477 research outputs found
Dynamical Spectroscopy of Transient He2 Molecules
The velocities of atoms can be manipulated by near-resonant laser light. After many absorption-spontaneous emission cycles momentum is transferred to the atoms in the direction of the laser beam. In this way atoms can be slowed down and cooled to milliKelvin temperatures and below. In three mutually orthogonal pairs of counter-propagating laser beams in combination with a magnetic field atoms can be cooled and trapped. In this so-called magneto-optical trap (MOT) two atoms can be excited to a
molecular state by absorption of a photon. This technique is called photoassociation spectroscopy. In our case we study metastable helium atoms, which have a large internal energy. When two metastable helium
atoms approach each other, one of the atoms ionizes and the ions are detected by a microchannelplates detector. When a probe laser beam is directed onto the MOT cloud and scanned in frequency, atom pairs can
be excited to several ro-vibrational states. In the excited state the metastable helium atoms are accelerated towards each other and ionize at short internuclear distance. The ro-vibrational states appear as peaks in
the ionization signal and from the position of these states information on the excited state interaction potential can be extracted
Photo induced collisions with laser cooled He* atoms
This paper presents an experimental investigation of cold collisions between metastable Helium
atoms in an optical trap at 1mK. Penning (PI) and associative (AI) ionization reactions are distinguished
using a mass spectrometer and studied under influence of near resonant laser light. Sensitive behavior of the
ion rate is observed when the laser is tuned close to resonance. Experimental ?ndings are well described,
on an absolute scale, by a semi-classical model we have developed for optical collisions and by a modi?ed
Julienne-Vigu´e model
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Rotationally induced Penning ionization of ultracold photoassociated helium dimers
We have studied photoassociation of metastable \tripS helium atoms near the
\tripS-\tripP asymptote by both ion detection in a magneto-optical trap and
trap-loss measurements in a magnetic trap. A detailed comparison between the
results of the two experiments gives insight into the mechanism of the Penning
ionization process. We have identified four series of resonances corresponding
to vibrational molecular levels belonging to different rotational states in two
potentials. The corresponding spin states become quasi-purely quintet at small
interatomic distance, and Penning ionization is inhibited by spin conservation
rules. Only a weak rotational coupling is responsible for the contamination by
singlet spin states leading to a detectable ion signal. However, for one of
these series Bose statistics does not enable the rotational coupling and the
series detected through trap-loss does not give rise to sufficient ionization
for detection.Comment: 7 pages, 4 figures, submitted to EuroPhysics Letter
On the Role of Penning Ionization in Photoassociation Spectroscopy
We study the role of Penning ionization on the photoassociation spectra of
He(^3S)-He(^3S). The experimental setup is discussed and experimental results
for different intensities of the probe laser are shown. For modelling the
experimental results we consider coupled-channel calculations of the crossing
of the ground state with the excited state at the Condon point. The
coupled-channel calculations are first applied to model systems, where we
consider two coupled channels without ionization, two coupled channels with
ionization, and three coupled channels, for which only one of the excited
states is ionizing. Finally, coupled-channel calculations are applied to
photoassociation of He(^3S)-He(^3S) and good agreement is obtained between the
model and the experimental results.Comment: 14 pages, 18 figures, submitted to the special issue on Cold
Molecules of J. Phys.
Frequency shifts of photoassociative spectra of ultracold metastable Helium atoms : a new measurement of the s-wave scattering length
We observe light-induced frequency shifts in one-color photoassociative
spectra of magnetically trapped He atoms in the metastable
state. A pair of ultracold spin-polarized helium atoms is excited into
a molecular bound state in the purely long range potential connected to
the asymptote. The shift arises from the optical coupling of
the molecular excited bound state with the scattering states and the bound
states of two colliding atoms. We measure the frequency-shifts for
several ro-vibrational levels in the potential and find a linear
dependence on the photoassociation laser intensity. Comparison with a
theoretical analysis provides a good indication for the s-wave scattering
length of the quintet () potential, nm, which
is significantly lower than most previous results obtained by non-spectroscopic
methods.Comment: 7 pages, 4 figure
Development of novel low-mass module concepts based on MALTA monolithic pixel sensors
The MALTA CMOS monolithic silicon pixel sensors has been developed in the
Tower 180 nm CMOS imaging process. It includes an asynchronous readout scheme
and complies with the ATLAS inner tracker requirements for the HL-LHC. Several
4-chip MALTA modules have been built using Al wedge wire bonding to demonstrate
the direct transfer of data from chip-to-chip and to read out the data of the
entire module via one chip only. Novel technologies such as Anisotropic
Conductive Films (ACF) and nanowires have been investigated to build a compact
module. A lightweight flex with 17 {\mu}m trace spacing has been designed,
allowing compact packaging with a direct attachment of the chip connection pads
to the flex using these interconnection technologies. This contribution shows
the current state of our work towards a flexible, low material, dense and
reliable packaging and modularization of pixel detectors.Comment: 5 pages + 1 page references,8 figure
MALTA monolithic pixel sensors in TowerJazz 180 nm technology
Depleted Monolithic Active Pixel Sensors are of highest interest at the HL-LHC and beyond for the replacement of the Pixel trackers in the outermost layers of experiments where the requirement on total area and cost effectiveness is much bigger. They aim to provide high granularity and low material budget over large surfaces with ease of integration. Our research focuses on MALTA, a radiation hard DMAPS with small collection electrode designed in TowerJazz 180 nm CMOS imaging technology and asynchronous read-out. Latest prototypes are radiation hard up to 2 × 1015 1 MeV neq/cm2 with a time resolution better than 2 ns
Timing performance of radiation hard MALTA monolithic Pixel sensors
The MALTA family of Depleted Monolithic Active Pixel Sensor (DMAPS) produced
in Tower 180 nm CMOS technology targets radiation hard applications for the
HL-LHC and beyond. Several process modifications and front-end improvements
have resulted in radiation hardness up to and time resolution below 2 ns,
with uniform charge collection efficiency across the Pixel of size with a electrode size. The MALTA2
demonstrator produced in 2021 on high-resistivity epitaxial silicon and on
Czochralski substrates implements a new cascoded front-end that reduces the RTS
noise and has a higher gain. This contribution shows results from MALTA2 on
timing resolution at the nanosecond level from the CERN SPS test-beam campaign
of 2021.Comment: 8 pages, 8 figures. Submitted to Journal of Instrumentation (JINST).
Proceedings of the 23rd International Workshop on Radiation Imaging Detectors
IWORID 202
Performance of the MALTA Telescope
MALTA is part of the Depleted Monolithic Active Pixel sensors designed in
Tower 180nm CMOS imaging technology. A custom telescope with six MALTA planes
has been developed for test beam campaigns at SPS, CERN, with the ability to
host several devices under test. The telescope system has a dedicated custom
readout, online monitoring integrated into DAQ with realtime hit map, time
distribution and event hit multiplicity. It hosts a dedicated fully
configurable trigger system enabling to trigger on coincidence between
telescope planes and timing reference from a scintillator. The excellent time
resolution performance allows for fast track reconstruction, due to the
possibility to retain a low hit multiplicity per event which reduces the
combinatorics. This paper reviews the architecture of the system and its
performance during the 2021 and 2022 test beam campaign at the SPS North Area
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