Geant4.9.5, and a cylindrical 45T liquid argon single phase detector surrounded by borosilicate glass mimicking PMTs as well as stainless steel and a water veto Simulation done with 1.65 times more thorium than uranium matching assayed values of glass (and old simulations) # SimulatedEvents keVee & >65 cm from wall & PSD cutMC single scatters in ROI USD Sources Borosilicate Conclusion : Shape differences are not a large effect, but at this assay value Sources would have 1.8x more neutrons produced than the USD code 18

Next steps Check SOURCES calculations having Talys cross section as inputs Check USD calculations having Empire inputs (is it possible?) Cross check EMPIRE and TALYS cross sections with other calculations Benchmarking against the experimental nuclear data. (SOURCES has already provided some comparison studies in the users guide but not for U/Th decay chains) 19

Conversion factor #->n/kg/y Here below the formula used for calculating the normalization factor needed to convert the number of neutron found in the IZip from simulation into a counting rate (n/kg/year). 20

Summary of the Comparisons Neutron yield agreement is within a factor of 2 for all materials compared so far The agreement in energy spectra from various materials is not good – Understand the cross sections including resonances – The calculated kinetic energy of out-going neutrons with respect to different excited states of the final nucleus 21

Neutron Screening Facility (LZ-veto) Gd-LS detector (Courtesy to the LZ collaboration)) 22

LZ-Veto as a Neutron Screening The goals of this device are: To further characterize the neutron and gamma ray environment adjacent to the LUX liquid xenon, so as to better constrain possible background contributions to any apparent nuclear recoil signal from WIMPs. To screen a variety of components for neutron and gamma activity. Of particular interest are components containing fluorine like polytetrafluoroethylene (PTFE) used in noble-liquid dark matter detectors, including the LUX liquid xenon detector. Fluorine is particularly susceptible to emission of neutrons caused by impinging alpha particles, from radioactive contamination both on its surface and intrinsically. Such neutrons can potentially contribute to the background for a WIMP signal. In addition, this device can screen some materials more accurately for gamma-ray emission than standard germanium screening devices. To develop and establish safe and effective designs and procedures for deployment of Gd-LS deep underground. To conduct searches for rare processes involving bursts of neutrons, for example, the spontaneous fission of 232 Th, or searches for super-heavy elements. 23

Future Work Continue to work internationally as a collaboration Monte Carlos Validation – Energy spectra in different materials – Specifically focus on resonances in cross section – Include 228 Th and 226 Ra – Materials such as PTFE Measurements – USD neutron detector for measuring the (α,n) neutrons from rock – Other measurements from various experiments – LZ-veto for measuring (α,n) the neutrons from various materials 24