Semiconductor Detectors

Solid state Ionization Counters

1/40 eV

4 valence atoms

No need to go to very low Temperature with Si:

Intrinsic carriers concentration

ISi ∼ 10-9 A, IGe ∼ 10-3 A,

Leakege current

At room temperature: Measurable with Si detectors NOT Measurable with Ge detectors

ISi ∼ 10-9 A, IGe ∼ 10-3 A, Leakege current

n-type p-type

To control the electrical conduction

p-type in contact with n-type give rise to a p-n junction

n-type p-type

Increase of depletion region by applying REVERSE BIAS ⇒ E = Eint+ eVext Ge: Vext = 1000-3000 V, Si: Vext = 300 V

∇2ϕ = −ρ /ε

Ε(x) = − dϕdx

∇2ϕ = −ρ /ε

p-type n-type

Used for charged particles: 1 MeV e-, range = 1 mm 5 MeV α, range = 0.02 mm

Electric contact

77 K (−196 °C) with LN2

High Purity Ge Detectors impurity concentration N ~ 1010 atoms/cm3

600 µm

0.3µm

eNVd ε2

≈

Characteristics size Ø~10cm, L~9cm shape coaxial n-type less sensitive to radiation damage operating temperature < 85 K rates ~ 10 kHz to prevent pile-up energy resolution 2 keV at 1.332 MeV (0.2 %) time resolution 4-5 ns (with CFD) 200-300 ns total rise time efficiency* up to 200% * relative to 7.5x7.5 cm NaI(Tl) for 1.33 MeV γ-rays emitted by 60Co source at 25 cm from detector (εa = 1.2 x 10-3)

active region d

V~2500-4500V

15%

150%

See also: Best choice HpGe detector, from ORTEC

Signal Pulse Shape depends on interaction point

V+

V-

V+

V-

Planar geometry

Coaxial geometry

most severe in p-type detectors

Trapping Effect: - Reduction of pulse amplitude due to capture of carrier by trapping centers -  Deterioration of energy resolution due to variable amount of of charge lost per pulse

Energy resolution versus Temperature FW

HM

[ke

V]

FWHM

[ke

V]

Temperature [K] Temperature [K]

@122 keV @1332 keV

band structure effect

T(LN2) = 77 K (−196 °C)

Pulse shaping

true pulses

from preamp τ ~ 50µs

after shaping

FET

Preamplifier : FET (at 130 K, to minimize noise) Amplifier: CR-RC shaping circuit

pile-up

energy

time

τ

τ

t

out etEE −=if C1R1=C2R2=τ

τ ~ 15 µs τ ~ 15 µs is a good compromise

between reduced pile-up and good energy resolution

(depending on large charge collection)

mV

V

Preamplifier

R=input resistance C=input capacitance+ detector capacitance + cables …

τ = RC

operation mode for time information, high rates, …

operation mode for energy information

tc charge collection time ~100 ns

τ =RC decay time ~ 50 µs

Amplifier (RC-CR shaping)

RC-integrator (low-pass filter)

)1(...

/τtout

outin

eEE

EiRE

−−=

+=

CR-differentiator (high-pass filter)

τ/

...t

out

outin

EeE

ECQE

−=

+=

γ-ray interaction

γσ EZpp ln2≈

ionization occurs in limited regions of the absorber

Ge

µ

ppCph σσσµ ++=

Linear attenuation coefficient (probability per unit path)

γ

γσEE

ZC

ln≈

54

5.3

−=

≈

n

EZ n

phγ

σ

I/I0

t

e-µt

Detector response We detect recoil electrons

and NOT photons !

)(256.02

/21

22

2

cmEifMeVcm

cmEE

EEE

ee

eCEgap

>>=≈

+=−=

γ

γ

γγ

Egap

Important characteristics: §  energy resolution: δEγ/Eγ = FWHM/Eγ

§  peak-to-total: P/T = Areapeak/Areatotal

Egap

Ge Response function (+ Anti-Compton Shield)

P/T~20%

P/T~60%

Anular detector

used material: BGO (Bi4Ge3O12) §  density ~ 7.3 g/cm3

§  Z = 83 §  3 times more efficient than NaI ⇒  ideal for very compact geometry (small spaces) N.B. in some cases NaI nose is used to improve the light output far away from PM tube

used with heavy metal collimators in front

incident γ

Compton scattering angular distribution

)cos1)(/(1 2'

θγ

γγ −+=

cmEE

Ee

high-energy γ-ray: forward scattering low-energy γ-ray: forward & backward

NaI nose: improvement of light output far away from PM tubes (low-energy γ-rays) BGO back-catcher: improvement of high-energy Compton scattering (high-energy γ-rays)