NUCLEAR INSTRUMENTS AND METHODS 126 (~975) 369-372; ~:) NORTH-HOLLAND PUBLISHING CO. ON THE USE OF COHERENT ELECTRON-PAIR PHOTOPRODUCTION AND BREMSSTRAHLUNG OBTAINED FROM A 400 GeV PROTON-SYNCHROTRON GIORDANO DIAMBRINI-PALAZZI, EZIO MENICHETTI and ALBERTO SANTRONI Istituto di Scienze Fisiche, Unieersitc'l di Genova, Italy lstituto Nazionale di Fisica Nucleare, Sezione di Genre:a, Ita(v Received 24 April 1974 It is shown that the coherent electron-pair production by very- high-energy photons in a crystal converter could be used in order to obtain a cleaner electron beam from the CERN SPS or Batavia 400 GeV proton synchrotron. Coherent electron-pair cross section in a tungsten crystal has been integrated over the photon energy spectrum which fits the inclusive reaction ISR data. Performances of several crystals have been evaluated before selecting the tungsten one. There are plans to produce photon and electron beams in the hundred-GeV region from both the NAL and SPS CERN 400 GeV acceleratorsl). According to the double-conversion scheme, pho- tons from the 7r ° decay produced by an external 200-400 GeV proton beam are converted in electron- positron pairs, after magnetic sweeping of charged particles. Then an electron or positron beam is obtained by selecting a suitable momentum channel. Such a beam could be used for experiments (electroproduction, elastic or inelastic scattering, annihilation, etc.) or could hit a radiator in order to produce a brems- strahlung beam which, once cleaned from charged particles, will be suitable for photoproduction exper- iments. As far as experimental exploitation is concern- ed, the quality of both the electron and photon beams is determined essentially by their intensity, angular divergence, and purity, i.e., low hadron contamination. In fact the converter in electron pairs is hit by a neutral beam with a relatively high hadrons-over- 7 ratio. Then the neutral hadrons (essentially neutrons) can interact with the nuclei in the converter by produ- cing charged hadrons, which will contaminate the electron beams. Likewise these charged hadrons will contaminate the photon beam by generating neutral hadrons in the bremsstrahlung radiator. We want to show in this paper that the use of crystals as converter in electron pairs and/or as radiator of bremsstrahlung has the effect to improve the "'quality" of the beams as far as hadron contamination and electron scattering are concerned. it is well known that the use of a thin crystal as radiator allows to produce a coherent bremsstrahlung spectrum with peaks of polarized photons. One of the authors (G. Diambrini P., see also ref. 2) suggested that the coherence effect in production of bremsstrahlung and electron pairs in crystals could improve the ratio between the numbers of photons or electrons and contaminating hadrons. The fact, that the atoms of the crystal inside the coherence length cooperate coherently in the act of generating a photon or an electron-positron pair, has the effect to increase the probability of the event, and so to shorten the radiation length with respect to the corresponding amorphous target. Contrary to this, the nuclear collision path is unaffected by the crystal structure, and very likely so the multiple electron scattering. When a mixed beam of Nh hadrons and Ne electrons (or photons), strikes a crystal, the ratio between the number of strong inter- actions, due to hadrons, and electromagnetic inter- actions, nh/n ~, is given by: nh Nh Xa Xcr Nh Xa o" i n~ N c ). X a N~ 2 a c+a i so the "contamination" ratio nh/n ~ can be improved by minimizing each of the three factors: a) the ratio Nh/N e by optimizing beam geometry; b) the ratio )La/2 between the amorphous radiation length, X,,, and the nuclear collision length, 2, by choosing high-Z elements; and c) by using a crystal with max- imum possible (a~+al)/a ~, where a~ and a~ are the coherent and incoherent part of the cross sections in a crystal. In case of an electron-beam-producing brems- strahlung in a crystal radiator, the factor (a~+aO/a ~ is available directly from the computed intensity spectra in the 100GeV region2), and it ranges between 20 and 100. 369