Germanium Detectors. Germanium, a special material. Detectors, big is beautiful. Operational features. Applications. Iris Abt

Transcription

1 Germanium Detectors Germanium, a special material Detectors, big is beautiful Operational features Applications Iris Abt

2 Si Si Si Si Si Si n silicon Si + P The Material Si Donor Si Si conduction valence conduction Structures P ž Li Lithium is drifted, not implanted Si ž Ge Si Si Si p silicon - Acceptor B Si valence B ž B Boron is implanted

3 The Material The properties of silicon and germanium are determined by the amount of impurities. Silicon wafers with a certain resistivity are ordered, resulting in a well defined full depletion voltage. Germanium is not so well under control. Crystal growing is done by very few people and it could be called alchemie of the 20th century. Germanium crystals are incredibly pure! Still the impurities are what counts.

4 N/cm³ e bandgap E/ e-h e speed h speed Purity The Material Silicon Germanium ²² ²² ns/100µm 8ns/100µm 1ns/100µm 1ns/100µm e ND Full depletion voltage = d² 2 e ND = 10 ¹² d=300 µm : 66V ND = 10 ¹º d=300 µm : 0.5V more charge carriers lower depletion voltage

5 Bandgap This is a severely simplified picture! Lukas Ge: 0.66 ev Probably all you ever need to know Bandgaps depend on crystal axes Drift speed also

6 Diode n bulk - + Lukas n and p-type material is in contact: charges drift across the border equilibrium with depletion There is always some leakage current.

7 Diode to Detector d Lukas n + contact n bulk Net doping concentration, N Apply external electric field and enlarge the depleted region. en B reverse biasing p + contact d 2ε V The better processed, the less leakage current.

8 A Detector Electrically active impurities: ¹º/cm³ compare to ²² Ge/cm³ p n Signal is e-h pairs 1.3 x larger in Ge than in Si Bias Amplify And 13N material Bias voltage 0.3V for a 300µm standard wafer Why don't we use germanium everywhere?

9 Germanium Detectors Germanium does not work at room temperature. This makes integrating germanium detectors into anything cumbersome.

10 Germanium Detectors Silicon is everywhere and germanium not. Types p and n: p-side: boron implants still work n-side: phosphor implants do not lithium is drifted 10 µm 10 mm scale Silicon provides very good spatial resolution.

11 Germanium Detectors Cannot compete with silicon spatial resolution. But Germanium Detectors can be big. 300µm 3 cm x 100² 66 V 0.3 V 660 kv 3 kv Big detectors are good for spectroscopy. But usually they are cylindrical.

12 Germanium Detectors E I use true coaxial detectors for illustrations. [simple field] Lukas

13 Germanium Detectors See all the energy with high resolution Compton Scattering HV Pair Production Most commercial detectors have caps. HV signal

14 Standard Detectors Cap geometry and cryostat Standard n-type germanium detector 6 cm diameter 6 cm long Mikesch has a special vessel.

15 Germanium Detectors Great spectra in the kev to MeV range. s: 2~3 kev/1~2 MeV 40K 208Tl Pa234m(U238) 969 Ac228(Th232) 911 Ac228(Th232) 1173 Co Bi214(U238) 1238 Bi214(U238) 1332 Co K Bi214(U238) intrinsic: 2615 Tl208(Th232) s(e)=öf 2.85eV E F= Energy(MeV)

16 Naive expectation: s But it is much better! Energy Resolution Intrinsic, i.e. from charge carrier creation: ² = #e-h pairs s(e)=öf 2.85eV E F= Fano factor Charge carrier creation is not purely stochastical. Reality: Electronics is what determines resolution! There is a term proportional to input capacity first amplifier stage on detector

17 6 10 Germanium Detectors PB annihilation photons 137Cs Th234(U238) 47 Pb210(U238) 93 Th234(U238) 186 U235 Ra Pb212(Th232) 295 Pb214(U238) 352 Pb214(U238) 511 Positron 583 Tl208(Tl232) 609 Bi214(U238) 662 Cs Bi214(U238) 767 Pa234m(U238) Energy(MeV)

18 Applications Germanium Detectors against terrorism Homeland Security Should somebody tell them about lead shielding?

19 Applications Germanium detectors against wine swindles 137Cs 40K: 30Bq/l 137 Cs is man-made 29 authentic wines as reference If a wine is prenuclear-testing, there is no 137Cs. Comptes Rendus Physique, vol.10, no. 7, 622 (2009)

20 Applications 137Cs Activity [mbq/l] 1Bq/l Nuclear Fallout If somebody wants to sell you expensive vintage wine, have it checked.

21 Applications Non destructive method! 137 Cs has 661 kev gamma line gammas make it easily through glass and wine 137Cs 40K

22 Applications

23 Applications

24 Applications Use the bottle itself got them

25 Applications The French like their salt radioactive!

26 Applications 226Ra mbq/kg French atlantic salts marine salts The true "fleur de sel" Portugese imposters 40K mbq/kg

27 Detector Systems Back to "normal" science: Nuclear Physics These things can built into large systems: But: Obviously there are some geometrical problems...

28 Gamma Tracking AGATA, the Advanced Gamma Tracking Array Cut crystals into shape needed and segment for tracking Form three groups of for packaging

29 Gamma Tracking AGATA, the Advanced Gamma Tracking Array 3 detectors in one cryostat 6 x 6 segments Very special detectors with extremely different segments and pulses. 90 mm 40mm

30 Germanium Detectors Let us come back to detectors: 3x3 cap true coax segmented homogenous electric field direct spatial information

31 p-type Detector Types n-type Passivation Lithium drift» 500 µm Boron implant» 500 nm p-type: holes centre n-type: holes mantle Segmentation mostly on n-type

32 Segmented Detectors p-type for a long time could not be segmented gracefully. Had to cut deep into the crystal... not good for field or durability! There is a way, but unproven.

33 Segmented Detectors n-type detectors are segmented in the implantation step: 3d segmented boron implantation is possible. [one reliable supplier only] Planar detectors are like silicon detectors [without industrial support].

34 Detector Types true coax cap coax point contact /BEGe weighting potential p p n + David Radford inverted coax does not have to be tempered n +

35 Pulse Shapes n-type The electrical field pulls the electrons to the core and the holes to the mantle. I use true coaxial detectors for illustrations. [simple field] Amplitude [arb. units] Core Amplitude [arb. units] Segment Get radial information from rise times Time [ns] Time [ns] 1~2 mm, but degenerate

36 Rise-time and Crystal Axis Amplitude [arb. units] Scan with Europium Source charge Time [ns] Avg. 10% - 90% risetime [ns] % f [DEG.]

37 Speed and Crystal Axis Speed Þ rise-time depends on angle between trajectory crystal axes Þ 110 electrons holes NIM A 569 (2006) 764

38 Trajectories Different speeds Þ bent trajectories Thesis: Jing Liu Electrons Holes

39 Pulse Shapes and Trajectories 40 Trajectory Holes Trajectory Electrons y [mm] segment boundary x [mm] Pulse Shapes are used to distinguish event classes. mirror charges

40 Final Remarks Germanium detectors are fascinating devices. They are wonderful tools to study radioactivity and trace gamma rays. Germanium detectors offer a lot of opportunity to further investigate Germanium properties: charge mobility, charge trapping, surface effects,.... They have many applications.