1. Design of a shielding system for the new n-TOF-Ph2 Actinide Sample Cannings in use with Total Absorption Calorimeter Design of a shielding system for the new n-TOF-Ph2 Actinide Sample Cannings in use with Total Absorption Calorimeter Carla Santos 1), D. Cano Ott 2), C. Guerrero 2), I.F. Gonçalves 1), T. Martinez 2), P. Vaz 1) ITN, Sacavém - Portugal CIEMAT, Madrid - Spain

2. Old and new setups Neutron absorber vacuum crystals PM’s Air  rays  No interior pipe windows  No Titanium canning  Exterior pipe windows closer to the TAC  Aluminum backing of the sample

3. Motivations for a new shielding system To achieve a significant reduction of the TAC background due to beam neutrons scattered in the beam pipe windows. Items studied: 1.Effect of the new beam pipe configuration 2.Different geometrical configurations 3.Three different materials for the new shielding

4. Method The CIEMAT Monte Carlo simulation code of the TAC with the full geometry (crystals, PMT, absorber) and event reconstruction software. Long time consuming simulations (~1 month) on CIEMAT’s LINCE cluster (>120 Pentium IV CPUs) ITN modeling of the various neutron shielding designs for the cannings. -Different geometries of the shielding elements -Different materials (doped polyethylene) of the absorbing elements Simple modeling of the experimental area for investigating its influence. The neutron sensitivity of the setup was computed for the different materials and geometries of the shielding elements as a function of the neutron energy.

5. Results – Effects of the new tube (no shielding elements) ----- Old Pipe Interior windows ----- Old Pipe Exterior windows ----- New pipe windows 1 eV < En < 10 eV A reduction factor less than 2 10 eV < En < 100 eV A reduction factor of 2 100 eV < En < 1 keV A reduction factor of 3

6. Studied geometries of the new shielding system 7 different geometries simulated Total of 13 configurations under study 118.6  2 dm 3 of polyethylene Each cylinder 50 cm long, 25 cm high 71.2  2 dm 3 of polyethylene Each cylinder 30 cm long, 25 cm high 31.7  2 dm 3 of polyethylene Each cone 30 cm long, 5 cm to 25 cm high 32.8  2 dm 3 of polyethylene Each small cylinder 6 or 7 cm long, 5 cm high

7. T hree different materials for the additional shielding blocks were considered: Polyethylene doped with 5% of natural Boron, Polyethylene doped with 10% of natural Boron, Polyethylene doped with 7.5% of natural Lithium.

8. Results Effect of the different Materials ----- Absorber with 7.5% Lithium ----- Absorber with 10% Boron ----- Absorber with 5% Boron In all energie ranges borated polyethylene is a better solution for a factor of ~2 The difference in the borated is more significant, 40%, in the first interval. Then this difference decreases to 24% and 17%.

9. Results Effect of the different geometries ----- Absorber: Reference Cylinder ----- Absorber: Cylinder ----- Absorber: Cone For neutron energies 1 eV < En < 10 eV The difference in the probabilities of detection between cone and reference cylinder is a factor of 2. No better than 30%.

10. Results Effect of the different geometries ----- Absorber: Cone ----- Absorber: Cone + small cylinder ----- Absorber: Cone + small cylinder + Lead For neutron energies 1 eV < En < 10 eV The difference in the probabilities of detection between cone + small cylinder and reference cylinder is a factor of 2. 100 eV < En < 1 keV Difference of 8%.

11. Old setup and new setup with shielding ------ Old tube interior windows ------ Absorber: Cone + small cylinder 1 eV < En < 10 eV A reduction factor of 4 10 eV < En < 100 eV A reduction factor of 9 100 eV < En < 1 keV A reduction factor of 14

12. Conclusions  The new beam pipe improves by a factor of ~2 in the energy range 1 eV < En < 100 eV, and the a factor of 3 in the energy range 100 eV < En < 1 keV, the rejection of the background due to the scattered neutrons in the windows.  The compromised solution for the material, considering also financial constrains, in the new neutron shielding, is polyethylene doped with 5% Boron.  The “recommended” geometry for the shielding element would consist of a conical shape in conjunction with an additional small cylinder to veto the small-angle scattered neutrons.

13. Conclusions These results allow to estimate a background reduction by:  a factor of 4 in the neutron energy interval 1 eV < En < 10 eV  a factor of 9 in the neutron energy interval 10 eV < En < 100 eV  a factor of 14 in the neutron energy interval 100 eV < En < 1 keV for the original background from scattered neutrons in the pipe windows.