Channeling effect in nuclear events of CsI ( Tl )

1. Channeling effect in nuclear events of CsI(Tl) 2009. 10. 22 KIMS Seoul National University Juhee Lee KPS in Changwon

2. Contents • Motivation & Objectives • Introduction to channeling effect • Investigation methods • Validity of our methods • Results • Conclusion • Summary • References KPS in Changwon

3. Motivation • KIMS (Korea Invisible matter search) has been trying to see the WIMP signal with CsI(Tl) crystal. We use PSD (Pulse Shape Discrimination) method to discriminate nuclear events with the gamma background. • DAMA saids that there is the channeling effect in crystal detector which induces higher quenching factor in nuclear events with some probability, so PSD method will remove that kind of real events. [1] KPS in Changwon

4. Objectives • How much the channeling effect can we expect in CsI(Tl)? • Can we measure that kind of event? KPS in Changwon

5. Introduction to the channeling effect • Channeling effect? : If the direction of a charged particle incident upon the surface of a crystal lies close to a major crystal direction, the particle suffer a small angle scattering passing through several hundreds or thousand of lattice spacing. [2] (a) Channeling effect (b) Blocking effect KPS in Changwon

6. Introduction to the channeling effect • Measurement of the channeling effect[2] (a) photographic reproduction (b) radial projection of the (111) face of the fcc crystal KPS in Changwon

7. Introduction to the channeling effect • Calculated interatomic potential[2] V(r) = (Z1Z2e2/r) (r/a) ,(r/a) : screening function (a) I -> Ag target <110> R - target atom’s position dot - the minima of potential KPS in Changwon

8. Introduction to the channeling effect • Calculation of the Channeling effect[1] C : 31/2 aTF : The screening length of Tomas-Fermi interaction d : Interatomic spacing E : Ion’s kinetic energy KPS in Changwon

9. Investigation Methods • Quenching Factor = Emeasured / Erecoil : Emeasured is reproduced by “Sum of Ionization energy * Scintillation efficiency at each penetration depth”. • Scintillation efficiency according to Stopping power ( MeV cm2/ g) (a) CsI(Tl) : Tl 0.046 mole%[3] (b) NaI(Tl) : Tl ~0.1mole%[4] KPS in Changwon

10. Investigation Methods • Ionization energy at each penetration depth (1) SRIM (The Stopping power and Range of Ions in Matter)[5] (2) MARLOWE (Computer Simulation of Atomic Collision in Crystalline Solids Ver. 15b)[6] KPS in Changwon

11. Investigation Methods [10] • Comparing quenching factors of SRIM & MARLOWE for CsI(Tl) (a) Stopping power distribution (b) Quenching factors for CsI(Tl) for the amorphous target [7] [8] [9] SRIM MARLOWE [3] [11] KPS in Changwon

12. Validity of our methods • Comparing of Ranges of MARLOWE & Experiments[11] ( Ion : 40keV 85Kr+ , Target : Al ) (a) Stopping power distribution (b) Ranges for the amorphous target <110> Al <100> Al 7 from <211> Al <111> Al Polycrystalline Al Amorphous Al2O3 Amorphous Al (b) SRIM MARLOWE KPS in Changwon

13. Results • Critical angle for CsI(Tl) Event selection – “Total ionization energy loss>Recoil energy/4” ex.) Cs 5keV in CsI(Tl) [100]  : 4.89 [111] [100] [110] KPS in Changwon

14. Results • Critical angle for CsI(Tl) (black : calculation, red: simulation) KPS in Changwon

15. Results • Events due to the channeling effect in CsI(Tl) in the neutron induced nuclear events (a) Radial penetration : depE (b) Measured E The amount of tail is ~2% KPS in Changwon

16. Results • Quenching factor (a) (Simulation) Emeasured (b) (Simulation) Emeasured with external ions penetrating with a lattice ion penetrating symmetric axis symmetric axis KPS in Changwon

17. Results • Critical angle & channeling effect of NaI(Tl) for I ion of 4keV KPS in Changwon

18. Conclusion In the case of Cs ion penetrating CsI(Tl) with few tens of keV, • The simulated critical angle of most open axis is larger than the others. But its energy dependence is different from the calculated one. • Although the angle from symmetric axis is in the critical angle, the probability of the dechanneling is significant. And the lower the ion’s energy, the higher the probability. • The recoil ion on a lattice site have less probability to be channeled than the incoming one from outside. • If this simulation is similar with the real situation, we cannot discriminate the channeling event in the nuclear event. KPS in Changwon

19. Summary • We check the possibility to use scintillation efficiency curve to simulate the measured energy and quenching factor. • SRIM can reproduce the range and energy loss of an ion penetrating an amorphous target. • MARLOWE has the merit to reproduce the crystal effect and estimate the channeling effect. • The experiment of channeling effect with neutrons is need to tell whether these results are true or not. KPS in Changwon

20. References [1] Eur. Phys. J. C 53 (2008) 205 [2] Rev. Mod. Phys. Vol. 46, No.1 (1974) [3] Phys. Rev. Vol. 131, No.2 (1963) [4] Phys. Rev. Vol. 122, No.3 (1961) [5] http://www.srim.org [6] http://www-rsicc.ornl.gov/rsiccnew/ CodesAvailableElsewhere.htm [7] Phys. Lett. B 536 (2002) 203 [8] Nucl. Inst. Meth. A 491 (2002) 460 KPS in Changwon

21. References [9] Astro. Phys. 11 (1999) 457 [10]Nucl. Instrum. Meth. A 557 (2006) 490 [11]Nucl. Instrum. Meth. A 500 (2003) 337 [12] Phys. Rev. Lett. Vol.10 (1963) 399 [13] arXiv:0706.3095v2 KPS in Changwon