institut max von laue - paul langevin fast real-time sans detectors charge division in individual,...

INSTITUT MAX VON LAUE - PAUL LANGEVIN

Fast Real-time SANS DetectorsFast Real-time SANS Detectors

Charge Division in Individual, 1-D Position-sensitive Gas Detectors

Patrick Van Esch.

INSTITUT MAX VON LAUE - PAUL LANGEVIN Patrick Van Esch S.D.N.TINX Septembre 2001

SANS-2MHz Millennium Project

• Project.– Goal: fast real-time detector for

small angle scattering.– Main specifications:

• > 2MHz at 10% dead time (actually: only 50 KHz).

• Resolution 128x128 on 1 m2.

• Efficiency ~ 75 % at 5 Angstrom.• Good gamma - neutron separation.• 5 microseconds time resolution for

thermal neutrons

– Approach:• 128 individual linear PSD.• Charge division per PSD.

• People– Bruno Guerard (detector group)

– Roland May (D22 responsible)

– Alexandre Sicard (PhD. student)

– Jean-Claude Buffet (mechanician)

– Frederic Millier (electronician)

– Patrick Van Esch (detector group)


Principle of Detector.

• Linear PSD detectors.

• 5 prototypes made by Reuter stokes.– 7.12 mm active diameter.

– 7.95 mm mechanical outer diameter.

– 1 meter long.

– About 10 bar He-3 gas.

Commercial 1 inch detector


Resistive Charge Division

Event current0.5 pCUp to 500ns

Resistive anode wire 5.6 KOhm

Current noise i1

Current noise i2

Voltage noise v1 Voltage noise v2

Transimpedance preamplifier 1 Transimpedance preamplifier 2

Gaussian Shaper 1 Gaussian Shaper 2

Baseline correction 1 Baseline correction 2

Peak detection And ADC

Johnson current noise


Principle of Resistive Charge Division

ZR

ZRLx

QA2

1

ZR

ZRLx

QB2

Charge Q collected at both ends divides in A and B when wire length is L, distance of impact from A is x, wire resistance is R and preamplifier impedance is Z.

ZR

R

L

xL

BA

BA

2

2

Extraction of position information: the fraction (A-B)/(A+B) codes the position in the interval (-1,1), reduced by the dynamic range.

Need for very low impedance (virtual ground).QBA


Electronic Noise and Position Resolution

RTk

Rviii Bn

nSMR1682 2

22

...21

RTk

Rviii Bn

nSMRSMR42 2

22

...2...1

iii nSMR

2

...212White current spectral noise

densities seen by both amplifiers at the input are correlated.

dffHdfffHV ii nnSMR

222...

The output R.M.S. voltage noise can be calculated from the input current noise spectral density. In the case of white noise, this simplifies to a factor related to the transimpedance function.

222

1BABABA

BA

BABA

BA

These voltage noise values can be propagated in the position calculation, resulting in the position resolution due to electronic noise.


Analogue Signal Processing

• Resistive charge division.– In contrast with capacitive

(single ended) detectors:• Faster shaping gives better S/N.

• Limited by overall conductance of wire (Johnson noise).

• Limited by integration time.

• Gaussian shaping.– Best compromise between:

• Time domain pulse width.

• Frequency domain noise bandwidth.

– Implementation as 4th order pure pole active filter, about 1MHz bandwidth.

• Unipolar pulse shaping:– Less dead time.

– But shift in baseline !

– Baseline correction:• Averages baseline over several

microseconds (eliminating noise).

• Corrects input signal with that amount.

• Reduction with a factor less than 1/10 of the initial baseline shift.

• No visible added noise.


Impulse response.

5´10-7

1´10-6

1.5 ´10-6

2´10-6timeHsL

0.5

1

1.5

2

1 pC response HVL

pC

VH ampli

1.2

HzMH noise975


Prototype Primary Charge

• HV bias: 100 V.

• Integration time: 10 microseconds.

• Using FET entry amplifier.

• Allows us to estimate absolute gain of detector.


Spectra

1100 V 1300 V

1400 V 1500 V


0 500 1000 1500 2000 2500BiasHVL1

5

10

50

100

500

1000

Absolute Charge Gain

Absolute Gain

8 mm Prototype 1 inch detector


Spectrum

spectrum

0

200

400

600

800

1000

1200

1400

1600

1 190 379 568 757 946 1135 1324 1513 1702 1891

channel number (~2mV/channel)

cou

nts

pCC 65.0

pC

VH 1.2

pC

C

42.011


Numerical application

HznVvn

0.1HzpA

in 6.1 KR 6.5

HzpA

SMRii 2.4...21

HzpA

SMRii 3.2...21

pCVH ampli

1.2

HzMH noise975

mVSMRVV 1.4...21 mV

SMRVV 2.2...21

pCQ 42.0

This results in a F.W.H.M. resolution of 5.5mm in the middle and 6.2mm on the borders.


Linearity of Position CalibrationCalibration 10 bits y = -0,9922x + 1009,3010

R2 = 1,0000

0

200

400

600

800

1000

1200

0 100 200 300 400 500 600position (mm)

chan

nel n

umbe

r

Calibration 8 bits y = -0,248x + 251,898

R2 = 1,000

0

50

100

150

200

250

300

0 100 200 300 400 500 600position (mm)

chan

nel n

umbe

r

Residues

-1,5

-1

-0,5

0

0,5

1

1,5

2

0 100 200 300 400 500 600

position (mm)

re

sid

ue

(m

m)

8 bits10 bits


Spatial Resolution

Spatial resolution

y = -0,0002x + 6,5674

y = -0,0004x + 5,7257

0

1

2

3

4

5

6

7

8

0 100 200 300 400 500 600

position (mm)

FW

HM

res

olut

ion

(mm

)

8 bits10 bits


Count Rate Issues• Estimated dead time:

– 770 nanoseconds

• no extra dead time due to detector effects

• Implication for count rates (10% dead time correction):– 130 kHz per tube

– SANS - 16.6 MHz

20000 40000 60000 80000true rate

20000

40000

60000

80000

observed rate Observed rate versus true rate

20000 40000 60000 80000true countrate

80

81

82

83

84

decrease of relative efficiency as a function of countrate


Spatial resolution at high count rates

• Large beam (35 mm FWHM) at about 100 KHz

• With and without a Cd sheet in beam length

• Spatial ‘resolution’ obtained ~9mm FWHM over entire length of detector.

• Reasonable upper estimate of true spatial resolution

200 mm : resolution

-5000

0

5000

10000

15000

20000

25000

30000

1 5 9

13 17 21 25 29 33 37

position (mm)

coun

ts

with Cd sheet

without Cd sheet

difference

gaussian fit

300 mm : resolution

-5000

0

5000

10000

15000

20000

25000

30000

35000

1 6 11 16 21 26 31

position (mm)

coun

ts

with Cd sheet

without Cd sheet

difference

gaussian fit


Spectral behaviour at high count rate

• The upper part of the spectrum suffers a degradation at high counting rates. This does not impair significantly the detector performance.

logarithmic spectra as function of count rate

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

1 296 591 886 1181 1476 1771 2066 2361 2656

energy (channel number)

log

ari

thm

ic c

ou

nt r

ate 11KHz

24,9KHz

42,5KHz

64,4KHz

80,8KHz

117,7KHz

140KHz


Efficiency Along Detector

• Scan along anode with narrow beam

• Very stable efficiency all over the detector

• Transition zone only about 4 mm !

efficiency - transition zone

0

5000

10000

15000

20000

25000

988 990 992 994 996 998 1000 1002

distance (mm)

coun

ts

efficiency along detector

0

5000

10000

15000

20000

25000

0 200 400 600 800 1000 1200

distance along wire (mm)

coun

ts


Conclusion

• Linear PSD based on the principle of resistive charge division offer great potential for building fast, large-scale neutron detectors.

• Resolution below 1cm at high count rates (>100kHz) can be obtained (6mm at low count rates).

• Very good linearity and uniformity.

• Electronics now exploits fully the potential of the detector.

• This opens up the possibility to have time resolution of the order of tens of microseconds.

institut max von laue - paul langevin fast real-time sans detectors charge division in individual,...

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