The performance of a muon radiography system designed to image the inner structure of a nuclear plant located at a distance of 64 m is evaluated. We conclude that there is an absence of fuel in the ...pressure vessel during the measurement period and profile the fuel material placed in the storage pool. The obtained data also demonstrate the sensitivity of the system to water-level changes in the reactor well and the dryer-separator pool. It is expected that the system could easily reconstruct a 2 m cubic fuel object. By operating multiple systems, typically four identical systems, viewing the reactor from different directions simultaneously, detection of a 1 m cubic object should also be achievable within a period of a few months.
A beam duct coated with NEG materials (Ti, Zr, V), which had been known to have a low secondary electron yield (SEY), was studied for the first time under intense photon irradiation using a positron ...beam at the KEK B-Factory (KEKB) to investigate a way to suppress the electron cloud instability (ECI). A 2.56
m test copper chamber was coated with the NEG materials (we call it NEG coating here) by magnetron sputtering. It was installed at an arc section of the KEKB positron ring, where the chamber was irradiated by direct photons with a line density of 6.5×10
14
photons
m
−1
s
−1
mA
−1. The vacuum pressure around the test chamber during a usual beam operation was lower than the case of non-coated copper chambers by a factor of 4–5. The number of electrons around positron bunches was measured by a special electron monitor up to a stored beam current of 1600
mA. The measured electron current, however, was almost the same as a non-coated copper chamber, especially at low-beam currents, and the effect of the NEG coating was smaller than expected. A simulation explained the result that abundant photoelectrons in the positron ring reduce the effect of the low SEY. The maximum SEYs of the NEG coating and non-coated copper were evaluated using a simulation as about 0.9–1.0 and 1.1–1.3, respectively, which were consistent with the values after a sufficient electron bombardment. Their photoelectron yields were also estimated as 0.22–0.28 and 0.26–0.34, respectively, and were in good agreement with the previous experimental results. The study indicates that the suppression of photoelectrons, by a beam duct with an antechamber, for example, is indispensable to make effective use of a surface with a low SEY, such as the NEG coating.
A beam duct with an antechamber scheme for high-current accelerators was designed and the test chambers were studied experimentally. The duct consists of two channels, i.e., a beam channel where a ...beam circulates and a Synchrotron Radiation (SR) channel (antechamber) aside where the SR passes through. By using the antechamber scheme, the maximum power density of SR can be diluted at the side wall. The impedance is small owing to the pumping ports not being at the beam channel, but at the SR channel. Photoelectrons inside the beam channel are also expected to be reduced, which would be a big merit for a positron ring to suppress the electron cloud effect since the photoelectron is a major source of electrons composing the cloud. Two copper test chambers were manufactured with different methods, by pressing and by drawing. These chambers showed a good static vacuum property, i.e., gas desorption rates with less than
3.5
×
10
-
9
Pa
m
3
s
-
1
m
-
2
after baking. After the installation to the positron ring of the KEK B-factory (KEKB), electron numbers in the beam channel, temperatures and pressures were measured during beam operation. The electrons in the beam channel were found to be reduced by a factor of 4 at 1.5
A compared to the case of the usual circular chamber. The reduction, however, was much larger, about 1/300, at a beam current of about 20
mA where the photoelectrons were dominant and the multiplication of electrons by the multipactoring was small. The temperatures were almost in agreement with the expectation. Vacuum scrubbing by photons proceeded almost smoothly, although pressure bursts were sometimes observed, especially for one test chamber, which was possibly due to discharges at the transverse joints in the beam chamber. Various instructive information had been obtained for future practical beam ducts for high-intensity accelerators.
A hybrid photodetector (HPD) consisting of a photocathode and a multipixel avalanche diode (MP-AD) was developed a few years ago. Our previous studies showed that its inherent potential for high ...resolution photon counting could be further enhanced by reducing fluctuations in charge loss in the dead layer at the entrance of the MP-AD. In this paper, we report on the improvement with the newly developed HPD whose encapsulated MP-AD has a thinner dead layer than before. It is demonstrated that the new HPD has much better energy resolution, which enables clearer counting up to nine photoelectrons. Further enhancement of the photocathode sensitivity of the HPD is also discussed.
Study of τ−→KSπ−ντ decay at Belle Epifanov, D.; Aihara, H.; Aulchenko, V. ...
Physics letters. B,
10/2007, Volume:
654, Issue:
3-4
Journal Article
Peer reviewed
We present a study of the decay τ−→KSπ−ντ using a 351 fb−1 data sample collected with the Belle detector. The analysis is based on 53,110 lepton-tagged signal events. The measured branching fraction ...B(τ−→KSπ−ντ)=(0.404±0.002(stat.)±0.013(syst.))% is consistent with the world average value and has better accuracy. An analysis of the KSπ− invariant mass spectrum reveals contributions from the K∗(892)− as well as other states. For the first time the K∗(892)− mass and width have been measured in τ decay: M(K∗(892)−)=(895.47±0.20(stat.)±0.44(syst.)±0.59(mod.)) MeV/c2, Γ(K∗(892)−)=(46.2±0.6(stat.)±1.0(syst.)±0.7(mod.)) MeV. The K∗(892)− mass is significantly different from the current world average value.
We report on a search for the lepton flavor violating τ−→μ−γ and τ−→e−γ decays based on 535 fb−1 of data accumulated at the Belle experiment. No signal is found and we set 90% confidence level upper ...limits on the branching ratios B(τ−→μ−γ)<4.5×10−8 and B(τ−→e−γ)<1.2×10−7.
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