New experiments on crystal assisted collimation have been carried out at the CERN SPS with stored beams of 120 GeV/c protons and Pb ions. Bent silicon crystals of 2 mm long with about 170 μrad bend ...angle and a small residual torsion were used as primary collimators. In channeling conditions, the beam loss rate induced by inelastic interactions of particles with the crystal nuclei is minimal. The loss reduction was about 6 for protons and about 3 for Pb ions. Lower reduction value for Pb ions can be explained by their considerably larger ionization losses in the crystal. In one of the crystals, the measured fraction of the Pb ion beam halo deflected in channeling conditions was 74%, a value very close to that for protons. The intensity of the off-momentum halo leaking out from the collimation station was measured in the first high dispersion area downstream. The particle population in the shadow of the secondary collimator–absorber was considerably smaller in channeling conditions than for amorphous orientations of the crystal. The corresponding reduction was in the range of 2–5 for both protons and Pb ions.
Experiments were performed at the CERN SPS with 120 GeV/c stored proton beams to assess the possibility of beam halo collimation assisted by bent crystals. A bent crystal was used to deflect ...horizontally by an angle of about 170 μrad the beam halo protons in channeling states directing them into a 60 cm long tungsten absorber. The halo loss rate due to nuclear inelastic interactions of protons in the aligned crystal was up to five times smaller than for its amorphous orientation. Channeled fractions, (75±4)% and (85±5)% for the two tested silicon crystals, were measured by intercepting the deflected beam with another collimator located between the crystals and the absorber. The pixel detector (MEDIPIX) installed in a Roman pot inside the beam pipe was used to obtain visual images of the deflected beam.
A precision measurement of the ratio RK of the rates of kaon leptonic decays K±→e±ν and K±→μ±ν with the full data sample collected by the NA62 experiment at CERN in 2007–2008 is reported. The result, ...obtained by analysing ∼150000 reconstructed K±→e±ν candidates with 11% background contamination, is RK=(2.488±0.010)×10−5, in agreement with the Standard Model expectation.
In ideal two-stage collimation systems, the secondary collimator–absorber should have its length sufficient to exclude practically the exit of halo particles with large impact parameters. In the UA9 ...experiments on the crystal assisted collimation of the SPS beam a 60 cm long tungsten bar is used as a secondary collimator–absorber which is insufficient for the full absorption of the halo protons. Multi-turn simulation studies of the collimation allowed to select the position for the beam loss monitor downstream the collimation area where the contribution of particles deflected by the crystal in channeling regime but emerging from the secondary collimator–absorber is considerably reduced. This allowed observation of a strong leakage reduction of halo protons from the SPS beam collimation area, thereby approaching the case with an ideal absorber.
A
bstract
The NA62 experiment at CERN, designed to study the ultra-rare decay
K
+
→
π
+
ν
ν
¯
, has also collected data in beam-dump mode. In this configuration, dark photons may be produced by ...protons dumped on an absorber and reach a decay volume beginning 80 m downstream. A search for dark photons decaying in flight to
μ
+
μ
−
pairs is reported, based on a sample of 1
.
4 × 10
17
protons on dump collected in 2021. No evidence for a dark photon signal is observed. A region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon masses between 215 and 550 MeV
/c
2
.
9Ni steels have been recently adopted in supercritical CO2 injection systems in deepwater oil fields. The manufacture of these reinjection systems involves multi-pass welding procedures, which ...produce an Heat-Affected Zone (HAZ) with a high heterogeneity level regarding the microstructural features and the local mechanical properties. An extensive microstructural and micromechanical characterisation was performed over the HAZ of three welded joints with different heat-input conditions to evaluate the effects of the reheating cycles and the welding parameters on the microconstituents. Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) analyses were performed to identify microstructural features that correlate to the local mechanical responses evaluated through an extensive microhardness mapping. Regarding the Coarse Grained HAZ (CGHAZ), the highest microhardness values for all welding conditions are found at the Supercritically Reheated CGHAZ (SCR-CGHAZ), characterised by its refined microstructure and a quite low area fraction of coarse martensite laths. The Subcritically Reheated CGHAZ (SC-CGHAZ) and the Intercritically Reheated CGHAZ (IC-CGHAZ) – regions where wider martensite blocks and higher coarse martensite lath area fractions were observed – composed the softer zones of the microhardness map. It was also found that reheating at intercritical temperatures induces the formation of supersaturated fresh martensite and may contribute to retained/reversed austenite particles’ C-enrichment, which may degrade the mechanical properties at the IC-CGHAZ.
Exclusive rho super(0)-meson electroproduction is studied in the HERMES experiment, using a 27.6 GeV longitudinally polarized electron/positron beam and unpolarized hydrogen and deuterium targets in ...the kinematic region 0.5 GeV super(2)<Q super(2)<7.0 GeV super(2), 3.0 GeV <W<6.3 GeV, and -t'<0.4 GeV super(2). Real and imaginary parts of the ratios of the natural-parity-exchange helicity amplitudes T sub(11) ( $\gamma*}_{T} \rightarrow\rho_{T}$ ), T sub(01) ( $\gamma*}_{T} \rightarrow \rho_{L}$ ), T sub(10) ( $\gamma*}_{L} \rightarrow\rho_{T}$ ), and T sub(1-1) ( $\gamma*}_{-T} \rightarrow\rho_{T}$ ) to T sub(00) ( $\gamma*}_{L}\rightarrow\rho_{L}$ ) are extracted from the data. For the unnatural-parity-exchange amplitude U sub(11), the ratio |U sub(11)/T sub(00)| is obtained. The Q super(2) and t' dependences of these ratios are presented and compared with perturbative QCD predictions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The possibility for optimization of crystal assisted collimation has been studied at the CERN SPS for stored beams of protons and Pb ions with 270 GeV/c per unit charge. A bent silicon crystal used ...as a primary collimator deflects halo particles in the channeling regime, directing them into a tungsten absorber. In channeling conditions a strong reduction of off-momentum particle numbers produced in the crystal and absorber, which form collimation leakage, has been observed in the first high dispersion (HD) area downstream. The present study shows that the collimation leakage is minimal for some values of the absorber offset relative to the crystal. The optimal offset value is larger for Pb ions because of their considerably larger ionization losses in the crystal, which cause large increases of particle betatron oscillation amplitudes. The optimal absorber offset allows obtaining maximal efficiency of crystal-assisted collimation.
Channeling is the confinement of the trajectory of a charged particle in a crystalline solid. Positively charged particles channeled between crystal planes oscillate with a certain oscillation ...length, which depends on particle energy. A crystal whose thickness is half the oscillation length for planar channeling may act as a mirror for charged particles. If the incident angle of the particle trajectory with the crystal plane is less than the critical angle for channeling, under-barrier particles undergo half an oscillation and exit the crystal with the reversal of their transverse momentum, i.e., the particles are “mirrored” by the crystal planes. Unlike the traditional scheme relying on millimeter-long curved crystals, particle mirroring enables beam steering in high-energy accelerators via interactions with micrometer-thin straight crystal. The main advantage of mirroring is the interaction with a minimal amount of material along the beam, thereby decreasing unwanted incoherent nuclear interactions. The effectiveness of the mirror effect for ultrarelativistic positive particles has been experimentally proven at external lines of CERN-SPS. The mirroring effect in a 26.5-μm-thick Si crystal has been studied in the experiment with a narrow beam of 400 GeV/c protons at the CERN-SPS. The reflection efficiency for a quasi-parallel beam is larger than 80%.
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