EISEYIER Nuclear Instruments and Methods in Physics Research A 400 (1997) 469-475

NUCLEAR INSTRUMENTS

&YETHoDs IN PHVSICS RESEARCH

Sechon A

MINIT: A new gas detector for very low threshold particle identification

S. Aiello”, P. Finocchiarob**, S. Pirrone”, G. Belliab3”, G. Cuttoneb, G. Politi”, A. Rovellib

‘INFN Sezione di Catania. torso Italia 57. 95100 Catania, Italy b INFN Laboratorio Nazionale de1 Sud, via S. Sofa 44. 95125 Catania, Ita!v

’ Universitri di Catania, Dipartimento di Fisica, torso Italia 57. 95100 Catania. Italy

Received 26 February 1997; received in revised form 10 June 1997

Abstract

We report on the first results of in-beam tests performed with the MINIT detection technique. Its modular structure of multilayer ionization chamber with proportional avalanche multiplication allows to detect and identify heavy ions with a very low energy threshold. Its simple mechanical structure, based on a glass microstrip plate, should allow the realization of cheap large-area arrays of similar detectors, in view of the next generation of experiments with radioactive ion beams.

1. Introduction

In the current generation of heavy-ion collision experiments at low and intermediate energy, there

is a still increasing interest in the detection of inter- mediate mass fragments (IMF) and target-like frag- ments (TLF) in a wide solid angle and energy range. So far, several multidetector systems have been developed in order to study the IMF emission in heavy-ion collisions [l-6]. These multidetectors,

based on telescopes, have some common features: large angular acceptance, particle identification ca- pability, good energy resolution, low energy thre- shold. Different strategies have been developed so

* Corresponding author: Tel.: + 39 95 542 284; e-mail:

finocchiaro@lns.infn.it.

far for particle identification, like AE- E telescopes

based on thin silicon detectors or ionization cham- bers, double scintillator (phoswich) techniques, time-of-flight measurement.

The new perspective of experiments with low- energy radioactive ion beams (RIB) at the INFN Laboratorio Nazionale de1 Sud (LNS) [7] will pose the same requirements on the detection systems, with a particular emphasis on the low energy thre- shold for particle identification.

This has been the starting point of our search for a particle identification technique that has lead to the development of the gas detector named MINIT

(MIcrostrip for Non-Intercepting Telescope), as a further improvement of the technique described by Gramegna et al. in Ref. [S]. In order to have a low-threshold telescope one can think of using a thin silicon as AE detector. A recent development

0168-9002/97/$17.00 Q 1997 Published by Elsevier Science B.V. All rights reserved

PI1 SO168-9002(97)00907-8

470 s. Aid/o rl al. /Nucl. Instr. und Meth. in Phw. Rrs. A 400 (1997) 469-473

has made possible to build a monolithic AE-E silicon detector with active AE thickness of z 1 urn and identification threshold (dead layer + AE thickness) of a few microns [S]. Unfortunately, the thinner the semiconductor, the higher its capacitance

and thus, a special preamplifier is needed; further- more, the achievable detector size is limited to few square millimeters, mainly due to the linear in- crease of the capacitance with the detector area.

This is why the ionization chamber is generally preferred as the AE stage of a large-area and low- threshold telescope. Such a detector has a good linearity, with variable thickness (gas pressure), can be rather large and cheap. Its intrinsic energy res- olution is poorer than silicon due to the higher average ionization energy ( z 30 versus 2 3 eV). The main sensitivity limit arising when operating the ionization chamber at very low threshold is the signal-to-noise ratio.

2. The detection technique

The MINIT detector is basically an ionization chamber with transverse drift field. The important improvement lays in the readout electrode: instead of a charge-collecting plate we exploit a glass

microstrip plate that allows operations in the ava- lanche proportional regime [9]. As already known the microstrip gas chamber (MSGC) has a good linearity, with a gain up to a few 103. So far it has been mainly employed in high-energy physics ap- plications, for the detection and tracking of min- imum ionizing particles, or in photon detection applications. In most cases, the MSGC plate is

perpendicular to the direction of the particle to be

detected. In our case, we have put the plate parallel to the

impinging particle direction, just in front of a large cathode plate, thus setting up a transverse drift field; the effective thickness of the active region can be decreased at will by decreasing the gas pressure. Moreover, at variance with the detector described in Ref. [S] and due to its different geometrical internal structure, the readout of the plate can be easily segmented. This implies that we can get in- formation from several independent thinner layers, thus realizing a self-standing multilayer telescope.

Fig. 1. Internal view of the MINIT prototype: (a) drift cathode:

(b) drift field shaping wires; (c) cubic support frame; (d) Frisch

grid; (e) glass microstrip plate.

The overall scheme of the technique is shown in

Fig. 1. When detecting heavy ions at low energy, the

“v lo3 gain results excessive, hence operation at gain between ten and a hundred is sufficient to get a comfortable signal-to-noise ratio, well fitting the

input characteristics of the preamplifier, and at the same time to operate the detector in a quite safe

and stable regime.

3. Our test prototype

Unlike the usual MSGC, the MINIT must be operated in a vacuum scattering chamber. Thus, the gas flowing inside the detector has to be kept at a pressure that the thin input and output windows can withstand. Usually, Mylar PTFE foils l-5 urn thick are suitable for a pressure of lo-lOOmbar, also depending on the shape and size of the win- dows. In order to strengthen the window a thin grid

S. Aiello et al./Nucl. Instr. and Meth. in Ph.vs. Rex A 400 (1997,) 469-475 411

of metal or plastic wires can be put just out of the gas box as further support. The gas has to be heavy and have a good electron mobility; moreover, it must be “clean” in the sense that it should give a low polymerization on the anodic electrodes.

In Fig. 1 we show the detector structure. On a 10 x 10 x 10 cm3 PVC skeleton, we have installed the drift cathode on one face and the microstrip plate on the opposite one. Thin copper wires, equally spaced between these two electrodes on the

a

d

Fig. 2. External view of the MINIT prototype housing de-

veloped at LNS: (a) 8 x 8 cm2 input (output) window with

strengthening grid; (b, e) gas inlet/outlet; (c) electronic connec-

tors; (d) flange for quick evacuation.

cathode (chromium)

other four faces, are kept at suitable voltages in order to perform a uniform field shaping; a few millimeters apart from the microstrip plate we have put a Frisch grid.

In Fig. 2 we show the external structure of the MINIT chamber prototype. The two 8 x 8 cm2 windows have been made in Mylar PTFE and strengthened by means of external grids made of thin plastic wires (the usual dental wire has been used); the window thickness has been chosen 3.5 urn in order to prevent the prototype from acciden- tal breaking, but thinner foils can be used when needed. On the top of this chamber there are a few connectors for signal feedthroughs. On the sides we have put the gas inlet and outlet, plus a large flange for quick evacuation by means of a bypass toward the larger scattering chamber. The chosen gas is CF4, widely used in ionization chambers for heavy- ion detection [IS].

The device has been entirely developed, built and assembled at INFN LNS and Sezione di Catania, with the only exception of the microstrip plate, whose mechanical features are shown in Fig. 3, realized by IMT [lo] with a photolithographic process in chromium on a D263 glass support.

4. In-beam test results

The active area of the tested microstrip plate was 8 x 8 cm2, the pitch between single electrodes

anode (chromium)

I

substrate 0263 glass

\ _, > ,_ , E 1

300 pm

J 4

Fig. 3. The mechanical structure of the used glass microstrip plate.

412 S. Aiello et al./Nucl. Instr. and Meth. in Phys. Rex A 400 (1997) 469-475

200 pm; however, the anodes have been grouped voltage was tuned ( < 10 V) in order to maximize ten by ten by the factory, so the useful pitch is the output signal amplitude as observed on an 2 mm. We have further grouped five elements to get oscilloscope. The gas-flowing system was regulated what we call the “thin” layer (1 cm) and 18 elements at an operating pressure of 90 mbar, with a stability for the “thick” one (3.6 cm). It is evident that on the of < l%, thus, the reduced drift field was E/P basis of the useful pitch one can build a person- z 0.5 V/(cm/mbar). The anode output signals were alized segmentation giving rise to a multi-layer collected by means of two Ortec 142C preampli- configuration. Unfortunately, due to mechanical fiers, in order to match the 350 and 1200 pF limitations of the first prototype, we could not capacitance of the thin and thick layers, respective-

avoid to have some “dead” space between the input ly; this gave a nominal equivalent noise of ~2000

window and the first active layer, as well as between and % 6000 electrons. The preamplifier output sig-

the two active layers. This configuration is sche- nals were shaped using two Ortec 571 spectroscopy matically shown in Fig. 4 (case a). amplifiers and then digitized.

In order to test the detector, we used a 80 MeV 12C beam delivered by the Tandem accelerator at LNS Catania. The target consisted of a 40 ug/cm2 carbon foil on which 50 ug/cm’ of UF4 was evapor- ated. The MINIT was placed inside a small scatter-

ing chamber with limited available space. For this reason we were forced to place it close to the target, at 25 cm distance. In order to avoid large variations of the active detection thickness, due to the wide angular opening of the different possible trajecto- ries, we placed a 2 cm diameter collimator in front of the input side of MINIT, at an angle of 21’ with respect to the beam axis.

We have built the thin-thick signal scatter plot, that is shown in Fig. 5. At least six different charged

particle types are evident (He, C, N, 0, F, Ne), each one placed on a characteristic locus, due to reac- tions between ‘*C and the different components of the target. It is also possible to see the peak due to the elastic scattering of ‘*C on U (briefly denoted carbon peak). In the upper part of the figure we can see counts probably due to uranium fission reac- tions, but the low statistics do not allow to deduce any further information.

The two groups of anodes were independently biased at + 400 V, while the cathode strips and the large drift electrode were grounded; the Frisch grid

For each particle type two operating regimes can be distinguished:

__ the AE-E regime, corresponding to the upper part of the curves (negative slope), with the low-energy particles crossing the thin detector and stopping into the thick one;

3.5 pn 7.1 cm 1CIll 2.6 cm 3.6 em microstrip plate mylar gas thin gas thick active area

dead active dead active 8x8 cm2 layer layer layer layer

Fig. 4. Layering setup of the test configuration (top view). The

central shadowed regions of 1 and 3.6 cm thickness were the two

active layers of the detector. See the text for the meaning of

(aHc).

_ the AE-AE regime, corresponding to the over- lapped lower part of the curves (positive slope), with the particles having enough energy to cross both detector layers.

The inversion point of each locus, indicated in

the figure, is usually called punching-through point and corresponds to particles with the minimum energy necessary to cross both detectors. Such a value is generally deduced by means of energy loss computations, and is used in order to make an energy calibration. We have performed this proced- ure on the five punching-through points available (the statistics were too low for Ne), and the result- ing calibration fits on the deposited energy for both detectors are shown in Fig. 6. The energy released by the elastic scattering of “C, not used for these

S. Aiello et al. /Nucl. Instr. and Meth. in Phys. Res. A 400 (1997) 469-475 473

.-:- Fission fragments

.

F _

,‘~II.IIII.l~~l11111111”‘l”“““l’ll’l”11”1”1 250 500 750 1000 1250 1500 1750 2000 225 2500

Ethick (arb.units)

Fig. 5. Scatter plot of the thin-thick layer energy signals. The punching-through points (see text) are indicated for He (4 MeV),

C (21.1 MeV). N (25.5 MeV), 0 (30.4 MeV), F (35.4 MeV), Ne.

fits, has been put on the plots as a cross-check point: the agreement appears nice.

In Fig. 7 we show the energy spectrum for the thin layer. The peaks corresponding to different Z values are clearly distinguishable. The FWHM energy resolution of the two detectors has been measured at the elastic-scattering peak (Gaussian fit), and is % 60 keV for the thin layer (inset of Fig.

7) and % 150 keV for the thick one. The electronic-

noise contribution to the measured resolution is negligible. In fact, for the thin layer, ~600 keV deposited by the elastic “C produced %20000

electrons. The chamber was operated with a gain of 30, so we get 6 x lo5 electrons that compared with the preamplifier noise of ~2 x lo3 equivalent elec- trons gives a signal-to-noise ratio of S/N % 10’.

We deduce that the main source of energy inde- termination comes from energy straggling; we

stress that this contribution is due to the whole gas thickness that is crossed. The Bohr formula for the

energy straggling [11] gives a value of A(FWHM) = 45 keV after the thin layer ( ~8 cm of gas cross- ed), to be compared with the ~60 keV measured indetermination. Nevertheless, with the next proto- type we are going to improve the internal mechan- ical arrangement of the detector, reducing the gas “dead” layer thickness to a few millimeters and hence reducing the energy straggling contribution.

5. Discussion

In this section, we comment on the quality of the results obtained by making some simple estimation of the attainable performance with an optimized MINIT detector.

414 S. Aiello et al.lNucl. Insfr. and Meth. in Ph_vs. Rex A 400 (1997) 469-475

2ooa ,‘, II ‘,‘I’[’

1800- a)

1600 -

800 -

6

Ethln fmev) &* WV)

Fig. 6. Energy calibration for the two MINIT layers. Five punching-through points, He, C, N, 0, F, have been used for the fit (circles);

the elastic peak of “C (square) is a cross-check to verify the correctness of the procedure: (a) thin layer; (b) thick layer.

2 1000

5 7 900 a

k m 900 E

a 0 700

600

500

400

300

200

100

0 irIm~f,,, 1’ ,,i~.~~lL 0 250 500 750 1000 1250 1500 1750 2000 2250 2!

E,,,(channels)

Fig. 7. Energy spectrum for the thin layer. The peaks corres-

ponding to different Z values are clearly distinguished. In the

inset the Gaussian fit to the elastic peak of “C is shown, whose

FWHM is -60 keV.

The nominal identification threshold of a tele- scope detector is the minimum energy necessary to cross the AE layer and to produce a useful signal into the E detector. This value obviously depends on the particle type, and so it is often convenient to express it in terms of silicon equivalent thickness. In this regard a typical threshold of silicon telescope is lo/100 pm; we have already mentioned, however, the recent development of a monolithic silicon tele- scope with a 1 pm AE thickness and a few microns threshold [S]. Such a device, as already said, is limited to few mm’ area.

The tested MINIT configuration (case a in Fig. 4) corresponds to z 15 pm silicon threshold. If

we put the two layers just behind the input window the threshold becomes equivalent to ~4 pm silicon (case b in Fig. 4). Finally, if we configure the two layers consecutively, i.e. with no gas dead layer in between, the threshold goes down to l-l.5 pm sili- con (case c in Fig. 4). Moreover, we have to con- sider that both the window thickness and the gas pressure can be decreased, giving rise to a

S. Aiello et al. /Nucl. Instr. and Meth. in Phvs. Rex A 400 (1997) 469-475 415

submicron threshold, in spite of a large active input

area. From these simple considerations we deduce that

large-area gas telescopes with very low particle identification threshold can be attained at a reason-

able price, without the necessity of special perform- ing electronics. The geometrical structure of the detector allows an easy composition of several elements, to cover a large solid angle, and a general- purpose gas layering. Its transparency can be useful for the installation of further detection elements as stop detectors for the higher-energy particles. As an example, we mention the possibility to make a Bragg’s curve measurement with a single detector setup, by exploiting the multi-layering feature of MINIT.

To conclude we cite another possible improve- ment we are planning. If a start signal is available, the drift time of the primary charge can be used to measure one coordinate of the hit point; the second coordinate could be obtained by means of a double readout of the first gas layer at the two ends of a resistive electrode, thus adding to the MINIT detector also the position sensitivity.

6. Conclusions

We have developed a prototype exploiting a technique for heavy-ion detection, that allows a modular realization of multilayer telescopes. The results obtained indicate that detector systems based on MINIT, with large solid angle and low identification energy threshold, are reliable and cheap. This opens good perspectives also in view of low-energy experiments with radioactive ion beams.

Acknowledgements

We thank R. Bellazzini and F. Gramegna for the useful suggestions, F. Sauli for the initial encour-

agement in the development of our project and for having allowed us the usage of the mask for the

microstrip plate layout. We would also acknow- ledge the contribution of G. Attina, C. Cali, S. Cardillo, G. Di Biasi, A. Giammo, N. Giudice, M. Gu, P. Litrico, S. Tringale, B. Trovato, S. Urso in the process of design, fabrication, assembling and testing of the MINIT prototype. Moreover, we are grateful to the INFN Gruppo V committee for having funded this development.

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