Channeling effect in nuclear events

of CsI(Tl)2009. 10. 22

KIMSSeoul National University

Juhee Lee

1KPS in Changwon

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Contents① Motivation & Objectives② Introduction to channeling effect ③ Investigation methods④ Validity of our methods⑤ Results⑥ Conclusion ⑦ Summary ⑧ References

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MotivationKIMS (Korea Invisible matter search) has been

trying to see the WIMP signal with CsI(Tl) crys-tal. 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]

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Objectives

How much the channeling effect can we ex-pect in CsI(Tl)?

Can we measure that kind of event?

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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

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Measurement of the channeling effect[2]

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Introduction to the channeling effect

(a) photographic reproduction (b) radial pro-jection of the (111) face of the fcc crys-tal

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Calculated interatomic potential[2] V(r) = (Z1Z2e2/r) (r/a) ,(r/a) : screening function

Introduction to the channeling effect

(a) I -> Ag target <110> R - target atom’s posi-tion dot - the minima of poten-tial

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Calculation of the Channeling effect[1]

C : 31/2 aTF : The screening length of Tomas-Fermi in-

teraction d : Interatomic spacing E : Ion’s kinetic energy

Introduction to the channeling effect

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Investigation MethodsQuenching Factor = Emeasured / Erecoil : Emeasured is reproduced by “Sum of Ionization en-

ergy * 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

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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]

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Investigation MethodsComparing quenching factors of SRIM & MARLOWE for CsI(Tl)

(a) Stopping power distribution (b) Quenching factors for CsI(Tl)

for the amorphous targetKPS in Changwon

SRIMMARLOWE

[7]

[9]

[8]

[3][11][10]

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Validity of our methodsComparing of Ranges of MARLOWE & Experiments[11] ( Ion : 40keV 85Kr+ , Target : Al )

(a) Stopping power distribution (b) Ranges for the amorphous target

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SRIMMARLOWE

<110> Al <100> Al7 from <211> Al <111> Al

Polycrystalline AlAmorphous Al2O3Amorphous Al

(b)

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ResultsCritical angle for CsI(Tl) Event selection – “Total ionization energy

loss>Recoil energy/4” ex.) Cs 5keV in CsI(Tl)

[111][100][110]

[100] : 4.89

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ResultsCritical angle for CsI(Tl) (black : calculation, red:

simulation)1keV 5keV 10ke

V

15ke

V

20ke

V

50ke

V

[111]20.3410.62

13.68.02

11.447.1

10.337.36

9.627.87

7.658.25

[100]18.316.27

9.444.89

7.945.32

7.185.01

6.685.33

6.895.05

[110]14.12

6.49.444.05

7.943.08

7.183.49

6.683.55

5.314.35

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ResultsEvents due to the channeling effect in CsI(Tl) in the neutron induced nuclear events

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(a) Radial penetration : depE

(b) Mea-sured E

The amount of tail is ~2%

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ResultsQuenching factor

(a) (Simulation) Emeasured (b) (Simulation) Emea-sured

with external ions penetrating with a lattice ion pene-trating

symmetric axis symmetric axisKPS in Changwon

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ResultsCritical angle & channeling effect of NaI(Tl) for I ion of 4keV Ref.[13] MARLOWE

[100] 6.4

[110] 4.9

[111] 2.8

(100) 4.1

(110) 3.2

(111) 2.7

Channeling ef -fect

~20% 4.1%

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ConclusionIn 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.

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Summary① We check the possibility to use scintillation effi-

ciency curve to simulate the measured energy and quenching factor.

② SRIM can reproduce the range and energy loss of an ion penetrating an amorphous target.

③ MARLOWE has the merit to reproduce the crys-tal effect and estimate the channeling effect.

④ The experiment of channeling effect with neu-trons is need to tell whether these results are true or not.

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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

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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