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| Experiments at CERN | |
| Title | Investigations of the coherent hard photon yields from (50-300) GeV/c electrons/positrons in the strong crystalline fields of diamond, Si, and Ge crystals |
| Experiment | NA43/2 |
| Greybook | See NA43/2 experiment |
| Approved | 07 February 1991 |
| Status | Finished |
| Collaboration | CHANNELLING |
| Accelerator | CERN SPS |
| Abstract | The aim of this experiment is to measure the influence of strong fields on QED-processes like: Emission of coherent radiation and pair-production when multi-hundred GeV electrons/positrons and photons penetrate single crystals near axial/planar directions. The targets will be diamond, Si, Ge and W crystals. QED is a highly developed theory and has been investigated experimentally in great detail. In recent years it has become technically possible to investigate QED-processes in very strong electromagnetic fields around the characteristic strong field E$_{0}$ = m$^{2}$c$^{3}$/eh = 1.32.10$^{16}$ V/cm. The work of such a field over the Compton length equals the electron mass. The theoretical description of QED in such fields is beyond the framework of perturbation theory. Such fields are only obtained in laboratories for a) heavy ion collisions b) interactions of multi-GeV electrons with extremely intense laser fields and in oriented crystals. In fact it turns out that crystals are unique for this type of experiment. The point is that the probabilities of processes in axial/planar fields are determined by the magnitude of these fields in the particle's rest frame. So the strong field parameter $\chi$ is given by $\chi = \gamma$E/E$_{0}$ where E$_{0}$ is given above, E is the local field from the crystals axis ($\sim 10^{11}$ V/cm) and $\gamma$ the Lorentz factor for the particle ($10^{5} - 10^{6}$). So for multi-hundred GeV electrons/positrons $\chi$-values of one or more are possible. During the last years dramatic effects on radiation emission, pair production (pp) and shower formation has been found. Radiation is enhanced more than two orders of magnitude, pp is enhanced around one order of magnitude and radiation lengths along axial directions are shortened 10-15 times compared to Bethe-Heitler values for shower formation. The dramatic enhanced emission could lead to a new $\gamma$-source. The photons should be polarized - investigations are in progress. For investigations of the predicted strong field effects the experimental setup used by NA43 in the H2 beam of the North Hall is unique. The two drift chambers 40m apart on the incident side gives an angular resolution of 3-4 $\mu$grad. There are two positions for crystal mounting on high precision goniometers inside dedicated vacuum chambers. In Vac. Chamber I one probes the crystal with $e^{+}/e^{-}$, in Vac. Chamber II with photons. These may be tagged with the combination of DC3, Bend3 and DC4. Crystal II may be cooled to liquid nitrogen temperature DC1-DC2-DC3 allows to measure the scattering taking place in crystal I. With C (=converter) and SSD is measured the average photon multiplicity. Finally the pair spectrometer represented by Bend4, DC5 and DC6 is used to determine single photon energies. The NA43 detector therefore is a multi-purpose setup in which may be studied many aspects of strong QED effects. In 1995 is studied conversion probabilities for 10-140 GeV photons in heavy, aligned and cooled crystals (W and Ir). Furthermore is measured for the first time single photon spectra in aligned diamond crystals. One will also measure the so-called Landau-Pomeranchuk-Migdal effect for high energies (> 25 GeV). Should likewise be studied the possible enhanced hadron production in aligned crystal as compared to an amorphous target. Finally will be measured photon initiated showering processes in thick Ge crystals (25 mm). |
| Related document(s) | CERN-SPSC-90-31 (SPSC-P-234-ADD.3) CERN-SPSC-92-61 (SPSC-M-512) |