The present paper deals with sequential designs intended to balance the allocations of two competing treatments in the presence of prognostic factors. After giving a theoretical framework on the ...optimality of balanced designs that can arise when covariates are taken into account, we propose a new family of covariate-adaptive randomized designs that represents higher order approximation to balance treatments, both globally and also across covariates. We derive the theoretical properties of the suggested designs in terms of loss of precision and predictability. The performance of this proposal is illustrated through a simulation study and compared with those of other procedures suggested in the literature.
We demonstrate efficient transverse compression of a 12.5 MeV/c muon beam stopped in a helium gas target featuring a vertical density gradient and crossed electric and magnetic fields. The muon stop ...distribution extending vertically over 14 mm was reduced to a 0.25 mm size (rms) within 3.5 μs. The simulation including cross sections for low-energy μ+-He elastic and charge exchange (μ+↔ muonium) collisions describes the measurements well. By combining the transverse compression stage with a previously demonstrated longitudinal compression stage, we can improve the phase space density of a μ+ beam by a factor of 1010 with 10−3 efficiency.
A novel device to compress the phase space of a muon beam by a factor of
10
10
with a
10
-
3
efficiency is under development. A surface muon beam is stopped in a helium gas target consisting of ...several compression stages, wherein strong electric and magnetic fields are applied. The spatial extent of the stopped muon swarm is decreased by means of these fields until muons with eV energy are extracted into vacuum through a small orifice. It was observed that a 20 cm long muon stop distribution can be compressed in the longitudinal direction to a sub-mm extent within
2
μ
s
. Additionally, a drift perpendicular to the magnetic field of the compressed low-energy muon swarm was successfully demonstrated, paving the way towards extraction from the gas and re-acceleration of the muons.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The high neutron detection efficiency, good gamma-ray discrimination and non-toxicity of
3
He made of proportional counters filled with this gas the obvious choice for neutron detection, ...particularly in radiation portal monitors (RPM), used to control the illicit transport of nuclear material, of which neutron detectors are key components.
3
He is very rare and during the last decade this gas has become increasingly difficult to acquire. With the exception of BF
3
, which is toxic, no other gas can be used for neutron detection in proportional counters. We present an alternative where the
3
He atoms are replaced by nanoparticles made of another neutron sensitive material,
10
B. The particles are dispersed in a gaseous volume, forming an aerosol with neutron sensitive properties. A proportional counter filled with such aerosol was exposed to a thermal neutron beam and the recorded response indicates that the neutrons have interacted with the particles in the aerosol. This original technique, which transforms a standard proportional gas mixture into a neutron sensitive aerosol, is a breakthrough in the field of radiation detection and has the potential to become an alternative to the use of
3
He in proportional counters.
The high neutron detection efficiency, good gamma-ray discrimination and non-toxicity of
He made of proportional counters filled with this gas the obvious choice for neutron detection, particularly ...in radiation portal monitors (RPM), used to control the illicit transport of nuclear material, of which neutron detectors are key components.
He is very rare and during the last decade this gas has become increasingly difficult to acquire. With the exception of BF
, which is toxic, no other gas can be used for neutron detection in proportional counters. We present an alternative where the
He atoms are replaced by nanoparticles made of another neutron sensitive material,
B. The particles are dispersed in a gaseous volume, forming an aerosol with neutron sensitive properties. A proportional counter filled with such aerosol was exposed to a thermal neutron beam and the recorded response indicates that the neutrons have interacted with the particles in the aerosol. This original technique, which transforms a standard proportional gas mixture into a neutron sensitive aerosol, is a breakthrough in the field of radiation detection and has the potential to become an alternative to the use of
He in proportional counters.
Muonic atom spectroscopy–the measurement of the x rays emitted during the formation process of a muonic atom–has a long standing history in probing the shape and size of nuclei. In fact, almost all ...stable elements have been subject to muonic atom spectroscopy measurements and the absolute charge radii extracted from these measurements typically offer the highest accuracy available. However, so far only targets of at least a few hundred milligram could be used as it required to stop a muon beam directly in the target to form the muonic atom. We have developed a new method relying on repeated transfer reactions taking place inside a 100 bar hydrogen gas cell with an admixture of 0.25% deuterium that allows us to drastically reduce the amount of target material needed while still offering an adequate efficiency. Detailed simulations of the transfer reactions match the measured data, suggesting good understanding of the processes taking place inside the gas mixture. As a proof of principle we demonstrate the method with a measurement of the 2
p
-1
s
muonic x rays from a 5
μ
g
gold target.
A new technique for detection of slow neutrons with gaseousdetectors using ultra-thin layers with 10B atoms ispresented. The reaction between a thermal neutron and a 10Batom releases two secondary ...particles, namely a 7Li ion and analpha particle, which due to momentum conservation are emitted inopposite directions, along the same line (back to back). Currentboron coated neutron detectors are equipped with 10B films withthicknesses of several micrometers, deposited on very thicksubstrate plates. However, since the ranges of the 7Li ion andthe alpha particle are of few micrometeres in most materials, one ofthese particles is always lost in the 10B layer orsubstrate. As such, these detectors lose the ability to reconstructthe reaction line of action and to precisely determine the neutronposition, as only one of the two secondary particles tracks can bemeasured. With the technique now presented, the sum of the 10Blayer and the substrate thicknesses is small enough to allow forboth secondary particles to escape and ionize the gas in oppositesides of the 10B converter foil. Independent readoutstructures, one on each side of the 10B converter foil, detecteach secondary particle and determine its track centroid and thedeposited energy. Since the two secondary particles are emitted backto back, the neutron position can be obtained by combining theinformation recorded by the two readout structures. Through GEANT4simulations, we verified that the spatial resolution can besignificantly improved: our results show that, by using a B4Clayer with a thickness of 1 μm on a 0.9 μm Mylarsubstrate, the spatial resolution can by improved by a factor ofeight, compared to conventional detectors with thick 10Bdetection layers.
Precision spectroscopy of light muonic atoms provides unique information about the atomic and nuclear structure of these systems and thus represents a way to access fundamental interactions, ...properties and constants. One application comprises the determination of absolute nuclear charge radii with unprecedented accuracy from measurements of the 2S - 2P Lamb shift. Here, we review recent results of nuclear charge radii extracted from muonic hydrogen and helium spectroscopy and present experiment proposals to access light muonic atoms with Z ≥ 3. In addition, our approaches towards a precise measurement of the Zemach radii in muonic hydrogen (μp) and helium (μ3He+) are discussed. These results will provide new tests of bound-state quantum-electrodynamics in hydrogen-like systems and can be used as benchmarks for nuclear structure theories.
The leak rate of helium gas through thin Kapton HN foils was measured for various temperatures between 310 and 150K and gas pressures ranging from 10 to 300mbar. From these measurements the ...permeability constant P(T) and its temperature dependence were determined. At room temperature the permeability constant for Kapton HN is P(T=296K)=(2.56±0.31) barrer. The temperature dependence of P(T) was verified to drop exponentially with decreasing temperature, causing a change in the permeability by more than two orders of magnitude in the range between 310 and 150K.
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