1,357 research outputs found
An adjustable focusing system for a 2 MeV H- ion beam line based on permanent magnet quadrupoles
A compact adjustable focusing system for a 2 MeV H- RFQ Linac is designed,
constructed and tested based on four permanent magnet quadrupoles (PMQ). A PMQ
model is realised using finite element simulations, providing an integrated
field gradient of 2.35 T with a maximal field gradient of 57 T/m. A prototype
is constructed and the magnetic field is measured, demonstrating good agreement
with the simulation. Particle track simulations provide initial values for the
quadrupole positions. Accordingly, four PMQs are constructed and assembled on
the beam line, their positions are then tuned to obtain a minimal beam spot
size of (1.2 x 2.2) mm^2 on target. This paper describes an adjustable PMQ beam
line for an external ion beam. The novel compact design based on commercially
available NdFeB magnets allows high flexibility for ion beam applications.Comment: published in JINST (4th Feb 2013
Critical Field of MGB2 : Crossover from Clean to Dirty Regimes
We have studied the upper critical field, Bc2, in poly-crystalline MgB2
samples in which disorder was varied in a controlled way to carry selectively p
and s bands from clean to dirty limit. We have found that the clean regime
survives when p bands are dirty and s bands are midway between clean and dirty.
In this framework we can explain the anomalous behaviour of Al doped samples,
in which Bc2 decreases as doping increases.Comment: 11 pages, 2 figure
Neutron Irradiation of Mg11B2 : From the Enhancement to the Suppression of Superconducting Properties
In this letter we present the effect of neutron irradiation up to fluences of
3.9 1019 n/cm2 on the superconducting properties of MgB2. In order to obtain a
disorder structure homogeneously distributed, the experiment was carried out on
bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 the
critical temperature is slightly diminished (36 K) and the superconducting
properties are significantly improved; the upper critical field is increased
from 13.5 T to 20.3 T at 12 K and the irreversibility field is doubled at 5 K.
For larger neutron fluences the critical temperature is suppressed down to 12 K
and the superconducting properties come out strongly degraded.Comment: 13 pages, 4 figures. Submitted to Appl.Phys.Let
A new application of emulsions to measure the gravitational force on antihydrogen
We propose to build and operate a detector based on the emulsion film
technology for the measurement of the gravitational acceleration on antimatter,
to be performed by the AEgIS experiment (AD6) at CERN. The goal of AEgIS is to
test the weak equivalence principle with a precision of 1% on the gravitational
acceleration g by measuring the vertical position of the anni- hilation vertex
of antihydrogen atoms after their free fall in a horizontal vacuum pipe. With
the emulsion technology developed at the University of Bern we propose to
improve the performance of AEgIS by exploiting the superior position resolution
of emulsion films over other particle de- tectors. The idea is to use a new
type of emulsion films, especially developed for applications in vacuum, to
yield a spatial resolution of the order of one micron in the measurement of the
sag of the antihydrogen atoms in the gravitational field. This is an order of
magnitude better than what was planned in the original AEgIS proposal.Comment: 17 pages, 14 figure
A beam monitor detector based on doped silica and optical fibres
A beam monitor detector prototype based on doped silica fibres coupled to
optical fibres has been designed, constructed and tested, mainly for
accelerators used in medical applications. Scintillation light produced by Ce
and Sb doped silica fibres moving across the beam has been measured, giving
information on beam position, shape and intensity. Mostly based on commercial
components, the detector is easy to install, to operate and no electronic
components are located near the beam. Tests have been performed with a 2 MeV
proton pulsed beam at an average current of 0.8 {\mu}A. The response
characteristics of Sb doped silica fibres have been studied for the first time
Enhanced flux pinning in neutron irradiated MgB2
We study the effect of neutron irradiation on the critical current density Jc
of isotopically pure polycrystalline Mg11B2 samples. For fluences in the range
1017-1018 cm-2, Jc is enhanced and its dependence on magnetic field is
significantly improved: we demonstrate that, in this regime, point-like pinning
centers are effectively introduced in the system proportionally to the neutron
fluence. Instead, for larger fluences, a strong suppression of the critical
temperature accompanied by a decrease of both the upper critical field Bc2 and
Jc is found.Comment: 13 pages, 3 igure
First results on proton radiography with nuclear emulsion detectors
We propose an innovative method for proton radiography based on nuclear
emulsion film detectors, a technique in which images are obtained by measuring
the position and the residual range of protons passing through the patient's
body. For this purpose, nuclear emulsion films interleaved with tissue
equivalent absorbers can be used to reconstruct proton tracks with very high
accuracy. This is performed through a fully automated scanning procedure
employing optical microscopy, routinely used in neutrino physics experiments.
Proton radiography can be used in proton therapy to obtain direct information
on the average tissue density for treatment planning optimization and to
perform imaging with very low dose to the patient. The first prototype of a
nuclear emulsion based detector has been conceived, constructed and tested with
a therapeutic proton beam. The first promising experimental results have been
obtained by imaging simple phantoms.Comment: Submitted to Journal of Instrumentation (JINST
Thermal conductivity of MgB in the superconducting state
We present thermal conductivity measurements on very pure and dense bulk
samples, as indicated by residual resistivity values as low as 0.5 mW cm and
thermal conductivity values higher than 200 W/mK. In the normal state we found
that the Wiedemann Franz law, in its generalized form, works well suggesting
that phonons do not contribute to the heat transport. The thermal conductivity
in the superconducting state has been analysed by using a two-gap model. Thank
to the large gap anisotropy we were able to evaluate quantitatively intraband
scattering relaxation times of and bands, which depend on the
disorder in different way; namely, as the disorder increases, it reduces more
effectively the relaxation times of than of bands, as
suggested by a recent calculation [1].Comment: 12 pages, 5 figure
Efeitos gĂȘnicos, heterose e depressĂŁo endogamica em caracteres de sorgo forrageiro.
Dois cruzamentos envolvendo tres linhagens de sorgo (Sorghum L. Moench); BR 007A x BR 501R e BR 008A x BR 501R originaram dois hibridos forrageiros BR 601 e BR 601. Estes, em conjunto com suas geracoes avancadas por autofecundacao F2 e F3, constituindo nove genotipos, foram avaliados em duas epocas de plantio (normal e sucessao), em diversos caracteres de rendimento e qualidade de forragem, obtendo-se as estimativas dos efeitos genicos, heterose media, heterobeltiose e depressao endogamica nas geracoes F2 e F3. Os resultados permitiram as seguintes conclusoes: (a) no plantio em sucessao, comparativamente ao normal, os hibridos forrageiros apresentaram menor altura de planta, florescimento precoce, reducao na materia verde e no rendimento de massa seca e maior porcentagem proteica, alem de maior proporcao do colmo e menor de panicula; (b) para os dois hibridos forrageiros, as estimativas de heterose foram positivas para rendimento de massa seca e negativas para proteina, nas duas epocas de plantio; (c) nao houve, em geral, depressao endogamica, com excecao da alteracao do rendimento de graos, na geracao F2 do hibrido BR 602, no plantio normal, inviabilizado a utilizacao comercial de sementes F2s; (d) os efeitos aditivos predominaram para a maioria dos caracteres, no cruzamento forrageiro BR 007A x BR 501R, ao passo que os efeitos aditivos e dominantes foram igualmente importantes para a determinacao dos caracteres de rendimento, para o cruzamento forrageiro BR 008A x BR 501R; (e) os desvios de dominancia explicaram as heteroses significativas de varios caracteres
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