Neutral Particle Detector Research Articles

Recent results from ELSA

One of the foremost challenges in hadron physics is to obtain an understanding of the structure of baryons. The main features of the baryon mass spectrum at low excitation energies are reasonably known and described quite accurately by models, but the high-mass spectrum is still rather obscure. With modern achievements in accelerators, highly polarized nucleon targets and hermetically closed detectors allowing for charged and neutral particle detection high-lying excitations can be investigated through the systematic study of single and multiple meson production. In this contribution the instrumentation and results of experiments from the ELSA accelerator at Bonn as well as their significance for baryon resonance analyses are described. In particular, data on photoproduction of vector and pseudoscalar mesons of different flavors are presented.

Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan

The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager 1 spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling” and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge-energy-mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ~ 2.4 cm2 sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5° full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (~0.05 cm2 sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E < 0.884 MeV in the forward direction (G ~ 0.02 cm2 sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ~ 0.4 cm2 sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-a measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ~60 RS every 2–3 h (every ~10 min from ~20 RS). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be <0.1 s, which is more than adequate for microsignature studies. Data obtained during Venus-2 flyby and Earth swingby in June and August 1999, respectively, and Jupiter flyby in December 2000 to January 2001 show that the instrument is performing well, has made important and heretofore unobtainable measurements in interplanetary space at Jupiter, and will likely obtain high-quality data throughout each orbit of the Cassini mission at Saturn. Sample data from each of the three sensors during the August 18 Earth swingby are shown, including the first ENA image of part of the ring current obtained by an instrument specifically designed for this purpose. Similarily, measurements in cis-Jovian space include the first detailed charge state determination of Iogenic ions and several ENA images of that planet’s magnetosphere.

Diagnostics of the Local Interstellar Medium using particles and UV radiation

The chain of processes which govern trajectories and ionization of neutral particles from the circumsolar interstellar gas to the inner heliosphere can now be studied by combining several types of diagnostics: solar backscattered radiation (H and He), direct detection of neutral particles (He), detection of pickup ions of interstellar origin (H, He, C, N and O, etc.). Moreover, continuous observations of the Sun bring the required solar parameters to be included in the modeling and, in parallel, considerable efforts have been made in model developments. A good example of such improvements is the synthetic work done on interstellar helium, detailed by Möbius et al. in this issue. This is useful because helium is a tracer of the initial state of the interstellar gas in the Local Cloud and serves as a zero level for the other species. In particular, the velocity distribution of interstellar neutral hydrogen measured by SOHO/SWAN can be compared with helium characteristics, to quantify the modifications experienced by the H flow at entrance in the heliosphere. This provides a measurement of the interstellar circumsolar plasma pressure. Here we focus on three aspects: • the recent EUVE data on the helium 58.4 nm glow, which provide an independent measurement of the flow direction and velocity, and help to understand how helium temperature has been overestimated when analyzing past glow data. • the recent SOHO/UVCS data which bring new constraints on the ionization processes close to the Sun, namely the significant role of electron impact and a possible explanation for the observed correlation between H + and He + pickup fluxes. • an update of the hydrogen–helium differentiation at the heliospheric interface.

The CEBAF large acceptance spectrometer (CLAS)

Abstract The CEBAF large acceptance spectrometer (CLAS) is used to study photo- and electro-induced nuclear and hadronic reactions by providing efficient detection of neutral and charged particles over a good fraction of the full solid angle. A collaboration of about 30 institutions has designed, assembled, and commissioned CLAS in Hall B at the Thomas Jefferson National Accelerator Facility. The CLAS detector is based on a novel six-coil toroidal magnet which provides a largely azimuthal field distribution. Trajectory reconstruction using drift chambers results in a momentum resolution of 0.5% at forward angles. Cherenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data-acquisition rates allow operation at a luminosity of 10 34 nucleon cm −2 s −1 . These capabilities are being used in a broad experimental program to study the structure and interactions of mesons, nucleons, and nuclei using polarized and unpolarized electron and photon beams and targets. This paper is a comprehensive and general description of the design, construction and performance of CLAS.

Scattering of atoms and molecules off a magnesium oxide surface

Neutral particle imaging in the low energy range will, when employed in future spacecraft missions, potentially contribute to enlarging our knowledge about the origin and evolution of our universe. The main difficulty of a neutral particle detector in this energy range is that the incoming neutrals must first be ionized such that they can be mass- and energy-analyzed by conventional methods. In this paper we report on the first observation of the formation of negatively charged ions upon reflection from a magnesium oxide surface. Experiments were performed at two different setups, one of them including a time-of-flight measurement of the specularly reflected particles, the other one allows angle resolved detection. We measured high fractions (12–30%) of negative ions and less than 1% of positive ions when scattering O +, O 0, O 2 + and O 2 0 at 90–3000 eV primary energy per atom off a MgO surface. When scattering H +, H 2 + and H 2 0 at 300–1500 eV per atom off the MgO surface, 3–7% of the particles were converted into negative ions and 2–10% into positive ions and when scattering C +and C 0 at 1500–2000 eV per atom around 3% of the particles were converted into negative ions and 3% into positive ions. Our results strongly suggest a complete memory loss of the incident charge state. Scattered molecules dissociated always completely. The mean energy loss was approximately proportional to the incident particle energy except for O − ions where an increased loss at lower energy was observed. MgO therefore meets the most important requirements for application on a space platform.

Scattering of atoms and molecules off a barium zirconate surface

Neutral particle imaging in the low-energy range will, when employed in future spacecraft missions, potentially contribute to enlarging our knowledge about the origin and evolution of our universe. The main difficulty for a neutral particle detector in this energy range is that the incoming neutrals must first be ionized such that they can be mass- and energy-analyzed by conventional methods. In this paper, we report on the first observation of the formation of negatively charged ions upon reflection from a barium zirconate surface. Experiments were performed at two different setups, one of them including a time-of-flight (TOF) measurement of the specularly reflected particles, the other one allows position-resolved detection. We measured high fractions (22–35%) of negative ions and no positive ions when scattering O +, O 0, O 2 + and O 2 0 at 500–3000 eV primary energy per atom off a BaZrO 3 surface. When scattering H +, H 2 + and H 2 0 at 300–1500 eV per atom off the BaZrO 3 surface, 3–5% of the particles were converted into negative ions and 2–8% into positive ions. Our results suggest a complete memory loss of the incident charge state. Scattered molecules dissociated almost completely. The mean energy loss was proportional to the incident particle energy. BaZrO 3 therefore meets the most important requirements for application on a space platform.

Sputtering efficiency of LiF surfaces on impact of low energy neutral helium (20–80 eV): Calibration of the interstellar neutral helium instrument on ULYSSES

For the detection of interstellar neutral helium particles entering the inner solar system, a novel detection technique has been developed using sputtering of secondary ions from lithium-fluoride (LiF) surfaces on the impact of He particles. In order to be able to determine the interstellar He fluxes, the detection efficiency had to be determined in an energy range Ekin≈20 to 200 eV, an energy range for which data were not available in literature. Therefore, a calibration had to be performed in a dedicated accelerator system providing a neutral beam in the required energy range. While precise values could be obtained for energies above about 50 eV, significant uncertainties in the efficiency remained for lower energies, due to the difficulty in determining the absolute flux of the primary beam after acceleration, as elastic scattering of particles out of the beam geometry during the charge exchange process becomes more important. Corrections have been applied using a theoretical simulation of this process. These uncertainties have subsequently been eliminated, using the inflight measurements of neutral He during the ULYSSES mission. Due to the orbital parameters of this spacecraft, the relative energy of the interstellar He particles in the instrument’s reference system varies in the range 20–80 eV. Assuming that the interstellar He flow is stationary, i.e., it shows only negligible time variations, constant count rates would be expected over this energy range. The energy dependence in the actually observed count rates has been used to refine the efficiency calibration at low energies (⩽40 eV).

Time-of-flight neutral particle analyzer for Alcator C-Mod

A neutral particle analyzer, based on the time-of-flight technique is scheduled to be installed on the Alcator C-Mod tokamak. The instrument was originally designed and used in the ZT-40 experiment, and later, briefly used on the CTX experiment at Los Alamos National Laboratory. The design consists of a chopper wheel mounted on a turbomolecular pump, a ∼2 m long flight tube, a Cu–Be secondary electron emitting surface, and an electron multiplier, which together are used as the neutral particle detector. Among the changes introduced in the system from the original design, of particular interest is the location of the instrument. The instrument is mounted behind a magnetic-type neutral particle analyzer and shares the same line of sight. Both of them can be scanned, poloidally down to the X point and tangentially to R/R0=0.7. This will enable us to compare neutral flux through two different techniques with an overlap in energies from ∼0.5–4 keV, with the instrument capable of detecting neutral particles with energies as low as ∼20 eV. This should aid in characterizing neutral and ion behavior at the edge of the tokamak plasma, especially near the X point. We will also describe changes made to the controls for improved versatility and ease of operation.

The cathode read-out of the DELPHI hadron calorimeter

To improve the identification and separation of leptons in the Hadron Calorimeter of DELPHI, one of the four LEP experiments at CERN, the possibility of a direct read-out of the cathodes of the 20,000 limited streamer tubes was studied and successfully tested on a small scale. A larger scale test started in June 1994. This new system which is independent of the present pad read-out provides a ``yes/no`` information. The combination of both read-out systems makes it possible to use the Hadron Calorimeter as a track detector. The result of these test show that the cathode read-out provides a better {pi}/{micro} separation, and improved detection of neutral long lived particles, enhanced discrimination of two showers and a more precise hadron energy measurement. It was decided to equip the whole detector with the new read-out, starting during the 94/95 shutdown.

Investigation of a laser-produced atomic beam

The acceleration of a thin layer mounted on the rear surface of a thick transparent slab target was shown to be feasible with a laser of moderate energy. Both long (about 1 ns) and short (about 25 ps) laser pulse interactions were investigated. Using a single laser beam with 2 J in a 1 ns pulse it was possible to accelerate an Al layer of thickness <or=1 mu m up to a velocity of 2*106 cm s-1. Atomic beam velocity distributions were measured with a time-of-flight neutral particle detector and a time-gated micro-channel plate neutral particle pin-hole camera. The expanding atomic beam was confined to a very narrow cone, being directed along the normal to the target rear surface and independent of the laser beam axis. Time-resolved imaging of the target rear surface in different spectral bands provided measurements of its temperature and the characteristic scale-length of the atomic beam emission area.

Two-stage parallel-plate energy analyzer for simultaneous detection of positive, negative, and neutral particles

We describe a unique apparatus that simultaneously measures the laboratory energy and angles of several charged particles formed in a single beam. The ability to separate and detect particles of opposite polarity allows one to measure them in coincidence. Equations for the trajectories of the particles are derived and discussed in detail. The expressions for the corresponding resolution of the detected particles are also presented. Data produced in recent experiments illustrate some of the analyzer’s more important features.

Imaging neutral particle detector

Abstract With the support of NASA's Innovative Research Program (IRP), we undertook the definition and initial development of an imaging neutral particle detector, a new instrument for global imaging of planetary magnetospheres. This instrument carries out remote sensing of magnetospheric plasmas by imaging and analyzing charge exchange neutrals, which are energetic neutral atoms (ENAs) created by charge exchange reactions between ambient neutrals and energetic ions. The imaging neutral particle detector can determine the composition of the charge exchange neutrals and therefore also the composition of the original hot plasma. Such an instrument, the Ion‐Neutral Camera of the Magnetospheric Imaging Instrument (MIMI), has been selected for flight on the NASA/ESA Cassini mission to Saturn and Titan, and a similar instrument is part of the baseline payload for the proposed Inner Magnetosphere Imager (IMI) mission to carry out remote sensing of Earth's magnetosphere. Remote sensing of charge exchange neutrals...

A secondary emission detector capable of preventing detection of the photoelectric effect induced by pulsed lasers

A secondary emission detector for neutral particles has been constructed. The detector is designed for an experiment to investigate the photodetachment process X-+h nu to X+e- where X- can be any negative ion. A beam of negatively charged ions is merged with a laser beam, and the rate of the photodetachment process is measured by monitoring the neutral atom production. The neutral atoms impinge on a coated glass plate where secondary electrons are formed which are detected with a channel electron multiplier. The laser beam strikes the glass plate at the same point as the neutral atoms do and the intense laser light produces electrons by the photoelectric effect. This gives rise to an unwanted signal which is much stronger than the desired signal induced by the neutral atoms. The neutral particle detector described is capable of preventing the detection of electrons produced by the photoelectric effect. It is shown that the detection of electrons produced by the light from pulsed lasers with pulse energies of up to 10 mJ and wavelengths down to 250 nm can be completely prevented without any decrease in the yield of the electrons produced by the neutral atoms. The detection efficiency of neutral atoms with an energy of 3 keV is shown to be 40%.

DAPHNE: a large-acceptance tracking detector for the study of photoreactions at intermediate energies

A large-acceptance (94% of 4π sr) hadron detector capable of handling multiparticle final states is described. The track reconstruction capability, energy resolution, particle identification capability and neutral-particle detection efficiency of the detector are discussed and the results of tests shown. Tests have been performed both with cosmic rays and in a realistic experimental situation using a 500 MeV photon beam impinging on hydrogen and deuterium targets.

Determination of the imploded mass in laser-imploded shell targets

A new method of experimental measurement of the imploded mass of laser-imploded shell targets is proposed. This method is based on simultaneous measurements of the velocity spectrum of the neutral atom flux emitted from the imploded part of the target, and the total energy carried by them. A semiconductor diode with a shallowly located p – n junction was used as the low energy neutral particle detector. By means of the time of flight method neutral atom fluxes with velocities υ ∼ (0·6 − 1·8) × 107cm/s were registered in experiments carried out on the 6-beam laser system “Delfin”.

Low-energy neutral-particle detection in the Mont Blanc LSD experiment

In this work we analyse the sources of background that can imitate a real antineutrino interaction in the Mont Blanc LSD detector. The sources are: fast neutrons from nuclear showers produced by muons in the rock, spontaneous fission of238U and the decay of products of thermal neutron capture by nuclei in the apparatus or in the surrounding rock.

A new method to determine the mean adsorption lifetime of alkali metal halide molecules on a hot metal surface

The emission current of Li + ions produced from LiI molecules incident upon a polycrystalline rhenium surface heated to 1157 K at a residual gas pressure of ∼ 2×10 −7 torr was measured as a function of elapsed time after opening or closing a sample beam shutter. Within ∼6 s after stopping a beam incidence (6.66×10 13 molecules cm −2 s −1), the Li + current increased by 3.86 times and then decreased to zero. This change was analyzed using our original theory on thermal ionization, thereby leading to the conclusions: (1) the increase is caused by an increase in work function up to 5.99 eV according to a decrease in surface density of LiI (neither Li nor Li +), (2) the percentages of those particles (Li +, Li, and LiI) emitted during the 6 s are 36.6, 2.1 and 61.3%, respectively, and (3) the mean adsorption lifetime of LiI on the surface covered with residual gas molecules is 3.23 s, which is much longer than that of Li + (<0.2 s). This new and simple method does not require the detection of emitted neutral particles (LiI and/or Li) and may be readily applicable to other sample-molecule/metal-surface systems under such an experimental condition that the emission current is appreciably increased after closing the shutter.

Radiative signal for light neutral supersymmetric particle production in e +e − annihilation

The process e +e − → γ + neutrals is analyzed as a means for the detection of light supersymmetric neutral particles.

Monte Carlo Calculation on Neutral Gas Transport and Impurity Production in a Toroidal Plasma

The spatial distribution of the neutral gas and the energy distribution of escaped neutral particles in a toroidal plasma are investigated using the Monte Carlo technique. These measurements are important in estimating the charge exchange loss of the fast ions produced in neutral beam injection heating, for data analysis in neutral particle detection, etc. Taking a fairly large value for the major radius of the torus, a comparison with other calculations in an infinite cylinder is carried out and satisfactory agreement is obtained within the range of statistical error. A method is also presented, for estimating the influx of impurities due to sputtering from the wall surface by escaped neutral atoms.

X-ray and energetic neutral particle emission from Saturn's magnetosphere

A description is presented of observations of the Voyager 1 spacecraft obtained a few days before encounter of the Saturn magnetosphere in November 1980. As Voyager 1 has no special experiment onboard for X-ray and neutral particle detection, the sensitivity and directional measurements of the low energy charged particle (LECP) analyzer is used to identify the neutral radiation above the charged particle background. The LECP telescope uses a silicon detector of 96.6 micrometers thickness and 0.08 sq cm area to accumulate counts in eight separate sectors near the ecliptic plane. The existence of radiation above the detector background emanating from the vicinity of Saturn, upstream from the bow shock is shown. The excess counts are most plausibly attributed to neutral radiation, that is, X rays and/or neutral energetic particles. However, in connection with quantitative considerations the observed excess counts are probably not due to X-rays. It is concluded that charge exchange of energetic ions with satellite tori is an important loss mechanism at Saturn as well as at Jupiter.