Linearity and resolution of a one dimensional position sensitive detector

N U C L E A R I N S T R U M E N T S AND METHODS

60 (i968)

201-204; © NORTH-HOLLAND P U B L I S H I N G CO.

LINEARITY AND RESOLUTION OF A ONE DIMENSIONAL POSITION SENSITIVE DETECTOR W. MELZER and F. P{]HLHOFER Max Planck Institut fiir Kernphysik, Heidelberg

Received 1 December 1967 Linearity and resolution of a one dimensional position sensitive Si surface barrier detector were investigated. The experimental results agree within 20% with those predicted theoretically 1, 2). The parameters of the detector were: active area 53 x 8 mm 2, junction capacitance 150 pF, resistance of the back layer 7.2 k#2. The pulse shaping network consisted of a 0.7 ffsec R C integration and 0.7 #sec single delay line clipping filter. The electronic noise

of the position information was measured to be 52 keV (0.5 mm at 5.5 MeV) fwhm Si. The electronic noise of the energy information was 31 keV. Using 5.5 MeV e-particles we measured 59 keV (0.57 mm) and 35 keV resolution respectively. The non-linearity of the characteristics was less than 1% indicating that the variation of the position dependent ballistic deficit was also less than 1%.

1. Introduction

and protected by a Si layer had a resistance o f 7.2 kf2.

One-dimensional position sensitive detectors (PSD) have been used mainly to replace nuclear track plates in magnetic spectrographs3). F o r the construction and application of these detectors it is necessary to have detailed knowledge o f the charge dividing process and the dependence of the linearity and resolution o f a P S D spectrometer on the detector parameters and the pulse shaping used. These questions have been discussed theoretically using a simple detector model1'2). In the following paragraphs we describe measurements on a PSD to show its efficiency and to c o m p a r e the results with those predicted theoretically.

3. Experimental The measurements were carried out with the detector cooled to - 1 0 ° C in order to reduce detector current. This permits a better comparison of the measured results with the values predicted by the theory, since the current was neglected there. The detector was irradiated by 5.5 MeV or-particles f r o m a 241Am source t h r o u g h a thin brass sheet containing equidistant slits, the distance between the slits being eight m m for figs. 2, 3, 4 and four m m for fig. 5. The width o f the slits was 0.4 mm. The charge pulses at the preamplifier outputs were detected using an oscilloscope, triggered by the energy signal, and a 1600 channel pulse height analyser after passing t h r o u g h variable filters (block diagram, fig. 1). Two characteristic quantities o f a PSD are its junction capacitance C and its back layer resistance R. The intrinsic time constant R C / n 2 is a basic value for the description o f the charge dividing process1).

2. Preparation of the detector A PSD with an active area of 5 3 x 8 m m 2 was prepared as described by Bock et al.3). The surface barrier was produced by evaporation of gold on 8000 f2. cm n-type Si. The resistive layer on the back side of the detector, produced by evaporation of Bi

PSD -10°C

Cf

energy contact

la

[

Au

n-Si(8QOO#cm )

7" .....7 .......... ;i

--~

E signal

, J preamplifiers

= ~__~

main amplifiers (filter)

E.x signal

]

.

• --- mutti =', "channel • analyser

position

Bi-

contact

backlayer

~

trigger amplifier I trigger input

"' l oscillos-

=i -- =

cope

Fig. 1. The block diagram shows the electronics used for testing the position sensitive detector (PSD).

201

202

w.

MELZER

AND

F. P L r H L H O F E R

T o g e t h e r with the preamplifier and the filter these quantities d e t e r m i n e linearity a n d r e s o l u t i o n o f the P S D . By varying t h e bias voltage o f t h e detector one can adjust C and therefore RCI ~2.

x/L 0.05 0.20 0.35 0;50 0.65 0.80 0,95

4. Intrinsic rise time, ballistic deficit and linearity of the P S D U s i n g a d e t e c t o r bias o f 25 V the j u n c t i o n c a p a c i t a n c e is 240 p F a n d the t i m e c o n s t a n t RC/rc z of the d e t e c t o r is 170 ns. This leads to a relatively slow rise time o f the charge pulses and the preamplifier and oscilloscope rise times can be neglected except for pulses b e l o n g i n g to particles incident near the p o s i t i o n terminals o f the d e t e c t o r ( x / L ~ 0 or ~ 1, L = length o f the detector).

x/L

~ 0,05 t0.95

I

E

-o o

~tUl.o

I

I

I

!

I

v ........ i

I

x/L=03

- -/ ~ J

o0,5

"o

~ 0

0.50

°

x/L =0,5 7

x/L =0.7 x/L =0.9 I

0 2

I

I

4

6

I

X,L 4 °

I

x/L=O x/L: 0.1

f ~ ~ /

O) t~ c-

0.35

0.65

I

1

0

0.20 0<80

f

'

......

time

t

RC/~ 2

Fig. 3. Same as fig. 2 for the position information.

!

' TD×

"

:

~ 0.5 1

2

3

4

, ,,

5 6 t time ,RC/~ 2

pos tion

Fig. 2. The voltage pulses of the energy information at the preamplifier output for different positions x/L are compared with pulses predicted by theory (lower curves). The time base of the oscilloscope is 100 ns/unit. The time constant RC/~ 2 has the value of 170 ns.

0 --

I n figs. 2 a n d 3 t h e pulses of t h e energy a n d t h e p o s i t i o n signals a t the preamplifiers o u t p u t s are shown. It is n o t possible to trigger the oscilloscope for all pulses fast enough, because there is noise a n d the energy pulses c o m i n g f r o m the m i d d l e o f the d e t e c t o r ( x / L 0.5) are delayed b y a p p r o x i m a t e l y 0.2 R C / ~ z .

"

""

C

I

0.5 Position of incidence(x/L)

computed

(

t : 12.4) RC/T[ 2

~+

measured

( t RC/~ 2

2)

measured

(

~ = 1) RC/TL 2

~

measured

(_ _ t

4 )

Fig. 4. The heights of the pulses displayed in figs. 2 and 3 at t/(RC/;~ 2) = l; 2; 4 as a function of position of incidence x/L. These curves, which describe the relations between pulse height and position of incidence, are called the characteristics of a PSD.

LINEARITY

E/ ,

,

,

i

i

203

AND RESOLUTION

i

i

i

i

t

PSD 52

i

I /

i

#

I

I

I

241Am

6O L 50

position spectrum ~)f

~ ~ ~ ~ ~ ~ ~ .. ~.

4o~ ,.+

3O0

o

3o ~_

=~ 2o0

•

u "

"

pulser 1/5

100

C o

u

-

0

"~

.

Z':

; .

"~ '

160

"~ ,:k-.

"

'

~;

• • rnm :

'

.

.'"

--~

, *

360~'

" '

,

.

L

--'-". ,

.'.

"

: "~

"

"~ -

4 5 0 " T"

"4-

,

?-

'

~.

6~)

700

,-

050ram

"."'

.:m-" v ~ :~,

;:

560

20

.'

- "4- - _

10 O

950 1000 Channel Number

Fig. 5. Position s p e c t r u m o f the P S D ; the detector being irradiated by ~4tAm c~-particles t h r o u g h 12 equidistant slits in a brass sheet. T h e width o f the slits was 0.4 m m a n d their separation 4 m m . T h e detector was operated at 75 V bias (junction capacitance 150 pF). T h e pulse s h a p i n g n e t w o r k consisted of a 0.7 ffsec RC integration a n d 0.7 #sec single delay line clipping filter•

The measured pulse shapes are to be compared with the calculated ones shown in the lower part of figs. 2 and 3. One finds a good agreement• A more quantitative comparison is possible by looking at the characteristics of the detector. In fig. 4 the pulse heights at different fixed times t in units of RC/Tz2 are shown as function of the position of incidence x/L. The values are taken from figs. 2 and 3. The curves clearly show that there is a considerable dependence of the ballistic deficit of the energy and position measurements when the pulse height is determined at times comparable to the intrinsic rise time RC/~2. We find good agreement between the measured characteristics and those predicted theoretically. For the deviations, however, we have no explanation. I

5. Resolution

As proposed by theoretical investigations ~'z) the parameters of the PSD were chosen to be: junction capacitance 150 pF (bias 75 V), back layer resistance 7.2 kf2, pulse shaping 0.7#see RC integration and 0.7 #sec single delay line clipping. The equivalent noise resistance of the preamplifier was 250 f2 4). For a nonlinearity of the characteristics of 1% these parameters should result in an optimum resolution• Under these conditions a position spectrum of the PSD was taken, shown in fig. 5. The characteristic has a non-linearity of about 1% which, in this case, is mainly due to inhomogeneities of the resistive layer on the detector• The energy spectrum of the 241Am source was also I ""

I

I

241Am PSD 52

energy

spectrum

5O0

~1/5

c c

U Q.

.

pulser

5.477 MeV "

400

300

--! j

g 2oo

L-- 3! key

35 keV

()o 100

1300

13'50

1400

1450 Channel

15'00 Number"

1600 ,,-

Fig. 6. Energy s p e c t r u m o f the P S D ; the detector being irradiated t h r o u g h a 0.4 m m wide slit at position x/L = 0.5. T h e detector was operated u n d e r conditions m e n t i o n e d in fig. 5.

204

W. M E L Z E R A N D F. P U H L H O F E R

m e a s u r e d (fig. 6). Here the d e t e c t o r was i r r a d i a t e d only t h r o u g h a single slit in the m i d d l e o f the d e t e c t o r to avoid a n a d d i t i o n a l line b r o a d e n i n g caused by t h e n o n linearity o f c o r r e s p o n d i n g characteristic. The p o s i t i o n r e s o l u t i o n (fig. 5) a n d the energy r e s o l u t i o n (fig. 6) o f t h e P S D are c o m p a r e d w i t h t h e expected values in t a b l e 1. The m e a s u r e d a n d the p r e d i c t e d values for the elecTABLE 1 Resolution of the position and the energy information. Measured

sured using different time c o n s t a n t s o f the filter. The results are c o m p a r e d with the theoretical values in t a b l e 2. TABLE 2

Noise of the position information as a function of filter time constant. Filter time constant ~sec)

0.7

Electronic noise of the measured 48 position signal (keV fwhm Si) theory 43

2.0 83 75

4.0 117 107

Theory

0~-

pulser Noise of position information (keY fwhm Si) Noise of position information (mm fwhm Si for a 53 mm long PSD and 5.5 MeV a-particles) Noise of energy information (keV fwhm Si)

52

0.5 31

particles

59

0.57 35

pulser

6. Conclusion

43

T h e charge dividing process in a one d i m e n s i o n a l surface b a r r i e r P S D seems to be described well by the m o d e l given in 1,2). N o n - l i n e a r i t y a n d r e s o l u t i o n are p r e d i c t e d with a n accuracy sufficient for c o n s t r u c t i o n a n d a p p l i c a t i o n o f a P S D spectrometer,

0.41

W e wish to t h a n k Prof. W. G e n t n e r a n d Prof. R. B o c k for their interest in this work.

26 Refer enees

tronic noise agree within 20%. The deviations are due to noise sources neglected in the calculations a n d c o n s i d e r a b l e stray c a p a c i t a n c e at the preamplifier input. T h e r e s o l u t i o n o b t a i n e d with a-particles is s o m e w h a t worse because o f t h e source w i d t h a n d the finite geometrical w i d t h o f t h e slits. To have a further test the electronic noise was m e a -

1) S. Kalbitzer and W. Melzer, Nucl. Instr. and Meth. 56 (1967) 301. 2) A. Doehring, S. Kalbitzer and W. Melzer, Nucl. Instr. and Meth. 59 (1968) 40. z) R. Bock, H. H. Duhm, W. Melzer, F. Ptihlhofer and B. Stadler, Nucl. Instr. and Meth. 41 (1966) 190. 4) S. Kalbitzer, W. Melzer, J. Kemmer and P. Walther, Z. Naturf. 21a (1966) 1178.