Semiconductor Detectors Solid state Ionization Counters

Semiconductor Detectors

Solid state Ionization Counters

1/40 eV

4 valence atoms

GROUP (IVA) same external electronic configuration

Z A

SILICON is the second most abundant element in the Earth crust (28.2%) GERMANIUM 54° most abuntant (1.5%)

(Most abundant is oxygen, 46.1%)

Abundance (atom fraction) of the chemical elements

in Earth's upper continental crust as a function of atomic number.

Not included are elements with extremely

low crustal concentrations:

• technetium (Z=43),

• promethium (61)

• all elements with Z > 83 (except thorium (90)

and uranium (92)).

No need to go to very low Temperature with Si:

Intrinsic carriers concentration

I_Si ~ 10^-9 A, I_Ge ~ 10^-6-10^-3 A Leakage current

At room temperature:

Measurable with Si detectors

NOT Measurable with Ge detectors

I_Si ~ 10^-9 A, I_Ge ~ 10^-6-10^-3 A, Leakage 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 Þ E = Eint+ eVext

Ge: Vext = 1000-3000 V, Si: Vext = 300 V

∇²ϕ = −ρ / ε

Ε(x) = −dϕ dx Fixed charges

!!!

Too small to use it for detection (high recombination) N_A

ND

n p

REVERSE BIAS DIRECT BIAS

(DIODE, current conduction majority carrier flow)

(Little current conduction minority carrier flow)

≈ 1 V Too small

Charge recombination

majority carrier flow

minority carrier flow

A decrease in potentialf(x) (defined for positive charge) corresponds to an increase in electron energies

p

p n

∇²ϕ = −ρ / ε

Condizioni al contorno per E e V

2 2

2 2

A

A

A N_Da = N_Ab

p-type n-type

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

Electric contact

77 K (−196 °C) with LN₂

i= Intrisic (non-doped) layer)

High Purity Ge Detectors

impurity concentration N ~ 10¹⁰ atoms/cm³

600 µm 0.3µm

eN d 2eV

»

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 g-rays

emitted by ⁶⁰Co source at 25 cm from detector (e_a = 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

Annealing process

Trapping Effect:

- Reduction of pulse amplitude

due to capture of carrier by trapping centers

- Deterioration of energy resolution due to variable amount of charge lost per pulse

Energy resolution versus Temperature

FW H M [ ke V] FW H M [ 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

t ~ 50µs

after shaping FET

Preamplifier :

to minimize noise) Amplifier: CR-RC shaping circuit

pile-up energy

time

t t t

out t e

E

E = ^-

if C₁R₁=C₂R₂=t

t ~ 15 µs

t ~ 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 …

t = RC

operation mode

for time information, high rates, …

operation mode

for energy information

t_c

charge collection time ~100 ns

t =RC

decay time

~ 50 µs

Amplifier

(RC-CR shaping)

RC-integrator (low-pass filter)

) 1

( ...

/t t out

out in

e E E

E iR E

- -

= +

=

CR-differentiator (high-pass filter)

t /

...

t out

out in

Ee E

C E E Q

= -

+

=

g-ray interaction

s_pp » Z²lnEg

ionization occurs

in limited regions of the absorber

Ge

µ

pp C

ph

s s

s

µ ^{= + +}

Linear attenuation coefficient

(probability per unit path)

g

s g

E Z E

C

» ln

5 4

5 . 3

-

=

» n

E Zⁿ

ph

g

s

I/I₀

t

e^-µt

Detector response

We detect recoil electrons and NOT photons !

) (

256 . 2 0

/ 2 1

2 2

2

c m E

if c MeV

m

c m E E E

E E

e e

e CE

gap

>>

=

»

= + -

=

g g

g g

E_gap

Important characteristics:

§ energy resolution: dE_g/E_g = FWHM/E_g

§ peak-to-total: P/T = Area_peak/Area_total

E_gap

Ge Response function

(+ Anti-Compton Shield)

P/T~20%

P/T~60%

Anular detector

used material: BGO (Bi

Ge

O

)

§ density ~ 7.3 g/cm³

§ Z = 83

§ 3 times more efficient than NaI Þ 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 g

Compton scattering angular distribution

) cos 1 )(

/ (

1 ²

'

g q

g

g = + -

c m E E E

e

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

NaI nose: improvement of light output far away from PM tubes (low-energy g-rays)

BGO back-catcher: improvement of high-energy Compton scattering (high-energy g-rays)