Parallel Plate Avalanche Counter Research Articles

Development of a parallel-plate avalanche counter to perform conversion electron Mössbauer spectroscopy at low temperatures

We describe an experimental setup based on a sealed parallel-plate avalanche counter, which allows conversion electron Mössbauer spectroscopy at temperatures down to 120 K. A study of the counting gas performances is carried out in order to find out the appropriate operating conditions. The detector can work in a wide range of pressures and voltages at room and low temperatures, for both iron- and tin-based Mössbauer transitions. An estimation of the gas gain is obtained by using a simple approach, which considers the avalanche gas process in parallel-plate geometry. The gas gain as a function of the filling pressure shows a maximum at room temperature and a monotonic increase at low temperature. The experimental setup has been tested with the determination of the Debye temperature of beta-Sn, SnO(2), and epsilon-FeSi.

Status and outlook of the neutron time-of-flight facility n_TOF at CERN

The neutron time-of-flight facility n_TOF at CERN, fully operational since 2002, combines a high instantaneous neutron flux with high energy resolution. The wide energy range and the high neutron flux per time-of-flight burst result in a much enhanced signal to background ratio for neutron capture of radioactive isotopes and makes this facility well suited for the measurement of high quality neutron-induced reaction cross-sections.Neutrons are created by spallation reactions induced by a pulsed 20GeV/c proton beam impinging on a lead target. A 5cm water slab surrounding the lead target serves as a coolant and at the same time as a moderator of the spallation neutron spectrum, providing a wide energy spectrum from 0.1eV to about 250MeV.By the end of 2005, a first phase of data taking has been successfully terminated. Fission and capture experiments have been performed on a variety of isotopes of interest for nuclear astrophysics, advanced nuclear technologies and for basic nuclear physics.The instrumentation developed for this facility consists of parallel plate avalanche counter and fission ionization chamber detectors for the fission experiments and of low mass C6D6 detectors and a 4pi BaF2 total absorption calorimeter for capture experiments. A new data acquisition system, based on sampling of the detector signals, has been developed to cope with the high count rates and to minimize the effective dead time to only a few tens of nanosecond.A second phase of data taking is planned to start in 2007, after an upgrade of the spallation target. On the longer term, the construction of a flight path at 20m resulting in an increased neutron flux of about a factor of 100 opens new possibilities.

Simultaneous measurement of (n, γ) and (n, fission) cross sections with the DANCE 4π BaF2 array

We have recently begun a program of high precision measurements of the key production and destruction reactions of important radiochemical diagnostic isotopes, including several isotopes of uranium, plutonium and americium. The detector for advanced neutron capture experiments (DANCE), a 4π BaF2 array located at the Los Alamos Neutron Science Center, will be used to measure the neutron capture cross sections for most of the isotopes of interest. However, neutron capture measurements on many of the actinides are complicated by the presence of prompt γ-rays arising from low energy neutron-induced fission, which competes with neutron capture to varying degrees. Previous measurements of 235U using the DANCE array have shown that we can partially resolve capture from fission events based on total γ-ray calorimetry (i.e. total γ-ray energy versus γ-ray multiplicity). The addition of a dedicated fission-tagging detector to the DANCE array has greatly improved our ability to separate these two competing processes. In addition to higher quality neutron capture data, the addition of a fission-tagging detector offers a means to determine the capture-to-fission ratio (σγ/σf) in a single measurement, which should reduce the effect of systematic uncertainties. We are currently using a dual parallel-plate avalanche counter (PPAC) with the target material electro-deposited directly on the center cathode foil. This design provides a high efficiency for detecting fission fragments and allows loading of pre-assembled target/detector assemblies into the neutron beam line at DANCE. Results from tests of the fission-tag detector, as well as preliminary results from measurements on 235U and 252Cf that utilized the fission-tag detector will be presented.

On spectrometric properties of transmission avalanche counters at moderate specific ionization

The purpose of this investigation is to determine the energy resolution of transmission avalanche counters at moderate specific ionization. Parallel-plate avalanche counters (PPAC) with an electrode spacing d from 0.1 to 0.4cm have been used to register alpha particles emitted by 241Am, at n-heptane vapour pressures of p⩾5Torr. An analysis of complex spectral distributions was carried out for the detected particles to determine the effect of such a fine energy structure of the recorded spectrum upon the properties of the counters under investigation. The derived empirical curves of PPAC energy resolution have a plateau range that gets longer with the increase in p and d. The absolute values of the actual gas gain corresponding to the plateau ranges amounted from 4.6×10 to 2.91×104, while d=0.2–0.3cm and p⩽30Torr. The obtained results have been discussed with respect to both veracity and conclusions involved with fluctuations of the charge generated in the interelectrode space of the detector.

Neutron in-beam Mössbauer spectroscopic study of iron disulfide at room temperature

An in-beam emission Mossbauer spectrum of 57Fe arising from the 56Fe(n, γ) 57Fe reaction in iron disulfide at room temperature was measured with a parallel plate avalanche counter. It was clearly observed that the nuclear reaction and the following process lead to the production of a new chemical species of iron different from the parent compound.

Energy dependence of proton induced fission cross sections for heavy nuclei in the energy range 200–1000 MeV

Total cross sections for proton induced fission of ${}^{\mathrm{nat}}\mathrm{Pb}$, $^{209}\mathrm{Bi}$, $^{232}\mathrm{Th}$, $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, $^{237}\mathrm{Np}$, and $^{239}\mathrm{Pu}$ nuclei were measured in the range 200--1000 MeV with an energy step of 100 MeV. The experiment was carried out at 1 GeV in a PNPI synchrocyclotron. Complementary fragments of the binary fission from a thin target were detected in coincidence by two parallel plate avalanche counters (PPAC) located close to the target. An assembly of the two PPACs and with the target in between them was placed directly into the beam, which provided a large solid angle acceptance. The beam monitoring system employed a scintillation counter telescope that was used for direct proton counting as well as for registration of the elastic $\mathit{pp}$-scattering events from the auxiliary (${\mathrm{CH}}_{2}){}_{n}$ target. The measured energy dependence of the total fission cross sections is presented. The results are compared with other available experimental data as well as with calculations in the frame of the cascade-evaporation model.

A New Look at the PPAC

The Parallel Plate Avalanche Counter (PPAC) has been used mostly for measuring the energy loss of single, heavily ionizing particles. A PPAC is even more effective with high-energy showers for which a single event causes hundreds of electrons to pass through it. For operation at reduced pressures, the alkanes such as isobutane, C4H10 are excellent PPAC gasses with the quality of performance increasing with the size of the molecule. Under some circumstances the perfluoroalkanes such as perfluoropropane, C3F8, may be preferred. For operation at atmospheric pressure, CF4 and mixtures containing Ar and CO2 work well.

The focal plane detector of the magnetic spectrometer PRISMA

The characteristics and performance of the focal plane detector system developed for the magnetic spectrometer PRISMA are presented. It is composed of a multi-wire parallel plate avalanche counter (providing X, Y and timing signals) followed by a multi-parametric transverse field ionization chamber array (providing multiple Δ E signals). The detector array has been built to match the very large acceptance of PRISMA and provides optimal performance in the complete identification (nuclear charge, energy and mass) of nuclei produced in heavy-ion binary reactions.

Evolution of the prompt dipoleγ-ray emission with incident energy in fusion reactions

MeV/nucleon, with the aim to probe its evolution with incident energy. These reactions populate, through entrance channels having different charge asymmetries, the 132 Ce compound nucleus at an excitation energy of 117 MeV with identical spin distribution. Fusion events were selected by detecting high-energy γ-rays in coincidence with evaporation residues. The present results can be compared directly with those related to the same reactions at Elab � 9 MeV/nucleon where the compound nucleus excitation energy was equal to 173.5 MeV [1]. This study could be of aid in the formation of superheavy elements. In fact, the emission of pre-equilibrium dipole photons could represent an efficient new cooling mechanism of the composite system in charge asymmetric ”hot” fusion reactions, increasing thus its survival probability against fission [2]. EXPERIMENTAL SET-UP The experiment was performed by using the pulsed beams of 32 S and 36 S provided by the heavy-ion 15 MV Tandem-XTU accelerator of the Laboratori Nazionali di Legnaro (Italy) at Elab = 196 MeV and 214.2 MeV, respectively, impinging on 550 � g/cm 2 thick 100 Mo ( 97.42%) and 96 Mo ( 95.9%) self-supporting targets. The beam consisted of � 2 ns wide bunches with a 400 ns separation and it was measured in a Faraday cup shielded with lead and paraffin to reduce the background due to γ-rays and neutrons. Beam current was about 10 nA. The γ-rays were detected by using six seven-pack clusters of BaF2 scintillators situated at 28 cm from the target at different θ angles with respect to the beam direction. The total solid angle covered by all the clusters was 1.6 sr. Each cluster was surrounded by a 3 mm thick lead shield which reduced the counting rate due to the low energy γ-rays to 50% and stopped the charged particles. The evaporation residues were detected by means of four position sensitive Parallel Plate Avalanche Counters (PPAC’s) located at θ = 7 ◦ symmetrically around the beam direction at 70 cm from the target. The total solid angle covered by the PPAC’s was 0.089 sr. They provided the time of flight (TOF) with respect to the radiofrequency signal of the accelerator, the energy loss (�E) and the position of the reaction products. The energy calibration of the γ-ray detectors was obtained by using the sources 60 Co, 88 Y, the composite source of 241 Am+ 9 Be and the 15.1 MeV γ-rays from the p+ 12 C reaction. Down-scaled single events together with coincidence events between a PPAC and at least one fired BaF2 scintillator were collected during the experiment. A coincidence event was accepted if the deposited energy in a BaF2 cluster was greater than � 5 MeV. The threshold of each BaF2 scintillator was set at � 100 keV. The coincidence request eliminated any cosmic ray contamination of the γ-ray spectra.

Analysis of Fe valence states in iron-gall inks from XVIth century manuscripts by 57Fe Mössbauer spectroscopy

The charge state of Fe in iron-gall inks was studied by 57Fe Mössbauer spectroscopy. Three ancient manuscripts from the 16th century and a set of model samples were characterized. The chemical fingerprint was found to be well defined for the ancient manuscripts whereas the model samples show a larger spread of chemical states. Two different modes of Mössbauer spectroscopy were used in this work: (1) the detection of γ-quanta in the usual transmission mode and (2) the detection of conversion electrons in backscattering geometry in a special parallel plate avalanche counter. The use of these two measuring modes allowed to reveal differences between the chemical features of the core and the surface of the iron-gall ink structures.

Detection of landmines by using 14 MeV neutron tagged beams

A large-area scanning system using 14 MeV tagged neutrons has been built. The associated 4He particles emitted in the D+T reaction are detected in an array of Parallel Plate Avalanche Counters that defines a scanning plane having about 10 × 100 cm 2 area. Coincident gamma-rays are detected by 10 BaF 2 scintillators. The capability of the system to determine the presence and the position of samples and to perform an on-line background subtraction is demonstrated. Test with landmines are also reported. This technique allows a significant improvement of the signal-to-noise ratio searching for hidden threat materials. The use with portable sealed-tube generators is foreseen.

A large area scanning system using 14 MeV neutron tagged beams for non-destructive assays

A large area scanning system using 14 MeV tagged neutrons has been built. The associated 4He particles emitted in the D + T reaction are detected in an array of Parallel Plate Avalanche Counters that defines a 10 × 100 cm 2 area in the scanning plane. Coincident gamma-rays are detected by 10 BaF 2 scintillators. The commissioning of the sensor has been completed. The capability of the system to determine the presence and the position of samples has been verified. Furthermore the possibility of on-line background subtraction is demonstrated.

Development and application of parallel-plate avalanche counter for in-beam Mössbauer spectroscopy

For Mossbauer γ-rays in the “in-beam” Mossbauer experiments using various beams such as heavy ion, secondary short-lived isotope, and neutron beams, it is important to develop a detector. A parallel-plate avalanche counter (PPAC) is the most suitable gas counter for on-line measurements, because PPAC collects the internal conversion electrons emitted by the Mossbauer effect even under high γ-background. We evaluated the influences of the pressure and flow rate of the counter gas against the counting efficiency of PPAC, and determined the optimum conditions for use in the in-beam Mossbauer experiments.

Method of Measurement of Cross Sections of Heavy Nuclei Fission Induced by Intermediate Energy Protons

A method of measurement of total fission cross sections and an experimental set-up are described. Experiments are carried out at the 1 GeV synchrocyclotron of PNPI with an updated beam system which allows one to obtain proton beams of various energies from 200 to 1000 MeV with intensity up to 107 protons/s. The proposed measurement method is based on the registration in coincidence of both fission fragments by two parallel plate avalanche counters. The counters together with the target to be investigated are placed directly on the proton beam which provides a large solid angle acceptance. The beam monitoring is performed by scintillation telescopes. The first preliminary results on energy dependences of total fission cross sections of 238U, 235U, 209Bi and natPb are presented.

Reaction dynamics and hot nuclei formation in the36Ar+98Moreaction at37AMeVstudied through light charged particle andγ-ray emission

Central and mid-central collisions which lead to the formation of a heavy residue in the 37A MeV ${}^{36}\mathrm{Ar}{+}^{98}$Mo reaction have been studied with the MEDEA multidetector array coupled to a Parallel Plate Avalanche Counter. The dependence of the high energy gamma ray and light charged particle production as a function of the linear momentum transferred to the fused system has been studied. The unique potentialities of the MEDEA detector have made it possible to follow the evolution of the reaction dynamics from the pre-equilibrium stage to the formation of a heavy compound nucleus. The analysis of the correlation between the most energetic photons and protons shows how both of them are mainly produced in the most energetic primary nucleon-nucleon collisions. The multiplicity of high energy ${(E}_{\ensuremath{\gamma}}>30\mathrm{MeV})$ $\ensuremath{\gamma}$ rays has been found to increase with the linear momentum transfer, showing the dominance of two-body dissipation in the transfer mechanism and giving a tool to correlate the momentum transfer with the centrality of the collision. Light charged particle kinetic energy spectra show how in these collisions, compound systems with excitation energies of more than $3A\mathrm{MeV}$ and temperature up to 7 MeV are formed. The experimental findings are compared with Boltzmann-Nordheim-Vlasov calculations. A scenario is found where the nucleus-nucleus interaction starts with two-body nucleon-nucleon collisions in the overlap region of nuclear densities. These collisions give rise to the production of high energy nucleons and $\ensuremath{\gamma}$ rays, and play a fundamental role in the energy transfer from the relative motion to internal excitation of the quasiprojectile and the target.

Light and heavy transfer products in58Ni+208Pbat the Coulomb barrier

Multinucleon transfer reactions in Ni+Pb have been studied at ELAB=328.4 MeV, corresponding to ' 3% above the Coulomb barrier. Light reaction products have been detected with the time-of-flight spectrometer PISOLO [1], while the associated heavy partners produced in the binary reaction have been detected in kinematic coincidence using a transmission-type multiwire parallel-plate avalanche counter [2]. The main motivation of the experiment was to simultaneously detect both light and heavy nuclei produced in multinucleon transfer reactions, for a suitable projectile and target combination where large number of neutrons and protons could be transferred at energies close to the Coulomb barrier. The Ni+Pb system is in fact a good candidate, since its Q-value matching conditions, close to optimum up to 8 proton stripping, allows one to investigate in detail the mass and charge partitions of the light reaction products and, at the same time, to see if the same transfer mechanism is suited for producing a significant yield of the corresponding high Z heavy partners [3,4]. A first report on the experiment is given in Ref. [5] while the final work including the comparison between data and theory is reported in Ref. [6]. We refer to the cited references (and references therein) for the details and for the discussion of the physics involved.

Measurement of Cross Sections of Heavy Nuclei Fission Induced by Intermediate Energy Protons in the Energy Range 200 - 1000 MeV

A method of measurement of total fission cross sections and experimental set-up are described. Experiments are carried out at 1 GeV synchrocyclotron of PNPI with an updated beam system which allows one to obtain proton beams of various energies from 200 to 1000 MeV with intensity about 106 protons/s. The proposed measurement method is based on the registration in coincidence of both fission fragments by two parallel plate avalanche counters. The counters together with the target to be investigated are placed directly on the proton beam which provides a large solid angle acceptance. The beam monitoring is performed by scintillation telescopes. The first results on total fission cross sections of 238U and 209Bi at some proton energies in the range from 200 MeV to 1000 MeV are presented.

Neutron-Induced Fission Cross Section Measurements between 1 eV and 250 MeV

The CERN n-TOF facility opens new opportunities due to an efficient neutron production for all energies between 1 eV and more than 200 MeV. The neutrons are produced by spallation of a lead target when bombarded with 20 GeV protons from the PS accelerator. Our experiments aim at measuring fission cross sections on a series of actinides and nuclei involved in the thorium cycle. The knowledge of these fission cross sections is of importance to evaluate the behaviour of such nuclei when introduced in a nuclear reactor since their fission cross sections have an impact on the reactivity of the reactor. Parallel Plate Avalanche Counters have been used as fission fragments detectors.

Efficiency of position sensitive PPAC for various ions

The detection efficiencies of a position sensitive parallel plate avalanche counter, measured with 40 MeV/u 17N beam bombarding on a 621 mg/cm 2 197Au target, were observed to be significantly different for different ions from helium to oxygen. Furthermore, for a given type of ion, the efficiency decreases with the increase of the incident energy.

Delay-line PPAC for high-energy light ions

We have developed a delay-line parallel-plate avalanche counter (PPAC) with high detection efficiency for high-energy light ions. A large signal-to-noise ratio was achieved by a newly designed pre-amplifier and the usage of C 3F 8 gas at a pressure of 30 Torr . The counter has been tested with beams of 90 A MeV 7 Li at RIKEN, 340 A MeV 12 C at GSI, and 254 A MeV 8 B at GSI. We achieved high detection efficiencies (>95%) for all of these ions. Furthermore, we achieved reasonable position and timing resolutions.