A scintillation drift chamber with 14 cm drift path

Nuclear Instruments and Methods 204 (1983) 371-377 North-Holland Publishing Company

A SCINTILLATION

DRIFT CHAMBER

WITH

371

14 c m D R I F T P A T H

M . S I M O N a n d T. B R A U N

University of Siegen, Physics Department, Adolf-Reichwein-Str., 5900 Siegen 21, Fed. Rep. Germany Received 30 April 1982

An Ar+2% N 2 gas scintillation drift chamber with a drift path of 14 cm has been exposed to fl- and t~-particles. The read-out was performed alternatively by the conventional electric charge signal from the sense wire or by the light signal from phototubes. Results from both read-out modes are presented and discussed. As a conclusion, with the light read-out the chamber can work in a self-triggering mode, it provides a spatial resolution of better than 1 mm and is superior to a conventional charge read-out in applications where space charge effects should be avoided (high rate, linearity).

1. Introduction Drift chambers are widely used in high energy physics since they provide the best spatial resolution among the various types of electronic track detectors. Their technique is well established and described in the literature [8]. In the last years attempts have been made to employ them in cosmic ray experiments. The problem under a high energy heavy ion exposure is due to 8-rays which accompany the primary heavy particles. In a previous drift chamber test at the heavy ion beam at Berkeley, California, we investigated this problem and found that there is a possibility to discriminate against 8-rays by running the drift chamber under conditions where one avoids space effects, Simon et al. [10]. Space charge effects are due to the charge multiplication at the sense wire. Since conventional drift chambers in general depend on a charge multiplication we pursued the attempt to investigate the use of a gas scintillation drift chamber for such an application. This type of detector is based on the fact that a penetrating charged particle not only liberates electrons by ionization but also excites the gas and produces scintillation light which can be measured. Position information is obtained from the measured time delay between two scintillation flashes detected by photomultipliers. These two scintillation flashes are: the primary scintillation light which originates from the direct penetration of the heavy particle and the secondary light which is produced after the electrons have drifted to a sense wire where they encounter an electric field strong enough so that they gain enough energy between collisions to produce further excitation of the medium but no or only a weak charge multiplication. This mode of operation has some principle advantages compared with conventional drift chambers: 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland

(1) High secondary light amplitudes can be obtained without or with only weak charge multiplication so that space charge effects are minimized. (2) The gas scintillation drift chamber works in a self-triggering mode so that no additional detector which would bring matter into the beam is needed to give a time-zero signal. (3) Since both light signals are proportional to the energy loss of the penetrating particle such a gas scintillation drift chamber may be used simultaneously as a d E / d x and a position sensitive detector. The principle of scintillation drift chambers was recently introduced and tested with small size detectors (Charpak et al. [1], Schelhaas et al. [9]). Our aim was to investigate the use of this method for larger dimensions and longer drift paths such as needed possibly in a cosmic ray experiment where one has to deal with high energy heavy ions. In the following we present the results from our gas-scintillation drift chamber which we tested with fl- and a-particles and which we ran alternatively in the electric and in the light read-out mode. 2. The scintillation drift chamber Fig. 1 illustrates schematically the experimental drift chamber configuration. The frame itself was made out of plexiglass which we coated from the inside with a wavelength shifter (Viehmann et al. [11]). N o reflecting paint was applied to the bottom and to the top plate. The bottom and the top plates were made out of 2 mm thick epoxy material on which we etched a 2.5 mm wide Cu-strips pattern with an equal spacing of also 2.5 mm. The potentials to these Cu-strips were supplied by a resistor network. The applied voltage was typically 8 KV (negative) providing a uniform electric drift field of

372

M. Simon, T. Braun / A scintillation drift chamber

153mm

i~ 30mm .i • :

E

t g a s out

~1~ "F 30 mm I

-

A

e~t...... lit ~2mm*lO::

~

~

6,

~

+

5

0ram £j:

s~iotess steel

C~-ae.

:':

y7;;'2/°

)

RCA 8850

" / / / / / / A EZ

o

SSZZ

t g a s /n

4 30mm~4

1L7mm 153mm

~t30mm.,

j I

T, l [ ~ ~ / /

~

2,5ramCu-strip /'~ 2'5 mims p a c i n ~

2 mm epoxy plate high

•

147 mm

voltage

voltage

Fig. 1. Schematic lay-out of the gas scintillation drift chamber. The chamber was optically wiewed from one end by 3 phototubes.

533 V / c m . The gap of the c h a m b e r was 3 cm a n d the m a x i m u m drift path was 14.7 cm. F o r the sense wire we used a 0.1 m m thick C u - B e - w i r e a n d the potential wire was 1.5 m m thick a n d m a d e out of stainless steel. The c h a m b e r was viewed edge on by 3 p h o t o t u b e s ( R C A 8850) from one side. A t three different distances (1.25, 6.85 and 12.95 cm) from the sense wire we placed 2 m m wide slits in the b o t t o m a n d in the top plate which formed a collimated b e a m for a- a n d /~-particles. Und e r n e a t h the b o t t o m plate a semiconductor detector or

two thin scintillators in coincidence served as b e a m forming start triggers, as shown in fig. 2. The stop signal we derived electrically from the sense wire (charge signal) and alternatively from the p h o t o t u b e s (light signal). Fig. 3 describes the electronic read-out in the selftriggering mode. T h e sum signal from all three phototubes was split into two channels. One channel went over a discriminator with a low threshold so that the primary light amplitude could pass to start the TAC. T h e other channel went over a discriminator with a

373

M. Simon, T. Braun / A scintillation drift chamber + high

voltage

preampl.

fl-source

-41 t~ - S

TAC

b

t,I SEN~-FE . 285

I PMT ~

¢t-

---~scmhllators

stop bl • ] (alternah'vely~ stop

Ource

I .....

I

start

~

~, sta

(alternat/vety )!

"~ semiconductor detector

MCA Canberra 3100

preamp[

~

oinci~ence

Fig. 2. Experimental test set-up and electric read-out. The drift chamber was tested with fl- and a-particles.

start~ low threshold

stop

high threshold

Fig. 3. Electric read-out in the self-triggering mode.

much higher threshold so that only the secondary light could pass to stop the TAC.

3. The scintillation drift chamber gas Gas scintillation based on the luminescence of noble gases and noble gases with admixtures of molecular gases, mainly N2, is well known and the current state of the art in the development of gas scintillators has recently been reviewed by Policarpo [6]. For a scintillation drift chamber one would like to find a gas which combines a good light yield with a short decay time for timing purpose and with a reasonable fast drift velocity.

As gas fillings we tried several pure noble gases and mixtures with nitrogen. As expected xenon provided the best light-emitting properties but only when special care was taken for constant purification (Gebauer [4]). With a SAES Getters 101/700 rare gas purifier we obtained stable gas conditions but without it the gas deteriorated rather fast. Since xenon is too expensive to be used in a constant flow mode we excluded it from consideration for our purpose. Pure argon provided also a reasonable light output - which was in our test chamber about half of that for xenon - but the drift velocity is rather slow, which we measured to be 0.45 cm//xs at a reduced field of 0.47 V / c m Torr, see fig. 4. This is about ten times smaller than in conventional drift chamber gases, such as Ar + 10% C H 4. According to Mutterer et al. [5] pure argon has also a slow decay time of the scintillation light, which is in the #s region, and which does not favour pure argon for timing purposes. G o o d results in various aspects were obtained from an a r g o n / n i t r o g e n mixture. Although the admixture of N 2 reduced the light yield by roughly a factor of two compared to pure argon this gas mixture has the following good properties suitable for our purpose. (1) It provides drift velocities which are four times as fast as in pure argon. Fig. 4 shows the measured drift velocity for an Ar 4- 2% N 2 mixture as a function of the reduced electric field. (2) The admixture of N z leads to a drastic decrease of the light decay time which makes this gas useful for time purpose [5]. We measured the rise time of the

374

M. Simon, T. Braun / A scintillation drift chamber

I

I

I

I

I

I

I

I

I

30 •

A! - D o r g a z e l l i , A r i y a r a t n e ,

•

F. S a u h ,

,',

th~s e x p e r / m e n t

B r e a r e , Nandl, 1980

1977

20

Nz - g a s

th

\

10

J, _ _

±

Ar -gas

J

0 0

I 0,2

I

[

0,4 E/p

/ V/cm

46 Tocc

I

]

Fig. 4. Measured drift velocities versus reduced electric field in pure argon gas and an Ar+2%

scintillation light from the Ar + 2% N 2 mixture at 760 Torr to be about 80 ns. (3) The argon/nitrogen mixture emits light in the visible region (3000-4200 .A,) which matches the maximum sensitivity of conventional phototubes and which makes wavelength shifters less important compared to pure xenon or argon which emit in the UV-region (Charpak [2]). The following results are obtained from an Ar + 2% N 2 mixture, although other gas mixtures are certainly worthwhile to investigate since an increase of N z concentration makes the light even faster, and Charpak et al. [2] claim to have obtained good results from a mixture of 48% Ar + 48% N 2 + 4% C O 2.

I

I 48

N2

mixture.

terminated with 50 ~2. The electric charge signal was fed through a self-made preamplifier. These measurements were performed with an ~-source (24JAm, 4 MeV, behind a gold foil) and at a reduced field of 0.7 V / c m Torr. As it can be seen in both cases the amplitude is a sensitive function of the applied sense wire voltage. But as expected one needs a relatively high voltage at the sense wire ( > I 0 0 0 V ) in order to obtain an electric charge signal above noise. The secondary light signal appears at a much lower sense wire voltage - even zero volt - indicating that the secondary light yield is high enough even when no or only a weak charge multiplication occurs. Thus for all applications in which a drift chamber is negatively effected by space charge effects (high rate, linearity) the light read-out should be of advantage.

4. Results 4.2. Position spectra 4.1. Light and charge amplification

As illustrated in fig. 2 the drift chamber could be read out in a conventional manner by using the electric charge signal from the sense wire or by utilizing the secondary light measured by the phototubes. The amplitudes from these two read-out modes are shown in fig. 5 as a function of the sense wire voltage. The light output refers to one phototube (RCA 8850, high voltage:2 KV) which was measured directly at the anode and

Fig. 6 shows position spectra from fl-particles which we obtained from both read-out modes. They were recorded in the way illustrated in fig. 2. The two 2 mm wide slits in the top and the bottom plates of the drift chamber formed a collimated beam and the two thin scintillators underneath the chamber provided in coincidence the start signal for the TAC. The stop came alternatively from the electric charge sense wire signal or from the phototubes. The TAC signal was finally

M. Simon, 72 Braun P

/

'

A scintillation drift chamber

375

I

1000

'00

23 t3

tO0

I

4=

t. C3

io

o

E (3

OFF

10

,

J

0,5

,

1,0 s e n s e w i r e - voltoge

15 [ kV]

J

,

2,0

2,5

I

Fig. 5. Measured signal amplitudes in an Ar+2% N 2 gas mixture as a function of the sense wire voltage. The left scale corresponds to the light read-out and the right scale to the charge read-out.

a n a l y s e d b y the M C A . T h e top s p e c t r u m c o r r e s p o n d s to the light read-out m o d e a n d the b o t t o m s p e c t r u m to the electric read-out. A s can be seen, in b o t h w a y s o n e l o c a t e s the p o s i t i o n of these two slits with respect to the

200 Ar +2%

Nz

hght read

out

.b2

~, 700

/

'1'1'

200

At÷2%Nz electric

~3

reod out

100

20

40

50

80

100

120

140

s [mm]

Fig. 6. Position spectra obtained from B-particles and measured in both read-out modes. The positions of these peaks correspond to the slits in the chamber, see figs. 1 and 2.

Fig. 7. Typical picture from a scope where both light components can be seen. This result has been obtained from an a-particle. The first small signal corresponds to the primary scintillation light and the delayed bigger signal belongs to the secondary light.

376

M. Simon, T. Braun / A scintillation drift chamber

sense wire (6.85 and 12.95 cm drift path) with equal precision. The widths (b2, b3) are consistent with 2 mm within the limits of our experimental errors. We conclude, therefore, that a spatial resolution of less than 1 mm is feasible for the electric read-out as well as for the ligh t read-out. For the electrons we needed a separate start signal from the scintillators because the primary light was too low to be measured by the phototubes. In order to run the chamber in a self-triggering mode, see fig. 3, we used a-particles from an Am 241 source. The primary scintillation light from these particles could easily be measured. The average amplitudes varied between 10 and 100 mV depending on the position in the chamber. Fig. 7 shows as an example both light components. The first, smaller signal belongs to the primary light and the delayed larger signal to the secondary light. This picture was obtained by delaying the PMTsignals and triggering the scope externally by a signal from a semiconductor detector which was placed underneath the bottom slit and which determined the zero time of the penetration of an a-particle, see fig. 2. The capability of locating particles in the self-triggering mode is demonstrated in fig. 8. We simply placed a-sources on all three slits and only utilized the signals from the phototubes, no additional start trigger or sense wire signals were involved, fig. 3. The peaks in fig. 8 correspond to the distances of the slits with respect to the sense wire similar to fig. 6. The widths of these distributions are necessarily somewhat broader than those of fig. 6, because the j~-particles were collimated by the scintillators whereas the a-particles enter the chamber through the 2 mm wide topslits under various angles.

b

T

4. 3. Signal-amplitude to drift path dependence From work with conventional drift chambers one knows that the amplitudes decrease somewhat with the drift distance, Rahman et al [7]. This attenuation of pulse height is due to a loss of drifting electrons, which attach to gas molecules, impurities or which could get deflected out of the drift path. The amount of these losses depends therefore on the gas properties and the special geometrical and electrical conditions of the chamber. In the present chamber we measured the variation of amplitude as a function of the drift path. Using a-particles, fig. 9 gives the relative variation of measured pulse heights versus drift distance for our tested gas scintillation drift chamber gas (Ar + 2% N2) and for the conventional drift chamber gas Ar + 10% C H 4. These measurements were performed in both read-out modes - charge and light read-out - and at a reduced drifting field of 0.7 V / c m Torr. The conventional drift chamber gas Ar + 10% C H 4 which was read out electrically showed only a weak attenuation of less than 2% over the total drift path of 12.95 cm which is in agreement with results of earlier measurements, Rahman et al. [7]. The scintillation gas Ar + 2% N 2 behaved very much different, since we measured a drastic decrease of amplitude over the distance of 12.95 cm. The amplitudes from a-particles which enter the chamber at a distance of 12.95 cm from the sense wire were on the average 2.5 times smaller than those from a-particles at a distance of 1.25 cm. This was true for both read-out modes. Since the conventional drift chamber gas Ar + 10% C H 4 does not show such a loss of drifting electrons, geometrical or electrical conditions of this cham-

i

sense wire voltage. 400 Volts reduced field 0 7 V / c m Tort At+2

% N2

200

105

0

J 20

40

60 drlftpath

80

100

120

740

[mm]

Fig. 8. Position spectra obtained from a-particles and in the self-triggering mode. The positions correspond to the slits in the chamber, see text and figs. 1 and 2.

M. Simon, T. Braun / A scintillation drift chamber

terms of light yield, drift velocity, emitting light spect r u m and decay time. F o r out test we solely used an A r - 2 % N 2 mixture, although other gas mixtures of a r g o n with CO 2 a n d N 2 have b e e n considered in the literature ( C h a r p a k et al. [2]). It could be shown that the light read-out works at al lower sense wire voltage than the charge read-out indicating that n o or only a moderate charge amplification appears at the sense wire in the light read-out mode avoiding space charge effects. In the limits of our experimental tests b o t h read-out modes, charge read-out a n d light read-out, provided the same spatial resolution a n d one millimeter should be no p r o b l e m in b o t h cases. In view of a cosmic ray application, one could run such a c h a m b e r in the self-triggering m o d e since the heavy ions produce enough primary scintillation light which serves as the zero time trigger. Cosmic ray experiments, on the other hand, d e m a n d relatively long drift paths in order to limit the power of c o n s u m p t i o n and the n u m b e r of read-out wires. By using A r + 2% N 2 long drift paths suffer an amplitude a t t e n u a t i o n which effects the efficiency for a fixed value of the discriminator threshold. In this respect the A r + 2% N 2 mixture is not good. F u r t h e r investigations on different gases are in progress.

~0

09

0.8

07

0.6

0.5 t3 Ac÷2 % N 2 (3

0.4

0.3 •

light r e a d o u t mode

o

c h o r g e r e o d o u t mode

0.2

r e d u c e d driftfield

0.7

V / cm T o r t

0.1

0

377

References

0

~ ~ J

[

50 drift~th

,

~ ~ i

[

too

,

i

~

750

[ram]

Fig. 9. Relative variation of measured pulse heights versus drift distance for the conventional drift chamber gas (Ar+10% CH4) and the scintillation drift chamber gas (Ar+2% N2) obtained from both read-out modes.

b e r should not cause this decrease. We ascribe therefore this effect to the A r + 2% N 2 drifting gas itself, which is certainly a disadvantage of this gas in drift chambers, with long drift paths.

5. Conclusion Our investigations show that gas scintillation drift c h a m b e r s can be applied advantageously to the location of ionizing particles. A r g o n / n i t r o g e n mixtures a p p e a r to provide good properties for such an application in

[1] G. Charpak, S. Majewski and F. Sauli, Nucl. Instr. and Meth. 126 (1975) 381. [2] G. Charpak, S. Majewski and F. Sauli, IEEE Trans. on Nucl. Sci. NS-23, No. 1 (1976) 202. [3] S.S. Dargazelli, T.R. Ariyaratne, J.M. Breare and B.C. Nandi, Nucl. Instr. and Meth. 176 (1980) 523. [4] D. Gebauer, Diplomarbeit, Universit~it Siegen, Fachbereich Physik (July 1980). [5] Mutterer, M., J. Pannicke, K.P. Schelhaas, J.P. Theobald and J.C. van Staden, IEEE Trans. on Nucl. Sci. NS-26, No. 1 (1979) 382. [6] A.J.P.L. Policarpo, Space Sci. Instr. 3 (1977) 77. [7] M. Rahman, J. Meyer, W.D. Dau, A. Stachmann, W. Stamm and H. Jokisch, Nucl. Instr. and Meth. 188 (1981) 159. [8] F. Sauli, CERN-77-09 (1977). [9] K.P. Schelhaas, M. Mutterer, J.P. Theobald, P.A. Schillack, G. Schrieder and P. Wastyn, Nucl. Instr. and Meth. 154 (1978) 245. [10] M. Simon, M. Henkel and G. Schieweck, Nucl. Instr. and Meth. 192 (1982) 483. [11] W. Viehmann and R.L Frost, Nucl. Instr. and Meth. 167 (1979) 405.