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CENTRE D'ETUDES NUCLEAIRES DE SACLAY CEA-CONF -10099 Service de Documentation F9I19I GIF SUR YVETTE CEDEX L7

THE FUTURE SCIENTIFIC POLICY IN FRANCE FOR NUCLEAR STUDIES WITH ELECTROMAGNETIC PROBES

BERGERE R.- GOUTTE D.

CLH Centre d'Etudes Nucléaires de Saclay, 91 - Gif-sur-Yvette Dept. de Physique Nucléaire

Communication présentée à : 5. Franco Japanese Colloquium: Results and Prospects on Nuclear PhysicJ uc Intermediary Energies

uogashimu \,JP) 26-30 5tip 1989 Participation au 5ëme Colloque Franco-Japonais : "Résultats et perspectives en physique nucléaire aux énergies intermédiaires" r\ f (Dogashima, Japon du 26 au 30 septembre 1989) V *) f S

Rapport DPn«N/Saclay n*2580 11/1989 THE FUTURE SCIENTIFIC POLICY 18 FRANCE FOR NUCLEAR STUDIES WITH ELECTROMAGNETIC PROBES R. Bergère et D. Goutte Département de Physique Nucléaire, CEN Saclay, 91191 Gif sur Yvette cedex, France THE FITURE SCIENTIFIC POLICY IN FRANCE I«X)R NUCLEAR STUDIES WITH ELECTROMAGNETIC PROBE.

R. Bergère and D. Goutte (DPh N. CEN Saclay)

In this talk, we want to explain, very briefly, what the frcnch nuclear physics community has decided for his future. We will describe, schematically, the present status of the project we have in mind, in terms of machine to be build as well as physics to be studied.

A few years, ago it appeared necessary to the French Nuclear Physics community to think about the lone term perspective in fundamental research. To carry out this reflection, the two main funding agencies, the National Center for Scientific Research (CNRS) and the Atomic Energy Commission (CEA) decided to create a joint commission to answer the following question :

" which topics in nuclear physics should be considered beyond 1905 and what facilities will they require ? " This commission concluded by giving :

"....the highest priority to the physics to be done with a superconducting electron accelerator with an energy superior to 3 GeV." The next step was initiated in 1987 by the creation of two joint CNRS/CEA commissions in charge of the precise definition of : 1) the physics 2) the machine

The conclusions of these two commissions will be given to the French Ministry of Research by the end of 1989 and a decision could be taken beginning of next year.

THE PHYSICS Among the topics recomipended by the joint CNRS-CEA commission let us mention:

Short distances The traditional picture of the nucleus is a description in terms of nucléons and mesons. For nuclear processes involving momentum lower than 1 GeV/c such a description seems to be valid. In fact, their is no absolute experimental evidence for the need of introducing new degrees of freedom. For higher momentum the situation is less clear. In addition to L

rolativistic offerts, one must take short range interactions into account: those are essentially not known. By increasing the momentum transfer to values larger than 1 GeV/c we will reach a domain whore a description of the nucléon-nucléon interaction will have to he consistent with the nucléon substructure. The high moment urn component of the nuclear wave functions will be studied by the extension of (ee'p) experiments ami the development of an (ee'n) program in more favorable conditions. The nucléon-nucléon correlations at short distances will be determined by a new generation of (")r,pp) or (ee',pp) experiments. '

Nucléon and resonances It is of great importance to understand the evolution of hadronic properties in the nuclear medium. In particular, we want to know if the nucléon dynamical behavior is modified in the nuclear medium. The free nucléon as well as its resonances where studied by inclusive experiment and a large amount of data are already available. However the precision of these data has most of the time to be improved. Moreover, in the nucleus, apart the A resonance, most of the nucléon excitation form factors are ignored. Exclusive experiments are needed to measure and fully understand the nature of these resonances and, eventually, their modification in the nuclear medium. A typical example could be the study of the transition form factors of the Sll excitation which offers the interesting property to decay mostly in r\ +N. The Sll can then be "tagged" without any ambiguity, even in a heavy mass nucleus. The experiment to be done is here a typical multicoincidence experiment (e,e'Ni/)

Strangeness in nuclei The strangeness degree of freedom should help to unravel quark effects in nuclei, since the study of the hyper-nucleon interaction enable us to distinguish some short-range features such as spin-orbit interaction and non-mesonic decays. The A-N interaction is particularly interesting since there is no single isovector meson exchange (p or n\ As a consequence, there is no long-range part such as that one which dominates the N-N interaction. This experimental program will mainly involve (e,e'K) experiments.

THE PROJECT To fulfil this recommended program we studied an electron accelerator reaching a maximum energy of 4 GeV and able to provide high intensity (100 /uA) and 100 % duty cycle eLctron as well as real photons beams. The experimental hafls are planed to allow multi-coincidence experiments. The machine The principle of this machine is based on recirculations through linear accelerators. It was decided to use superconducting technology for the linacs. Figure 1 is a schematic representation of the proposed machine. Two superconducting liaars (500 and 300 MeV) will accelerate the electron beam up to 4 GeV by five recirculations. This will allow the extraction of two beams with different (but correlated) energies.

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Figure 1 : Schematic representations of the 4 GeV electron accelerator The experimental rooms

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Figure 2 : The high luminosity experimental hall

The actual plans are to build two different experimental halls; one devoted to high luminosity experiments (virtual photons) and the other one to low luminosity (real photons). As shown in figure 2, the first one will be equipped with two classical spectrometers of 4 GeV/c and 1 GeV/c maximum momentum and 2 GeV/c toroide spectrometer with very large acceptance (320 msr).

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détecteur absorption totale (fétiquetoçe de photons polarisés détecteur toroidal détecteur arrêt de particules faisceau de très grande \ impulsion cage détecteur ^ de Faraday de neutrons ^o

Figure 3 : The low luminosity experimental hall s

The low luminosity hall (figure 3) will be a muJti-purpose one with a basic equipment composed of a photon tagging system and a large acceptance hadronic detector.

Technical studies on accelerating techniques In the mean time, we studied at Saclay, with the help of the Nuclear Physics Institute (IPN) of Orsay the technic of superconducting cavities. The GECS (Groupe d'Etude des Cavités Supraconductrices) was created in 1986 with the goal to reach the state of the art in terms of supra conducting technology and then to improve the performances of the cavities to be used in our future machine. This technical investment allows us to take now, as starting parameters for our machine an accelerating field of 10 MeV/m (instead of 5 MeV/m for American project CEBAF). The resulting design, a five cells cavity, is represented figure 4 and a cryomodule containing four of these cavities is presented figure 5.

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Figure 4 : A five cells cavity

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Figure 5 : A four cavities cryomodule The next step of this technical studies will be the construction of MACS (Module Accélé- rateur à Cavités Supraconductrices) which is a. set of three cryomodules wnere an electron beam will be injected an recirculate in order to reach an energy of about 100 MeV (see figure 6). The construction of this basic module has been approved and funded in 1989 an will be achieved by 1992.

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Figure 6 : The MACS project

This prototype will give us the essential know-how to build, hopefully as fast as possible, the final machine.