Simulation of Neutron Backgrounds in the ILC Extraction Line Beam Dump

1. Simulation of Neutron Backgrounds in the ILC Extraction Line Beam Dump Siva Darbha University of Toronto SLAC, ILC BDS Supervisors: Lewis Keller and Takashi Maruyama SULI Presentation

2. The International Linear Collider (ILC) • 500 GeV center of mass energy • Precision measurements • Clean signal to noise ratio SULI Presentation

3. Extraction Line and Water Dump x (cm) Concrete Collimator Most n are directed forward n 40 cm e+ A e- e- 250 GeV A’ H2O Concrete Tunnel FLUKA was used for all simulations, ROOT for analysis and some particle generation z (cm) SULI Presentation

4. Fluence at IP x (cm) Tunnel Wall • Flux was treated as isotropic from -1 < cosθ < -0.99 • Flux for 1.5 cm radius scoring plane at z=0 was found from flux in 2 m radius scoring plane At A-A’ (on the surface of the dump) Quadrupole aperture y (cm) Scoring plane at z=0 SULI Presentation

5. Three types of biasing were used: Particle Biasing • Leading particle biasing • simulating a full EM shower requires long CPU time • to save time, take only the most energetic secondary and remove all others • applied to e+,e-, and γ’s < 2.5 GeV • Photonuclear interaction length • #n produced proportional to lσ • σ was increased by a factor of 50 • ‘weight’ associated with each n produced from this was decreased by a factor of 50 to compensate γ A → n + X (σ, l) SULI Presentation

6. Particle Biasing (continued) x (cm) • Splitting/Russian roulette • Dump divided into 10 regions • Each region given a factor of 2 larger importance • As e+, e-, or γ crosses a boundary, their number is increased or decreased on average by the ratio of importances on either side of the boundary • ‘weight’ is adjusted accordingly e- … 8 4 2 1 z (cm) SULI Presentation

7. Cutoffs • Production • e+, e- > 50 MeV • photons > 10 keV • Transport • e+, e- > 50 MeV • photons > 10 keV • neutrons > 10 keV (group number < 48) SULI Presentation

8. Computation Time SULI Presentation

9. Fluence at IP 1010 n/cm2 at the VXD would cause displacement damage to CCD Si detectors However, not all neutrons that reach the IP will hit the inner detector SULI Presentation

10. Neutron Energy Distribution • Information was gathered on the neutron distribution in the backward direction and was used to generate 106 neutrons to study the real flux at the VXD 10 MeV bins In first 10 MeV bin SULI Presentation

11. Detector x (cm) All n’s were given a 7 mrad trajectory towards the detector C 2.4 cm quadrupole 2.4 cm 7 mrad n 3.0 cm C’ Be Si VXD W BeamCal SULI Presentation z (cm)

12. Initial position of n’s • n’s randomly and uniformly distributed within the quadrupole bore C-C’ SULI Presentation

13. CCD Si VXD with Be beampipe A B x (cm) x (cm) B-B’ A’ z (cm) B’ A-A’ y (cm) SULI Presentation

14. Results: Fluence at VXD • The BeamCal acts as a collimator for neutron backscattering from dump • With the W BeamCal, the nominal fluence at Layer 1 of VXD is: 4.28*108 n/cm2/year No BeamCal W BeamCal Black BeamCal SULI Presentation

15. 1 MeV Neutron Equivalent Fluence • However, the amount of displacement damage done to CCD Si detector by neutrons is a function of neutron energy • When relative damage to Si is considered, normalized to 1 MeV, the fluence is: 9.27*108 n/cm2/year • When e+ beam is considered also, value is doubled to 1.85*109 n/cm2/year • A value of 1010 n/cm2 would damage the CCD Si detector by this measure 1 MeV SULI Presentation T. M. Flanders and M. H. Sparks, “Monte Carlo calculations of the neutron environment produced by the White Sands Missile Range Fast Burst Reactor,” Nuclear Science and Engineering, vol. 103, pp. 265 – 275, 1989.

16. BeamCal Radius Dependence • Values are not normalized to 1 MeV fluence • Values should be doubled to incorporate positron beam SULI Presentation

17. Acknowledgements • Takashi Maruyama and Lewis Keller • Tom Markiewicz • Nan Phinney • Mario Santana • Nicholas Arias • SLAC • ILC BDS Group • DOE, Office of Science SULI Presentation