Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium...

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Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg, 03-07/03/2008 Introduction to GERDA experiment Test stand for GERDA with naked Ge detector Results with IR- & UV-light Outlook

Transcript of Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium...

Page 1: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

Effect of IR- & UV-lighton naked Germanium detector

Xiang Liu

Max-Planck-Institut für Physik, München

DPG Tagung, Freiburg, 03-07/03/2008

Introduction to GERDA experiment

Test stand for GERDA with naked Ge detector

Results with IR- & UV-light

Outlook

Page 2: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

GERmanium Detector Array (GERDA) search for 0νββ decay in 76Ge

clean room

Muon veto

detector array

water tank (10m)

LAr cryotank (4m)

water Č veto

14m

Ge detectors directly submerged in LAr

page 2

new segmented detectors (phase-II)

Located at LNGSunderground lab(3500m.w.e)

Page 3: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

Ge detectors see IR- & UV-light

page 3

need to understand Ge detector reaction to IR- & UV-light.(UV light from LAr not an issue, due to low energy deposit.)

LAr scintillation light (128nm)

IR radiation

Page 4: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

n-type non-segmented detector

page 4

From Canberra France.65mm diameter, 77mm high.Operated 61x in LN2, 4x in LAr,no obvious deterioration.

Ge crystal

Teflon layerfor protection

FET

preamp.

HV

“cooling finger”

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detector cooling

page 5

Page 6: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

Eu152 energy spectra

page 6

FWHM 3.5-4keV at 1.3MeV (dominated by electronic noise).Leakage current (LC) <20pA at -3500V.

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Detector n+ & p+ layer

page 7

Depleting voltage (-3500V) applied to the outer surface

passive coating

B implantation0.3μm (p+)

Li drift600μm (n+)

naked Gep-n junction at outer surface

p+n

n+

Page 8: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

detector exposed to IR- UV-light

page 8Photons could reach active detector in two different ways.

naked Gecoating

LED

Page 9: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

Leakage current (LC)↑ when IR- UV-light ↑

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slope = ΔLC / ΔI_LED [pA/mA]

UV(360nm)s=1.3 pA/mA

IR(1550nm)s=2500 pA/mA

Why IR & UV havedifferent slopes?

How does slope dependon bias voltage?

(Bias Voltage -3500)

Page 10: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

Bias V dependence: LC induced on surface vs. LC induced in bulk

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λ [nm] α [cm-1] 1/α induce LC at

0.02μm surface

surface & bulk1mm

0.6E6

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expected slope dependence on bias V

360 weak

1550 strong

Germanium optical property:

B implantation0.3μm (p+)

Li drift600μm (n+)

depleted volume(active volume)

n-type

Bias V =0 Bias V increase

Page 11: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

slope = ΔLC / ΔI_LED at different bias voltage

page 11

360nm

1550nm

bias HV s(Vbias) s(Vbias=-3500V)

as expected

Page 12: Effect of IR- & UV-light on naked Germanium detector · Effect of IR- & UV-light on naked Germanium detector Xiang Liu Max-Planck-Institut für Physik, München DPG Tagung, Freiburg,

Why IR slope >> UV slope?

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material λ [nm] 1/α n

coating 360 6.7nm 0.397

1.44

1.21

coating 1550 7.7nm

LN2 IR-UV

(total reflection at 19°)

naked Gecoating

360nm UV 1550nm IR

Main reason:coating reflectsmore UV than IR.

Most UV lights reach active volume through the naked Ge surface.

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To prove that UV gets in through naked Ge surface

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Detector shielded with extra 2mm-thick Al-plate.

Center hole with different sizes, Φ=0, 5, 10, 15, 20mm.

360nm

1550nm

UV slope almost independent on hole size.

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Conclusion & outlook

page 14

UV- & IR-light effect on n-type naked Ge detector as expected.Only qualitative study possible with current test stand.New test stand appropriate for this study under construction.

3D scan with γ, α and laser.

vacuum tank

LN2 feedthrough

LN2 tank

stage 2

stage 1

stage 3

source & collimator

detector (inside IR shield)