Neutron and 9Li Background Calculations Peter Fisher/Jon Link Oct. 27, 2005 Peter Fisher - MIT

Checks on neutron background calculations Outline Previous results Checks on neutron background calculations 9Li production: reinterpetation of Hagner experiment Peter Fisher - MIT

Previous Results GEANT4 Calculation: 0.8 neutron/day at vessel All neutrons originating from closer than 6 m are vetoed; high premium on veto efficiency Peter Fisher - MIT

Simple Calculation (J. Link): Pilcher-Hurwitz muon energy spectrum and angular distribution at 450 MWE Based on Wang et al. neutron energy spectrum MARS used to transport neutrons through rock (~150/ton/day in rock surrounding the detector) 0.93% survive passage through 1m of heavy concrete and 0.5 m mineral oil 20% of neutrons reaching vessel have 0.8<T<8 MeV 88% of neutrons reaching vessel have veto muon 0.47 unvetoed fast neutrons/day reaching the detector Peter Fisher - MIT

9Li (J. Link) Scaled from KamLAND, 1.5 GeV tag on muons losing more than 1.5 GeV in the detector tags 85% of all 9Li A 0.5 s veto would eliminate 85% of all 9Li decays, giving overall efficiency of 72% for getting rid of 9Li decays with 7% deadtime. The Veto Working group has spent the last few months checking these calculations and understanding 9Li production. Peter Fisher - MIT

Neutron background: comparison between hand calculation (Noel Stanton) and GEANT4 (Peter Fisher) for very specific muon source geometries located just outside the shield: Line source Sheet source Peter Fisher - MIT

Unvetoed, 1<En<10 MeV* 101 86 Sheet (x 10-6n/m) Line Unvetoed, 1<En<10 MeV* 101 86 Simple calculation 130 250 *Roughly 10% of neutrons have energies greater than 10 MeV Intuitively, one would expect the number of neutrons form the line source to be greater than from the sheet. More work needs to be done. Peter Fisher - MIT

Where do the neutrons come from? Toy model spreadsheet calculation (Spherical symmetry; point production of neutrons; neutrons travel in straight lines; effective neutron attenuation lengths; H&P muons; Wang neutrons/muon) Sources of neutrons (spherical geometry): Fiducial volume, r=260 cm Oil buffer, thickness 85 cm Steel vessel, thickness 5.0 cm CHESS shield, thickness 100 cm, “counters” on outside Dolomite rock, starts 100 cm outside shield Parameters: effective attenuation lengths in each medium; muon detection inefficiencies of “counters”, fiducial volume, oil buffer Peter Fisher - MIT

Toy model: calculate neutrons/day into fiducial volume Note: Fake IBD rate has further reduction factors, ~1/5 “Base” rate: contribution without shielding or tagging “Mitigated” rate: includes passive shielding Untagged rate: includes also muon tagging The two examples below use extremes of  for oil; same tagging effic We must understand effective  for H-rich stuff (oil, water) ! Peter Fisher - MIT

Toy model neutrons/day into fiducial volume (continued) Examples below have optimistic  for oil, varying tagging efficiency Looks like we want shield “counter” efficiency > 99% Peter Fisher - MIT

9Li Production m 9Li m n p 9Li 12C(n,n3p)9Li Hagner et al. (Astropart. Phys. 14(2000)33 measured σ(μ+12C->9Li+X) assuming 9Li was produced directly in μ-nucleon interactions: m 9Li However, it seems unlikely to produce 9Li directly for Eμ>10 GeV. From our studies, it appears the process looks like this: m Deep inelastic scattering n p 9Li Neutrons are produced ~20 cm from muon axis. 12C(n,n3p)9Li Peter Fisher - MIT

For the case of the multi-step process and, since the neutron energy spectrum is relatively independent of Em for Em>10 GeV, the production of 9Li breaks into two parts: Production of neutrons by muons, the number of neutrons increases with muon energy but shape of neutron spectrum remains the same(peaks around 100 MeV) Production of 9Li by neutrons (integral of s(12C(n,n3p)9Li) over neutron energy spectrum Bottom line: if we determine #2 from 9Li production anywhere from a known muon source, we know it at any depth Peter Fisher - MIT

If 9Li production is a multi-step process, what did Hagner measure anyway? Hagner measured 9Li production in scintillator target cell from 190 GeV muons. Result: This is geometry dependent and hence not really a cross section! However, can use this to extract a cross section for 12C(n,n3p)9Li. Peter Fisher - MIT

Calculations and measurements collected by Steve Dytman Common thread: All have approximately linear rise starting at 50 MeV and flattening at 100 MeV. Roughly, can think of single parameter: cross section at 100 MeV. Peter Fisher - MIT

Analog of hagner setup using Braidwood GEANT4 MC. Rock Use (100MeV)=0.2mb 20 million incident muons gives: 369 9Li events total 14 9Li events in target cell 47 9Li event in Z slice. Number of events in target cell corresponds to =2.2 b consistent with Hagner Using Hagner cross section would underestimate the total number of 9Li by a factor of three! Rock 240 cm Shield 100 cm Vessel Z slice Buffer 88 cm  Target Cell Peter Fisher - MIT

Simple Calculation (Noel Stanton) R(9Li)=(F *G * M) * f * n * <L> *  Muon flux (P&H) Neurons/muon/g/cm2 (Wang) Detector mass Takes into account rise in Cross section from 50-100 MeV Target density Attenuation weighted neutron Pathlength (=69 cm) Using 9Li rate from KamLAND, =0.64 mb Applying to Braidwood gives a rate of 7/day Peter Fisher - MIT

Conclusions Given the limitations of the calculations (statistics for GEANT4 and approximations for simple calculation), the agreement for neutrons is reasonable. Results for the sheet source will be investigated further. For 9Li, there is a factor of three difference in extracted cross section, again consistent with the limitations of the calculations. For purposes of future calculation, a cross section of (12C(n,n3p)9Li, En>100 MeV)=0.64 mb should be used. The Hagner cross section should not be used. We plan to use our GEANT4 code to estimate the 9Li rate for Braidwood based on the cross section in Point 2. After that, we will retire our code and use RAT. Peter Fisher - MIT

Conclusion (cont.) Our next simulation task will be to validate RAT with previous results. Noel Stanton has made a first cut at how measuring backgrounds in situ and we plan to use RAT to continue this study. Peter Fisher - MIT