New target and detection methods: active detectors

Nuclear Physics A722 (2003) lOc-16c

ELSEVIER

www.elsevier.comllocate/npe

New target

and detection

methods:

active

detectors

W. Mittig”, H. Savajol?, C.E Demonchy”, L. Giota, P. Roussel-Chomaz”, G.Ter-Akopianb, .4.Fomichevb, M.S. Golovkovb, S.Stepansovb, R.Wolskib, A. DrouartC, A. Gillibert”; V. Lapoux’ and E. PollaccoC

H. Wang”, N. Alamanos’,

a GANIL (DSM/CEA, IN2P3/CNRS), BP 5027, 14076 Caen Cedex 5, France bFLNR,JINR, Dubna, P.O. Box 79, 101 000 Moscow,

Russia

“CEA/DSM/DAPNIA/SPhN, Saclay, 91191 Gif-sur-Yvette Cedex, France The study of nuclei far from stability interacting with simple target nuclei, such as protons, deuterons, 3He and 4He implies the use of inverse kinematics. The very special kinematics, together with the low intensities of the beams calls for special techniques. In july 2002 we tested a new detector, in which the detector gas is the target. This allows in principle a 4~ solid angle of the detection, and a big effective target thickness without loss of resolution. The detector developped, called Maya, used isobuthane C4H10 as gas in present tests, and other gases are possible. The multiplexed electroncics of more than 1OOOchannels allows the reconstruction of the events occuring between the incoming particle and the detector gas atoms in 3D. Here we were interested in the elastic scattering of ‘He on protons for the study of the isobaric analogue states (IAS) of ‘He. The beam, in this case, is stopped in the detector. The resonance energy is determined by the place of interaction and the energy of the recoiling proton. The design of the detector is shown, together with some prelimiary results are discussed. 1. INTRODUCTION If we want to study the nature of nuclei far from stability, it is best to have interactions with simple particles, such as electrons, protons and other light particles of well understood structure. The lifetime of the nuclei far from stability being too short to prepare targets in nearly all cases, it will be necessary to inverse the role of target and projectile, and targets of H and He will be needed. The experiments in this domain need a very good sensitivity in order to detect rare events with high efficiency, combined with high resolution in order to have the maximum of information possible with low statistics. High resolution is often necessary, too, to have a very good signal to background ratio in order to find for example a process with one event per day in a background of thousands of events per second. 0375-9474/03/$ - see front matter 0 2003 Published by Elsevier Science B.V. doi:lO.l016/S0375-9474(03)01328-9

FVMittig et al. /Nuclear

Physics A722 (2003) lOc-16c

llc

In the study of reactions in inverse kinematics, the information of interest can be deduced by measuring either the kinematical characteristics of the heavy residue and/or of the light fragment. In the case of the heavy residue, the detection efficiency is increased by the forward focusing of the reaction, and the large velocity allows for the use of relatively thick targets. However, the detection of the heavy fragment is possible only for the reactions where it is bound or has a lifetime long enough to reach the detection system. Moreover, the angular center of mass resolution which can be obtained becomes rather poor, as soon as the mass of the projectile exceeds a few mass units. In these cases; the measurement of the energy and diffusion angle of the light recoil fragment allows to reconstruct the kinematics of the reaction. For targets simple nculei are preffered, such as H;He. They imply the use of gas targets, eventually at kryogenic temperatures to increase the density. Several examples of the use of cryogenic targets developed at Ganil and Dubna can be found in this conference. Here we want to show only some results with a newly developped active target detector. A recent revue on this subject can be found in [l]. 2. ACTIVE

TARGETS

2.1. General considerations Active targets were developped since a long time in high energy physics, such as bubble chambers. In the domain of secondary beams, the archetype is the detector IKAR [a]. A discussion of the use of this detector for elastic scattering at GSI energies can be found in [3]. The use of this detector was limited to Hz at a pressure of IOatm. For the domain of lower energies, and for the use of of various gases, we developped a new detector called MAYA[4], that we will describe here. 2.2. Objectives We want to illustrate the possibilities of such a device by two examples: Isobaric Analogue states (IAS), and transfer reactions. IAS can be studied via resonnant scattering of protons. As a first study we performed an experiment on the ‘He(p,p) reaction. The resonance corresonding to the groundstate of ‘He is expected at a center of mass energy of about 1.8MeV. The question of the spin of the groundstate of ‘He is still open to debate, and l=O andl=l are probable candidates. Several rather narrow excited states have been observed in this nucleus [5] but without spin and parity assignement even for the ground state. Some calculations predict that the ground state of ‘He should be l/2+ instead of l/2- or 3/2- expected in the normal shell ordering. The same parity inversion is well known in r’Be . For a recent discussion see e.g.[6,7] and references cited. A level scheme showing the relation of IAS in ‘Li is shown on figure 1. ‘The expected resonance structure is shown on figure 2,3 As can be seen a clear difference in the interference pattern between resonant and nonresonant amplitudes is expected at 90 degrees whereas at 180deg very simular structures are obtained. An experiment was performed in july 2002 with a 3.1MeV/nucleon ‘He beam from Spiral, an energy that allows to cover the expected groundstate and 1st excited state resonances (see fig. l), the incident beam being stopped in the detector gas at 1 atm. The data are being analysed now.

W Mittig et ai. /Nuclear

12c

Figure

1. Energy

relations

Physics A722 (2003) 1 Oc-16~

of the IAS of ‘He in ‘Li.

Another example of the use of this detector is in an experiment where very exotic reactions are searched[8]. As was reported in this conference resonances were observed in the 5H and tentatively in the ‘H systems. A possible alternative reaction is 1zC(sHe,7H)13N, with an ‘He beam from Spiral at 15MeV/ nucleon, and the C target being the detector gas C4H10. In this reaction, the very low energy recoil nucleus 13N must be observed. In standard techniques this is quite impossible, the target being too thin, and the recoil energy so low that standard detetctors will not allow particle identification. In this case the detector pressure may be diminished to 20mbar, and the recoil range of the 13N will be several centimeters. These are just two exemples of the possibilities of such a detector. Other detector such as deuterium or3He are possible, and will allow a broad range of experiments.

gases

2.3. The detector MAYA The detector has an active volume of 28(width)*26(lenght)*20(heighth) cm2. It can be filled with pressures up to 4atm. The detection scheme is illustrated by the figure 4. For a two body reaction, scattered and recoiling particle are in a plane. The electrons from the ionisation of the gas by the particles are drifting down to the amplifying wires. The wires are parellel to the beam. Therefore their diameter can be different in the region of the beam, to adjust for different ionisation densities of beam and recoil particles. In the experiment with ‘He we used a gain of l/10 in this central region. The spacing of the

13c

PF!A4ittig et al. /Nuclear Physics A722 (2003) IOc-16c

1 1.2 1.4 1.6 1.8 2

Figure 2. The elastic function at 90 degrees.

scattering

excitation

2.2

2.;;its;ja

Figure 3. The elastic scattering function at 180 degrees.

3

excitation

wires should be quite small, in principle less than the drift straggling of the electrons. We used a distance of 3mm. The wires corresponding to a row of pads are connected to the same preamplier. As can be seen on the scheme; the angle of the reaction plane can be determined by the drift time to the wires. The amplified signal is induced in the pads below. The distance between the wires and the pads determines the width of the induction pattern. A distance of 1Omm was chosen in order to have best position resolution that is obtained if the two nearby lateral pads have about half the amplitude of a central pad. A hexagonal structure was choosen for these strips, in order to have best conditions for the reconstruction of the trajectory, independant of the direction. This results in a honeycomb structure. A matrix of 35 by 34 pads, this 1190 pads constitues this anode. The pads are arraged in rows below the wires, in order to have a precise time relation between the wire signal and the pad signal. The border pads are not read by the electronics, and thus 1056 are conneced to preamplifiers. The pads are connected to gassiplex Asics. These provide a multiplexed readout, limiting the numbers of connections from the detector to the outside. The gassiplex are mounted on the back of the anode. The gassiplex need a track and hold signal, that is provided by the wire signal, treated by classical electronics. A typical charge induction pattern is shown on figure characterized as a CPC: Charge Projection Chamber.

5.

The detector

can thus be

As pointe out above, inverse kinematics genrate recoil particles in a great energy domain. High energy light paticles such as protons cannot be stopped in a reasonable gas volume. For escaping particles, the range information is lost, and hence total energy and particle identification is lost. Essentially two solutions exist; one is the use of magnetic fields, producing a curvature of the trajectories, or the use of ancillary detectors outside of the active volume. We adopted the second solution adding CsI wall of 20*25cm2, covering about 45 degrees around the beam for events in the middle of the detector. It can be put on forward angles for forward peaked reactions such as (p,p), (p,d), . . or at backward angles for reactions such as (d,p). . It is devided in 20 modules, and read out is done

14c

Figure 4. Detection the direction of the the incident beam. amplified by wires pattern constitued

W Mittig et al. /Nuclear

Physics A 722 (2003) 1 Oc-16~

scheme for the Maya detector. For a two body reaction, seen here in beam, the recoiling and scattered particles are in a plane that contains The electrons created by the particles in the gas drift down, and are below the Frisch grid. The wire signal is induced in a two dimensional by individually read out hexagonal pads.

by photodiodes. A first direct test of range resolution can be made by the determination of range for beam events, this is incident beam trace events without a reaction. This is shown on figure 6. From the charge induction patterns similar to the one on figure 5, without recoil particles, the range was calculated as the center of gravity of the derivative of the charge after the maximum corresponding to the Bragg peak. A FWHM of 1.2mm was obtained with 0 = 0.8mm. This is therefore an upper limit of the detector range resolution, range stragling included. This value is much smaller than the range straggling predicted from the code Srim, that gives u = 6.3mm. The range measured is identical to the one predicted by Srim. A similar measurement was carried out for 13C at ll.SMeV/nucleon, and a = 2.5mm was obtained, as compared to the predicted value of 7.4mm. Experimental determination of range straggling seems thus an important issue. The 3 dimensional determination of the trajectories needs a quite important development of software. This will be available in next future, and we hope to have soon the results of the first experiment with this detector.

?%Mittig et al. /Nuclear

Physics A722 (2003) 1 Oc-16~

15c

Figure 5. Charge pattern induced in the MAYA anode for an elastic scattering event ‘He(p,p). The beam is incident from the left. Each dot corresponds to a pad, with the area of the dot correponding to the charge induced. Note that wire diameter in the central region was bigger, resulting in a gain a factor 10 lower for y=+-lcm around the beam.

Range [l/lOmm]

Figure 6. The experimental range determined from the center of gravity of the derivative of the charge induced by ‘He at 3.9 MeV/nucleon in 1 atm of isobuthane.

l4TMittig et al. /Nuclear Physics A722 (2003) lOc-16~

16c

3. CONCLUSION As a complement to the construction of cryogenic targets, providing high density targets of gaseous, liquid or solid H,D, andHe, we developped a new active target detector, that takes full advantage of modern electronics, using Asics for cheap and compact electronics. Energy dynamics is increased by the use of a CsI wall, that allows to detect particles not stopped in the gas. The results obtained look promising and full analysis of the experience concerning IAS of ‘He is in progress as thesis work of C.E.D. We think that such type of detectors is an excellent tool for studying the properties of exotic nuclei by low intensity secondary beams, where highest efficiency and good resolution in inverse kinematics is essential.

REFERENCES 1. 2. 3.

4. 5. 6. 7. 8.

P.Roussel Chomaz and W.Mittig, NuclPhysics A 693 (2001) 495 A.A.Vorobyov et al., NIM 119 (1974) 509. P.Egelhof, Proc. Int. Workshop on Physics with Unstable Nuclear Beams, Serra Negra, Sao Paulo Brazil, ed. C.A.Bertulani et al., World Scientific 1997, ISBN981-02-29267,page 222 P.Gangnant, C.Spitaels, G.Fremont, P.Bourgault, J.F.Libin report Ganil 27.2002 W. von Oertzen et al., Nuclear Physics A588, 129 (1995). N.K.Timofeyuk, Phys.Rev. C65, 064306 (2002) P.Navratil, B.R.Barret, Phys.Rev. C57, 3119 (1998) H.Savajols, spokesman of an experiment for Spiral, tentatively sheduled for mid-2003.