Phys.Rev.ST Accel.Beams 11:041001,2008 The processes of coherent bremsstrahlung (CB) and coherent pair production
(CPP) based on aligned crystal targets have been studied in the energy range
20-170 ...GeV. The experimental arrangement allowed for measurements of single
photon properties of these phenomena including their polarization dependences.
This is significant as the theoretical description of CB and CPP is an area of
active theoretical debate and development. With the theoretical approach used
in this paper both the measured cross sections and polarization observables are
predicted very well. This indicates a proper understanding of CB and CPP up to
energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to
determine the polarization parameters in our measurements. New technologies for
high energy photon beam optics including phase plates and polarimeters for
linear and circular polarization are demonstrated in this experiment. Coherent
bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger
enhancement for hard photons than CB for the channeling orientations of the
crystal. Our measurements and our calculations indicate low photon
polarizations for the high energy SOS photons.
The distribution of the energy released in a thin silicon detector placed on the downstream side of a thick germanium single crystal bombarded with a 150 GeV electron or pion beam along directions ...close to the 〈110〉 axis or along random directions has been investigated. In view of a possible application to very high energy gamma ray astronomy and particle physics, the intrinsic capability of such a device to reject, on the basis of energy discrimination, unwanted events due to charged hadrons together with the resulting loss of efficiency for the detection of showers initiated by high energy electrons, is determined as a function of the chosen energy threshold.
Nucl.Instrum.Meth. B234 (2005) 128-137 The CERN-NA-59 experiment examined a wide range of electromagnetic processes
for multi-GeV electrons and photons interacting with oriented single crystals.
The ...various types of crystals and their orientations were used for producing
photon beams and for converting and measuring their polarisation.
The radiation emitted by 178 GeV unpolarised electrons incident on a 1.5 cm
thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the
String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised
photon beams.
A new crystal polarimetry technique was established for measuring the linear
polarisation of the photon beam. The polarimeter is based on the dependence of
the Coherent Pair Production (CPP) cross section in oriented single crystals on
the direction of the photon polarisation with respect to the crystal plane.
Both a 1 mm thick single crystal of Germanium and a 4 mm thick multi-tile set
of synthetic Diamond crystals were used as analyzers of the linear
polarisation.
A birefringence phenomenon, the conversion of the linear polarisation of the
photon beam into circular polarisation, was observed. This was achieved by
letting the linearly polarised photon beam pass through a 10 cm thick Silicon
single crystal that acted as a "quarter wave plate" (QWP) as suggested by N.
Cabibbo et al.
We present new results regarding the features of high energy photon emission
by an electron beam of 178 GeV penetrating a 1.5 cm thick single Si crystal
aligned at the Strings-Of-Strings (SOS) ...orientation. This concerns a special
case of coherent bremsstrahlung where the electron interacts with the strong
fields of successive atomic strings in a plane and for which the largest
enhancement of the highest energy photons is expected. The polarization of the
resulting photon beam was measured by the asymmetry of electron-positron pair
production in an aligned diamond crystal analyzer. By the selection of a single
pair the energy and the polarization of individual photons could be measured in
an the environment of multiple photons produced in the radiator crystal.
Photons in the high energy region show less than 20% linear polarization at the
90% confidence level.
The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20-170 GeV. The experimental arrangement ...allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active theoretical debate and development. With the theoretical approach used in this paper both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high energy SOS photons.
We present the first experimental results on the use of a thick aligned Si
crystal acting as a quarter wave plate to induce a degree of circular
polarisation in a high energy linearly polarised ...photon beam. The linearly
polarised photon beam is produced from coherent bremsstrahlung radiation by 178
GeV unpolarised electrons incident on an aligned Si crystal, acting as a
radiator. The linear polarisation of the photon beam is characterised by
measuring the asymmetry in electron-positron pair production in a Ge crystal,
for different crystal orientations. The Ge crystal therefore acts as an
analyser. The birefringence phenomenon, which converts the linear polarisation
to circular polarisation, is observed by letting the linearly polarised photons
beam pass through a thick Si quarter wave plate crystal, and then measuring the
asymmetry in electron-positron pair production again for a selection of
relative angles between the crystallographic planes of the radiator, analyser
and quarter wave plate. The systematics of the difference between the measured
asymmetries with and without the quarter wave plate are predicted by theory to
reveal an evolution in the Stokes parameters from which the appearance of a
circularly polarised component in the photon beam can be demonstrated. The
measured magnitude of the circularly polarised component was consistent with
the theoretical predictions, and therefore is in indication of the existence of
the birefringence effect.
The cross section for coherent pair production by linearly polarised photons
in the 20-170 GeV energy range was measured for photon aligned incidence on
ultra-high quality diamond and germanium ...crystals. The theoretical description
of coherent bremsstrahlung and coherent pair production phenomena is an area of
active theoretical debate and development. However, under our experimental
conditions, the theory predicted the combined cross section and polarisation
experimental observables very well indeed. In macroscopic terms, our experiment
measured a birefringence effect in pair production in a crystal. This study of
this effect also constituted a measurement of the energy dependent linear
polarisation of photons produced by coherent bremsstrahlung in aligned
crystals. New technologies for manipulating high energy photon beams can be
realised based on an improved understanding of QED phenomena at these energies.
In particular, this experiment demonstrates an efficient new polarimetry
technique. The pair production measurements were done using two independent
methods simultaneously. The more complex method using a magnet spectrometer
showed that the simpler method using a multiplicity detector was also viable.
We describe the case of a patient with unstable angina and left main coronary artery occlusion, yet with good collateral circulation from the right coronary artery, and with normal left ventricular ...function. We identified an anomalous vessel from the ascending aorta ensuring a good supply to the left coronary artery.
To verify the advantages and the systematic use of 5 French catheters in femoral percutaneous coronaroventriculography, 500 consecutive unselected patients with confirmed or suspected coronary artery ...disease underwent this procedure. In one case only (0.2%) we had recourse to 7 French catheters on account of the inability to enter the left coronary ostium. There were no major local complications. General complications were represented by vagal reactions in 20 cases (4%) and by transient minor motor aphasia in one case (0.2%). The cardiac complications were represented by: -1 (0.2%) atrio-ventricular isorhythmic dissociation; -1 (0.2%) complete right bundle branch block; -3 (0.6%) atrial fibrillation; (all had a rapid and spontaneous regression) -1 (0.2%) ventricular fibrillation treated successfully with D.C. shock; -1 (0.2%) non fatal acute myocardial infarction (at the end of the procedure); -2 (0.4%) attacks of cardiac asthma treated successfully with medical therapy. 5 French catheters have proved to be extremely efficacious, with some technical experience, in percutaneous coronaroventriculography via femoral approach. The possible advantages could consist of a reduction in local complications, a patient's precocious mobilization and a minor traumatism on the arterial wall. The major complications are comparable with those observed using 7-8 French catheters. In the future it might be possible, in selected patients, to use 5 French catheters to perform ambulatory cardiac catheterization with reduction of both costs and hospitalization period.
