1. Neutron exposure at CERN Mitsu KIMURA 19 th July 2013

2. First trial of neutron exposure Aim to expose the neutron with a few MeV - study radiation exposure in cancer treatment - reference signal in DM search experiment There is a 2.5 MeV neutron source in CERN - Maximum beam intensity : 1 x 10 6 n/sec in 4  sr. - X-ray background : no information First trial on 16 – 18 July - Measurement for neutron flux and X ray background flux Participants : - T. Asada (Nagoya), C. Regenfus, M. Kimura

3. Neutron source by DD nuclear fusion 2.5 MeV neutron -> recoil proton range is about 70  m. A base track cannot be produced. 90 cm CollimatorNo collimator 9.5 cm 32 cm 38 cm

4. Estimated neutron flux (in 4  ) 6 – 7 x 10 5 n/sec Measured by Christian Measured by company Electric current : 8 mA ( ≒ the amount of deuteron plasma) Accelerating voltage : 80 kV Exposure condition :

5. Time schedule 1 st exposure (7/16 17:40 – 7/17 14:50, 21 hr) - with collimator, - Nagoya sample (distance from production point : 45, 200 cm) 2 nd exposure (7/17 16:20 – 7/18 12:00, 20 hr) - no collimator, - Bern + Nagoya sample (45, 200 cm) 3 rd exposure (7/18 12:30 – 15:00, 2.7 hr) - no collimator, - Bern sample (7 cm) 4 th exposure (7/18 15:24 – 18:00, 2.6 hr) - no collimator, - Nagoya sample (7 cm)

6. Setup of Bern sample 7cm45cm200 cm Distance from production point Neutron X ray with less than 80 keV 1 2 3 DistanceSampleSizeExposureShield by lead plateExp time 17 cmOPERA New gel 5 x 10 cm 2 2 x 2 cm 2 Horizontal, Vertical Vertical Cover whole area2.7 hr 245 cmOPERA New gel Full size 2 x 2 cm 2 Vertical½ : OPEN, ½ : cover Cover whole area 20 hr 3200 cmOPERA New gel Full size 2 x 2 cm 2 Vertical½ : OPEN, ½ : cover Cover whole area 20 hr

7. 7 cm 45 cm 200 cm Nagoya Bern

8. Failure The sample at the 45 cm fell to the ground during beam exposure Before exposureAfter exposure

9. Development Participants : A. Ehtesham, M. Kimura OPERA film - 19 th July 9:30 – 12:20 New gel - 23 rd July 10:30 – 13:30 - Development for 20 min - Fix for 90 min

10. Conclusions Neutron source with a few MeV is very important for medical application and DM search experiment We performed neutron exposure to nuclear emulsions at CERN Exposure and development are completed We plan 2 nd exposure in 21 – 23 August

12. Recoil proton range by SRIM proton 68  m

13. Neutron cross section 2.2 barn @ 2.5 MeV JENDL 4.0

14. Estimated number of recoil protons Number of hydrogen N target =  x V x  x N A / A = 2.2 x 10 22 - Gel density  = 2.77 g/cc - Volume V = 10 cm x 12.5 cm x 44  m = 0.55 cc - Mass fraction of hydrogen in OPERA gel  = 2.4% Number of recoil protons N event =  x N target x J (s -1 ) x T (s) = (2.2 x 10 -24 cm -2 ) x (2.2 x 10 22 ) x (7 x 10 5 x  ) x T DistanceNumber of recoil protons in 10 x 12.5 cm 2 Np / view (= 0.3 x 0.3 mm 2 x 44  m) 7 cm5.3 x 10 5 3.8 45 cm9.6 x 10 4 0.7 200 cm4.9 x 10 3 0.04

15. X ray in a lead plate 80 keV Main process in material Rayleigh scattering : absorption Compton scattering : to generate electron track - Absorption length : 0.4 g/cm 2 @ 80 keV - Living probability after Pb 1mm thick exp (- L / ) = exp{- (11.34 g/cc x 0.1 cm) / 0.4 g/cm 2 } = 0.059

16. Neutron in a lead plate 8 barn @ 2.5 MeV Cross section  = 8 barn Number of atoms in 1 mm lead N target =  x l x N A / A = 11.34 g/cc x 0.1 cm x 6.02 x 10 23 x 207.2 = 3.3 x 10 21 Interaction probability Prob =  x N target = (8 x 10 -24 ) x (3.3 x 10 21 ) = 0.027 Living probability 1 – Prob = 0.973 JENDL 4.0

17. The effect of lead shield Neutron flux Electron flux