A gas scintillation drift chamber for heavy ion detection

Nuclear Instruments and Methods in Physics Research 225 (1984) 407-412 North-Holland, Amsterdam

407

A GAS SCINTILLATION DRIFT CHAMBER FOR HEAVY ION DETECTION K.D. M A T H I S , M. S I M O N a n d M. H E N K E L

University of Siegen, Physics Department, 5900 Siegen, W.-Germany Received 6 December 1983 and in revised form 6 March 1984

A gas scintillation drift chamber has been exposed to the heavy ion beam at Berkeley, Ca, USA. The drift velocity of an Ar(98%)-N2 (2%) gas mixture as a function of the reduced electric field has been measured. The detector provided a spatial resolution of 500 #m and an energy resolution of 6% for penetrating 518 MeV/nuc Fe-particles.

1. Introduction Conventional drift chambers are widely used in high energy physics and their technique is well established and described in the literature. A new type of drift chamber, the gas scintillation drift chamber, GSDC, has recently gained more attention. The basic concept of a GSDC is described in more detail in the next section. We studied a GSDC in various tests at our laboratory. Results from an a- and fl-exposure have been published earlier [1]. Here we present results from a heavy ion exposure at the Bevalac in Berkeley, Ca., USA.

2. Working mode of a GSDC To illustrate the working mode of a GSDC it is useful to remember the basic concept of a conventional drift chamber: The ionization electrons liberated at a particle path in the drift chamber gas, drift in an electric field towards a sense wire where charge multiplication occurs. The time that elapses between the particle's passage and the avalanche pulse appearing at the sense wire indicates the impact position. This working mode requires an external trigger and a charge multiplication at the sense wire. The working mode of a GSDC is different: a penetrating charged particle not only liberates electrons by ionization, but also produces scintillation light by excitation (primary light). As in conventional drift chambers the ionization electrons drift to a sense wire. In the vicinity of the sense wire they only encounter a weak electric field, so between collisions the electrons gain enough energy to excite the gas, but no or only a weak charge multiplication occurs. This leads to a secondary scintillation light. Both light signals can be measured with phototubes. The time between the prompt 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

primary light and the secondary light, delayed by the drift time, indicates the impact position of the particle. This working mode has the following interesting properties. 1) High secondary light amplitudes can be obtained without or at only weak charge multiplications. So, space charge effects are minimized. This is important in applications of high rate and large dynamic range in d E / d x measurements. 2) The GSDC operates in a self-triggering mode. The prompt primary light provides the time-zero signal. 3) Both light pulses are proportional to the energy loss of the penetrating particle and can be used for a d E / d x measurement. Thus a GSDC can be used as a self-triggering position sensitive d E / d x detector with a large dynamic range.

3. Mechanical configuration of the GSDC Fig. 1 illustrates schematically the experimental configuration of the GSDC we exposed to the heavy ion beam in Berkeley, Ca., USA. The detector has a gap of 5 cm and a sensitive area of 13 c m × 4 0 cm. The maximum drift path, the distance between the potential wire and the sense wire (Cu-Be: 0.1 mm, each) is 20 cm. The scintillation light is detected by three PMTs (RCA 8850) from both ends through 3 mm thin UVT-plexiglass windows which form a part of the detectors frame. The side wails of the chamber are made of 10 mm thick epoxy bars and the top and the bottom of I mm thick epoxy plates. These epoxy plates show a pattern of 16 mm wide AI strips with a spacing of 4 mm. This leads to an aluminium coverage of 80%, improving internal light reflection. A resistor network connects the AI strips with a high voltage power supply to form a uniform electric drift field within the chamber.

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4. Experimental test set-up Fig. 2 illustrates the experimental set-up for the heavy ion exposure. In front of the G S D C we placed a

Fe- beam "PW

-

beam telescope which consists of six individual conventional drift chambers. Results from these conventional drift chambers in terms of &ray suppression and spatial resolution have been published separately [2]. The telescope made it possible to determine for individual events the impact position in the GSDC. This makes it possible to measure the spatial resolution of the GSDC.

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Fig. 2. Experimental test set-up, consisting of a drift chamber telescope and the GSDC. The telescope was used to determine the spatial resolution of the GSDC (see sect. 6.4).

Fig. 3 illustrates the GSDC, the beam telescope and the electronic readout. The Fe beam (518 M e V / n u c ) penetrates the GSDC, the six drift chambers of the telescope and the two scintillators, Scl and Sc2, which had a width of 5 ram. Either these two scintillators in coincidence or the primary light of the G S D C provided the start signal to an 8-bit 100 MHz time-to-digital converter for the drift time measurement in the conventional drift chambers and to an 8-bit 25 MHz time-todigital converter for the drift time measurement in the GSDC. Both time-to-digital converters were built in our laboratory. The chamber signals were fed via LeCroy preamplifiers and discriminators (7791, MVL 100) to the stop entrance of the time-to-digital converters. The TDCs involve asynchronous counters, providing

K.D. Mathis et aL / Gas scintillation drift chamber

409 Monitor

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Fig. 3. Block diagram of the electronics used for processing signals from the drift chamber telescope and the GSDC. The scintillators Sc 1 and Sc2 define a beam profile of 5 mm. Alternatively, the primary light of the scintillators provides the start signal to the TDCs.

a clock period of 10 ns (beam telescope chambers) and 40 ns (GSDC). The estimated average time error was calculated to be g2DC = p 2 / 6 ( p = clock period), which leads to

AtTD C = ~ ( 1 0 2 / 6 ) -- 4 ns, for the beam telescope and At.~Dc = ~ ( 4 0 2 / 6 ) -- 16 ns, for the G S D C , respectively. The measured drift times were transferred to a microcomputer system, Eurocam II, and stored on discettes. In an on-line data analysis the drift-time distributions of all chambers, or correlations between drifttimes of different chambers, could be displayed on a TV-monitor. The more detailed data analysis was performed on a P D P 1 1 / 4 5 at the University of Siegen.

6. Results

6.1. Drift velocity of an Ar / N2-gas mixture For a G S D C one has to find a gas which combines the following properties: good light yield, short decay times for timing purpose and a sufficiently high drift velocity. Pure rare gases are not the ideal filling for a G S D C since they provide a small drift velocity and emit scintillation light in the VUV-reglon. We used an A r / N 2 - g a s mixture in our G S D C . The admixture of N 2 to Ar not only increases the drift velocity, it also shifts the wavelengths of the scintillation light from the UV-

region beyond 2500 ,~ to a region around 3500 A [3], which corresponds to the sensitive region of most standard glass photomultiplier tubes. Fig. 4 shows the drift velocity in the A r ( 9 8 % ) - N 2(2%) mixture as a function of the reduced electric f i d d ( E / p ) as measured in our G S D C . The drift velocity is still roughly 4 times smaller than in an Ar(90%)-CH4(10%) gas mixture often used in conventional drift chambers, but is roughly 4 times higher than in pure argon. The curve in fig. 4 shows interesting features. At low E / p values the drift velocity increases fairly steep, reaches a maximum, decreases again, and goes through a brought minimum with a further increase to higher E / p values. Thus fairly high drift velocities can be obtained at rather small electric fields, i.e. E / p <~0.1 V / c m . Torr. In order to obtain a faster drift, E / p values of more than 0.45 V / c m . Torr. are needed. This may be important for G S D C s of large drift paths. For example for a driit path of 50 cm one needs more than 17 kV to obtain a higher drift velocity which one already gets at 3.8 kV. We refer to this, since the noise increased sensitively with the high voltage, we applied to the chamber. Although the drift velocity attained at about 0.2 V / c m . Torr is 20% smaller, it might be a good working point, because the drift velocity shows only a weak dependence on electric field variations in this range.

6. 2. Light yield as a function of the sense wire voltage The amplitude of the secondary light measured behind the phototubes, as a function of the sense wire voltage (U~w) is shown in fig. 5. The high voltage power

410 vo

K.D. Mathis et al. / Gas scintillation drift chamber

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supply of the phototubes (Upm) was fixed at 1800 V. As can be seen, a reasonable light yield can be obtained at rather low sense wire voltages. Fig. 6 shows for example the amplitude distribution of the primary and secondary light as obtained from 518 M e V / n u c Fe-particles, at a sense wire voltage of 250 V. At this voltage certainly no charge multiplication occurs around the 100/~m thick sense wire. As can be seen both light signals can be used for a d E / d x measurement. But it is obvious that the secondary light

500

PM

provides much better resolution than the primary light, although the absolute values of 20% for the primary light and 6% for the secondary light as measured here depend on the sense wire voltage, the special design and the light collecting power of the detector. Better resolutions are certainly feasible.

= 1.8 kV

~ 3oo <

200 100

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Fig. 5. Light yield of the secondary light (A) as a function of the sense wire voltage (Usw). The voltage of the photomultipliers was set to 1.8 kV.

Fig. 6. Amplitude distributions of the primary and the secondary light (sense wire voltage: 250 V). The measurement was recorded with an MCA Canberra series 30.

K.D. Mathis et aL

/

411

Gas scintillation drift chamber

6.3. 8-ray suppression 150 -

Since we exposed our drift chambers to 518 M e V / n u c Fe-particles, which correspond to an energy deposition of about 11.9 MeV in the G S D C , we faced a problem which is due to 8-rays. A heavy ion beam penetrating our drift chambers liberates fast electrons, the so called 8-rays, which pass the drift chambers at the same time at various positions and angles. If the readout electronic is sensitive' to 6-ray signals, wrong drift times would be measured. This effect is discussed more in detail in ref. [2] where we studied the efficiency of 6-ray suppression in our conventional drift chamber telescope by means of an amplitude discrimination. This 8-ray effect and its amplitude discrimination can also be seen in the G S D C . Figs. 7a and b show two drift time distributions generated by a collimated Fe-beam (518 M e V / n u c , collimation width of 5 mm). Both measurements were accomplished under the same conditions (sense wire voltage: + 2 5 0 V, potential wire and Al-strip resistor network: - 2 0 0 0 V), except for the discriminator threshold. The distribution with the lower threshold shows a tail to smaller drift times, which is due to 6-rays. By increasing the threshold the erroneous stops from 8-rays disappear and a small and symmetric distribution remains. The probability of erroneous 8-ray stops as a function of the discriminator threshold is shown in fig. 8. Thus all 8-rays are sufficiently suppressed with a threshold of 10 mV. Since the Fe-signals itself have an amplitude of 63 mV a 100% detection efficiency of the Fe-particles will be obtained. This is not necessarily the case for heavy ions of lower charge, since they themselves produce signals of smaller amplitudes. Therefore a fixed discriminator threshold limits the charge range, that can be detected, see also ref. [2].

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6.4. S p a t i a l resolution

If a heavy ion traverses the experimental configuration (see fig. 2) perpendicular to the wire planes the drift times measured in a beam telescope chamber and in the G S D C can be combined, as expressed in the following equation D = VDCt I + VGSDCt2,

= tl + ( VGsDc/VDc)t 2 .

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75-

(1)

with Vt~c the drift velocity in the drift chamber, Vcsr~c the drift velocity in the G S D C , and for D, t 1, t 2, see fig. 2. Since D, VDC and VGsrx: were kept constant, eq. (1) leads to: T= D/VDc

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The width of the distribution of this quantity T reflects the time errors of the measured drift times t 1 (drift chamber) and t 2 ( G S D C ) and allows to deduce the spatial resolution ox of the G S D C if the spatial resolu-

1

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08

10

I 1.2

time [/us]

Fig. 7. Drift time distributions generated by a collimated iron beam of 518 MeV/nuc, collimation width 5 mm, taken at a discriminator threshold of 5.5 mV (a) and of 12 mV (b).

412

7 --

K.D. Mathis et aL / Gas scintillation drift chamber

This compares with resolutions we obtained from the beam telescope chambers [2]. For this measurement the drift velocity in the G S D C was 1.09 cm//~s, and the average drift path was 4.45 cm.

0

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Fe (518 MeV I n u c ) :

63 mV

7. Conclusion

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VDC.

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Fig. 9 shows a measured T-distribution. We measured AT, At1, and V o c to: A T = 37 ns (fwhm of T-distribution, see fig. 9), A q = 18 ns (fwhm of a drift time distribution of a beam telescope chamber), VDc = 40 m m / # s (drift velocity i n a beam telescope chamber). This leads to a spatial resolution of:

In our tests the G S D C has shown its ability to work as a position sensitive heavy ion detector. The properties of the detector are: - The G S D C can be run in a self-triggering mode, i.e. no external trigger pulse is needed to start the drift time measurement, because the trigger is produced within the chamber itself by the primary light. - The G S D C provides a spatial resolution of about 500 #m, which is comparable to conventional drift chambers. - G o o d d E / d x measurements can be performed by the primary or the secondary light. The basic functioning of a G S D C has now been studied. The detector turns out to be well suited for heavy ion detection, and should be a useful detector in cosmic ray experiments. So we intend to build a G S D C of reasonable size and compare its properties to those of established techniques, such as large area scintillators, Cherenkov counters and ionization chambers which are the detectors most frequently used in cosmic ray experiments. We would like to thank the Lawrence Berkeley Laboratory, Ca., U S A for giving us access to the heavy ion beam. In particular we thank H. Crawford and F. Bieser for their help at the LBL. This work has been supported by the Deutsche Forschungsgemeinschaft ( D F G ) under contract Si 290/12-1.

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References

I

w

L, 8

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Fig. 9. T-distrivuUon [see eq. (2)], to determine the spatial resolution of the GSDC, The average drift length was 4.45 cm.

[1] M. Simon and Th. Braun, Nucl. Instr. and Meth. 204 (1983) 371. [2] M. Simon, M. Henkel, R. Hundt, K.D. Mathis, G. Schieweck and Th. Suck, submitted to Nucl. Instr. and Meth. [3] T.D. Strickler and E.T. Arakawa, J. Chem. Phys. 4t (1964) 1983.