Search Results (17 total)

A theoretical methodology promising improved design of vacuum insulation in high-voltage pulsed-power systems is described. It consists of shaping the electromagnetic fields within the system in such a way that charged particles which can in principle initiate vacuum surface breakdown are deflected away from the insulator surface, and secondary electrons, if emitted, are prevented from restriking the surface. Thus, vacuum surface breakdown is prevented before it is able to develop. Our methodology is presented here by a set of case studies.

We summarize the results of two experimental programs at the Alternating Gradient Synchrotron of BNL to measure the nuclear transparency of nuclei measured in the A(p,2p) quasielastic scattering process near 90^° in the pp center of mass. The incident momenta varied from 5.9 to 14.4  GeV∕c, corresponding to 4.8<Q²<12.7  (GeV∕c)². Taking into account the motion of the target proton in the nucleus, the effective incident momenta extended from 5.0 to 15.8  GeV∕c. First, we describe the measurements with the newer experiment, E850, which had more complete kinematic definition of quasielastic events. E850 covered a larger range of incident momenta, and thus provided more information regarding the nature of the energy dependence of the nuclear transparency. In E850 the angular dependence of the nuclear transparency near 90^° and the nuclear transparency deuterons were studied. Second, we review the techniques used in an earlier experiment, E834, and show that the two experiments are consistent for the carbon data. E834 also determines the nuclear transparencies for lithium, aluminum, copper, and lead nuclei as well as for carbon. A determination of the (π⁺,π⁺p) transparencies is also reported. We find for both E850 and E834 that the A(p,2p) nuclear transparency, unlike that for A(e,e^′p) nuclear transparency, is incompatible with a constant value versus energy as predicted by Glauber calculations. The A(p,2p) nuclear transparency for carbon and aluminum increases by a factor of two between 5.9 and 9.5  GeV∕c incident proton momentum. At its peak the A(p,2p) nuclear transparency is ∼80% of the constant A(e,e^′p) nuclear transparency. Then the nuclear transparency falls back to a value at least as small as that at 5.9  GeV∕c, and is compatible with the Glauber level again. This oscillating behavior is generally interpreted as an interplay between two components of the pN scattering amplitude; one short ranged and perturbative, and the other long ranged and strongly absorbed in the nuclear medium. A study of the A dependent nuclear transparency indicates that the effective cross section varies with incident momentum and is considerably smaller than the free pN cross section. We suggest a number of experiments for further studies of nuclear transparency effects.

We studied the ¹²C(p,2p+n) reaction at beam momenta of 5.9, 8.0, and 9.0  GeV/c. For quasielastic (p,2p) events p_f, the momentum of the knocked-out proton before the reaction, was compared (event by event) with p_n, the coincident neutron momentum. For |p_n|>k_F=0.220  GeV/c (the Fermi momentum) a strong back-to-back directional correlation between p_f and p_n was observed, indicative of short-range n-p correlations. From p_n and p_f we constructed the distributions of c.m. and relative motion in the longitudinal direction for correlated pairs. We also determined that 49±13% of events with |p_f|>k_F had directionally correlated neutrons with |p_n|>k_F.

Beams of protons and pions of 5.9 GeV/c were incident on a C target. Neutrons emitted into the backward hemisphere, in the laboratory system, were detected in (triple) coincidence with two emerging particles of tranverse momenta p_t>0.6 GeV/c. We determined that for (46.5±3.7)% of the proton-induced events and for (40.8±4.5)% of the pion-induced events with the two high-p_t particles, there is also at least one backward emitted neutron with momentum greater than 0.32 GeV/c. This observation is in sharp contrast to a well- established universal pattern from a large variety of earlier inclusive measurements with hadrons, electrons, photons, neutrinos, and antineutrinos where the probability for backward nucleon emission was in the 5 to 10 % range. We present also a measurement of the momentum spectra for the backward going neutrons. The spectra have the same universal shape observed in the inclusive reactions. We speculate that the enhanced backward neutron emission in this semi-inclusive region could be an indication for a strong dependence of the cross section on the squared total center-of-mass energy (s) and for the importance of short-range nucleon-nucleon correlations.

The transparency of carbon for (p,2p) quasielastic events was measured at beam momenta ranging from 5.9 to 14.5 GeV/c at 90° c.m. The four-momentum transfer squared (Q²) ranged from 4.7 to 12.7 (GeV/c)². We present the observed beam momentum dependence of the ratio of the carbon to hydrogen cross sections. We also apply a model for the nuclear momentum distribution of carbon to obtain the nuclear transparency. We find a sharp rise in transparency as the beam momentum is increased to 9 GeV/c and a reduction to approximately the Glauber level at higher energies.

We measured simultaneously pp elastic and quasielastic (p,2p) scattering in hydrogen, deuterium, and carbon for momentum transfers of 4.8 to 6.2 (GeV/c)² at incoming momenta of 5.9 and 7.5 GeV/c and center-of-mass scattering angles in the range θ_c.m.  =  83.7°–90°. The nuclear transparency is defined as the ratio of the quasielastic cross section to the free pp cross section. At incoming momentum of 5.9 GeV/c, the transparency of carbon decreases by a factor of 2 from θ_c.m.≃85° to θ_c.m.≃89°. At the largest angle the transparency of carbon increases from 5.9 to 7.5 GeV/c by more than 50%. The transparency in deuterium does not depend on incoming momentum nor on θ_c.m..

Cross sections, pion momentum spectra, and angular distributions for the reactions ¹²C(π^-,π⁺π^-) and ¹²C(π^-,2π^-) at a bombarding energy of 292 MeV were measured wth a 360° spectrometer. An enhancement of the cross-section ratio of ¹²C(π^-,2π^-) to ¹²C(π^-,π⁺π^-) relative to that of the elementary cross sections was observed, and is discussed in terms of various nuclear-medium effects including a double-Δ production channel.

Pion absorption by ³He was studied at T_π=165 MeV in a kinematically complete experiment. The cross section for absorption on a (pn) pair of nucleons, σ_pn(π⁺), was found to be 17.0±2.6 mb; that for absorption on a (pp) pair, σ_pp(π^-), 0.91±0.20 mb. The angular distribution in the πNN center-of-mass system for σ_pn(π⁺) resembles that for the π⁺+d→p+p reaction while the angular distribution for σ_pp(π^-) is strongly backward peaked. Evidence that a significant fraction of the absorptions involves all three target nucleons is seen in the angular correlation between the two detected nucleons as well as in the momentum distribution of the unobserved nucleon. For π⁺ and π^- absorption, the three-body cross sections were found to be 9.6±2.1 and 4.2±01.2 mb, respectively. Neither initial- nor final-state interactions appear to be major contributors to the observed three-body absorption, though initial-state interactions may be contributing to the enhancement of the three-body π⁺ absorption at the Δ resonance.

The reaction ³He(π⁺,pp)p was measured at T_π=350 and 500 MeV in a kinematically complete experiment. Separate contributions from two-body and three-body pion absorption are clearly seen in recoil momentum distributions. The two-body angular distributions closely follow the shape of the free πd→pp cross section, although the number of deuterons estimated from the two-body yield is larger than the value 1.5 expected from isospin coupling and observed previously at lower energies. Comparison with a simple impulse approximation model suggests some of the enhancement may arise from binding of the quasideuteron, although absorption on T=1 (pn) pairs and the higher density of ³He may also be a factor. The strength of the three-body component falls off with bombarding energy with roughly the same slope as for the two-body component, and accounts for ≊45% of the total absorption cross section, a larger fraction than reported previously below the (3,3) resonance. The three-body matrix element ‖M₃‖² displays an angular dependence at both energies which is suggestive of initial-state interactions. No evidence of final-state interactions is seen.

Limits on the lepton-flavor-violating reactions μ^-+Z→e^-+Z and μ^-+Z→e⁺+(Z-2), muon-electron and muon-positron conversion, have been obtained from a search performed at TRIUMF using a time-projection chamber. Upper limits (90% C.L.) for the branching ratios compared to ordinary muon capture for a titanium target are R—(Ti)=Γ(μ^-Ti→e^-Ti)/Γ(μ^-Ti capture)<4.6×10^-12 and R₊(Ti)=Γ(μ^-Ti→e⁺Ca*)/Γ(μ^-Ti capture)<.1.7×10^-10 A smaller data set obtained using a lead target yielded R_-(Pb)<4.9×10^-10. The implications of these results for extensions of the standard model which allow lepton-flavor violation are discussed.

A search has been performed for the lepton-flavor-nonconserving reaction μ^-+Ti→e^-+Ti using a time-projection chamber. No candidate events were observed, resulting in a limit of Γ(μ^-+Ti→e^-+Ti) / Γ(μ^-+Ti→capture) <1.6×10^-11 (90% C.L.) for this reaction relative to ordinary muon capture.

The first measurement of pion double charge exchange at low energy is reported. The reaction ¹⁴C(π⁺,π^-)¹⁴O was measured at an incident pion energy of 50 MeV. Differential cross sections of the double analog transition are given in the angular range 50°-120°. Strong nonanalog transitions were also observed.

The inclusive pion inelastic scattering and true absorption cross sections at 50 MeV were measured for π⁺ on natural Li, C, Fe, Nb, and Bi and for π^- on C, Fe, and Bi. The results show that π^- cross sections are much larger than π⁺, the difference being significantly larger than expected from a simple Coulomb calculation. In particular, in ¹²C the absorption of negative pions is about twice that of positive pions.

NUCLEAR REACTIONS Li, C, Fe, Nb, Bi (π⁺, π^+′) and C, Fe, Bi (π^-, π^-′), E_π=50 MeV; measured σ(θ_π^′), transmission; deduced σ_absorption.

The cross section for true absorption of pions in nuclei was obtained from experiments at 85, 125, 165, 205, 245, and 315 MeV for positive pions and at 125 and 165 MeV for negative pions. The results are compared with theoretical calculations. The inclusive pion scattering angular distribution was also measured, and the results indicate that quasifree scattering plays an important role for backward scattering. The total pion-nucleus cross section is decomposed into its major channels: elastic scattering, inclusive inelastic scattering, true absorption, and single charge exchange, as a function of pion energy and charge. For light nuclei, the true absorption cross section has a strong energy dependence, reflecting the shape of the (3,3) resonance. The A dependence of the true absorption cross section is much stronger than that of the inclusive inelastic scattering.

NUCLEAR REACTIONS Li, C, Al, Fe, Nb, Bi (π^±,π^′); measured σ(θ_π^′), E_π⁺=85, 125, 165, 205, 245, and 315 MeV, E_π^-=125, 165 MeV; deduced σ_inelastic, σ_absorption; decomposition of σ_tot.

The cross section for true absorption of positive pions in carbon was obtained from experiments at 85, 125, 165, 205, and 245 MeV. The results are compared with theoretical calculations. The total π⁺-carbon cross section is decomposed into its major channels: elastic scattering, inclusive inelastic scattering, true absorption, and single charge exchange, as a function of pion energy. The inclusive π⁺ angular distribution was also measured and the results suggest that quasifree processes play an important role for backword scattering.

NUCLEAR REACTIONS ¹²C(π⁺, π^+′), (π⁺, absorption) E=85-245 MeV, measured σ(E, θ), deduced decomposition σ_react.

The cross section for absorption of 125-MeV positive and negative pions in Li, C, Al, Fe, Nb, and Bi was measured. The results were obtained by combining a transmission experiment, a measurement of the angular distribution for pion scattering, and an estimate of the charge-exchange cross section. Information about inelastic scattering and decomposition of the total pion-nucleus cross section into its contributions from the major channels is also obtained.

Angularly integrated cross sections for the reaction ⁷Li(π⁺, π⁰)⁷Be^* (429 keV) were measured over the energy range of 90 to 210 MeV by a selective "prompt-γ" detection method. These results, combined with the summed cross sections to the ground state and excited state, yield the cross section for the ground state alone. The excitation function for the excited state is flat with cross sections of about 0.3 mb. For the ground-state transition the cross sections are between 1.2 and 1.9 mb. The data are compared with theoretical calculations.