Fission Fragment Beams Research Articles

Characterization of Diamond and Silicon Carbide Detectors With Fission Fragments

Experimental fission studies for reaction physics or nuclear spectroscopy can profit from fast, efficient, and radiation-resistant fission fragment (FF) detectors. When such experiments are performed in-beam in intense thermal neutron beams, additional constraints arise in terms of target-detector interface, beam-induced background, etc. Therefore, wide gap semi-conductor detectors were tested with the aim of developing innovative instrumentation for such applications. The detector characterization was performed with mass- and energy-separated fission fragment beams at the ILL (Institut Laue Langevin) LOHENGRIN spectrometer. Two single crystal diamonds, three polycrystalline and one diamond-on-iridium as well as a silicon carbide detector were characterized as solid state ionization chamber for FF detection. Timing measurements were performed with a 500-µm thick single crystal diamond detector read out by a broadband amplifier. A timing resolution of ∼10.2 ps RMS was obtained for FF with mass A = 98 at 90 MeV kinetic energy. Using a spectroscopic preamplifier developed at INFN-Milano, the energy resolution measured for the same FF was found to be slightly better for a ∼50-µm thin single crystal diamond detector (∼1.4% RMS) than for the 500-µm thick one (∼1.6% RMS), while a value of 3.4% RMS was obtained with the 400-µm silicon carbide detector. The Pulse Height Defect (PHD), which is significant in silicon detectors, was also investigated with the two single crystal diamond detectors. The comparison with results from α and triton measurements enabled us to conclude that PHD leads to ∼50% loss of the initial generated charge carriers for FF. In view of these results, a possible detector configuration and integration for in-beam experiments has been discussed.

Beijing Isotope-Separation-On-Line Neutron-Rich Beam Facility and its application prospects

Radioactive-ion beam (RIB) experiments open up new exciting fields and frontiers in nuclear physics and nuclear astrophysics. The experiments can provide unprecedented opportunities to explore new phenomena such as new magic numbers and probe the isospin mechanisms occurring in shell evolution. Other experiments probe the limits of nuclear stability on the neutron-rich side, provide insights into the synthesis of superheavy elements and nuclear giant halo structures, and offer the possibility to investigate new nuclear decay modes as well as the nucleosynthesis of nuclides beyond Iron. To achieve this goal, many nuclear physics laboratories all over the world are building large-scale powerful new RIB facilities. The “Beijing Isotope-Separation-On-Line Neutron-Rich Beam Facility (BISOL)” is a joint project proposed by the China Institute of Atomic Energy (CIAE) and Peking University. This facility will combine two techniques: Projectile fragmentation (PF) and isotope separation on line (ISOL). The BISOL facility will operate based on a dual driver system, including reactor driving and intense deuteron-beam driving modes. The existing high flux research reactor, the China Advanced Research Reactor at CIAE provides reactor driving. At the BISOL facility ISOL separation of fission fragments, post acceleration, and fragmentation of neutron-rich fission fragment beams will be available. By combining ISOL and PF techniques in one facility the technical advantages of both can be merged. The BISOL will deliver intensive rare ion beams of very neutron-rich isotopes; the intensity of these beams will be of the order of at least 1-2 magnitudes higher than currently available ones. Intense deuteron-beam driving is an established technology, which provides an intense neutron beam source and can be operated independently in the facility. Characterizing material performance and performing lifetime experiments are the challenges facing researchers working on the current and next-generation nuclear energy systems. Intense radiation sources, especially neutron sources, are indispensable for these experiments. The intense deuteron accelerator proposed in this project will be able to generate intense neutron beam. These beams will be able to probe nuclear materials and will provide vital results to push forward the boundaries of nuclear physics.

A Novel Approach to β-delayed Neutron Spectroscopy Using the Beta-decay Paul Trap

A new approach to β-delayed neutron spectroscopy has been demonstrated that circumvents the many limitations associated with neutron detection by instead inferring the decay branching ratios and energy spectra of the emitted neutrons by studying the nuclear recoil. Using the Beta-decay Paul Trap, fission-product ions were trapped and confined to within a 1-mm3 volume under vacuum using only electric fields. Results from recent measurements of 137I+ and plans for development of a dedicated ion trap for future experiments using the intense fission fragment beams from the Californium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory are summarized. The improved nuclear data that can be collected is needed in many fields of basic and applied science such as nuclear energy, nuclear astrophysics, and stockpile stewardship.

Unexpected results in neutron-rich radioactive beams induced fusion

The fission-fragment beams at HRIBF provide a unique opportunity for studying the mechanisms of fusion involving nuclei with large neutron excess. To explore the role of transfer couplings, fusion excitation functions have been measured using neutron-rich radioactive 132Sn beams incident on 40Ca and 58Ni targets. The sub-barrier fusion enhancement for 132 Sn+40Ca is larger than that for 132Sn+58Ni although the neutron transfer Q-values are similar for the two reactions. The fusion excitation function for 46Ti+124Sn has been measured in an attempt to resolve the differences observed in 132Sn+40Ca and 132Sn+58Ni.

Radioactivity distribution in the MAFF beamline

The Munich Accelerator for Fission Fragments (MAFF) facility planned at the research reactor FRM-II in Munich is dedicated to produce, cool and accelerate high-intensity neutron-rich radioactive beams. The radioactive beam of fission fragments results from bombarding a uranium target by the thermal neutrons emerging from the reactor. Part of the nuclides is released from the target/ion source unit in a non-ionized state. This contribution is concerned with simulating the distribution of the non-ionized radioactivity along the MAFF beamline. The calculations are performed with the help of the computer program MOVAK3D. The calculated fraction of non-ionized radioactivity at the border of the experimental area is at the level of about 10 −7, relative to the activity produced in the target.

Study of the N=50 shell closure close to 78Ni

The PARRNe (Production d'Atomes Radioactifs Riches en Neutrons) experimental program has been part of the recent R&D efforts for the design of the SPIRAL2 project at GANIL. The PARRNe2 set-up installed at IPNOrsay allows the use of the Tandem as a deuteron driver in order to produce mass separated fission fragment beams. The measured fission fragment yields proved to be high enough to undertake a series of beta-decay experiments dedicated at studying the structure of proton-deficient nuclei close to N=50. We report here on two such experiments, the first being dedicated at the observation of the decay of 83Ga->83Ge (Z=32, N=51), 84Ga->84Ge (Z=32, N=52) and the second of the decay of 81Zn->81Ga (Z=31, N=50) allowing for the fist time the identification of a few gamma lines with transitions in the daughter nuclei thus providing the material for a preliminary glimpse of their structure.

Studies of the behaviour of shell closures far from stability

The structure of light neutron-rich nuclei around N=14,16,20,28 has been investigated at GANIL. In-beam gamma spectroscopy using fragmentation reactions of36S and48Ca beams on a Be target has allowed to obtain interesting results. γ-decay of excited states have been measured for the first time in nuclei around44S. Level schemes are discussed for a large number of these neutron-rich nuclei. The PARRNe2 set-up installed at IPN-Orsay allowed the use of the Tandem as a deuteron driver in order to produce mass separated fission fragment beams. The measured fission fragment yields proved to be high enough to undertake a series of β-decay experiments dedicated at studying the structure of neutron-rich nuclei close to N=50.

Studies of the behaviour of shell closures far from stability

The structure of light neutron-rich nuclei around N=14,16,20,28 has been investigated at GANIL. In-beam gamma spectroscopy using fragmentation reactions of 36S and 48Ca beams on a Be target has allowed to obtain interesting results. γ-decay of excited states have been measured for the first time in nuclei around 44S. Level schemes are discussed for a large number of these neutron-rich nuclei. The PARRNe2 set-up installed at IPN-Orsay allowed the use of the Tandem as a deuteron driver in order to produce mass separated fission fragment beams. The measured fission fragment yields proved to be high enough to undertake a series of β-decay experiments dedicated at studying the structure of neutron-rich nuclei close to N=50.

Production of intense beams of fission fragments at MAFF

At the new Munich high flux reactor FRM-II, the Munich Accelerator for Fission Fragments (MAFF) is under development. The main objective will be the production and study of new very heavy elements (Z>100). To obtain this goal, intense beams of neutron rich isotopes (70<A<160) are required. Thermal neutron induced fission is considered the most suitable method to produce these isotopes due to the large fission cross section and high thermal neutron flux (>1014 n/s*cm2) available at the new reactor. The target ion source design is based on the ANUBIS source at OSIRIS in Studvik, optimized for very high neutron fluxes. Using 1 g of U235 diluted in a graphite target, intensities of several 1011 ions/s for Kr91, Sn132, or Cs144, e.g., are expected after mass separation. These singly charged ions will be charge bred in an electron cyclotron resonance ion source and then injected into the MAFF-LINAC to reach the energies at the Coulomb barrier. The production of intense ion beams of neutron rich isotopes by thermal neutron induced fission, the development of the target ion source, and the development of the fission target will be presented.

RNB production with thermal neutrons

Thermal neutron induced nuclear fission is the most suitable method to produce neutron-rich isotopes (70⩽ A⩽160) due to the large fission cross section and the high thermal neutron fluxes in modern reactors. Intensities of mass separated neutron rich nuclei of some 10 11 ions/s are expected, e.g. for 91Kr, 132Sn or 144Cs from 235U diluted in a porous graphite target. Several front runners with low-energy fission-fragment beams exist like OSIRIS in Studsvik. In order to get beams of neutron-rich nuclei at the Coulomb barrier, the PIAFE project worked out a first concept of production and mass separation of high-intensity beams of fission fragments. At the new Munich high-flux reactor FRMII, the Munich Accelerator for Fission Fragments (MAFF) is under development to make use of post accelerated beams of neutron rich isotopes for experiments in many different fields of nuclear physics, solid state physics and medicine. One key experiment will be the production and the study of very heavy elements. An overview of the production method of neutron-rich isotopes by thermal neutron induced fission, and of the expected yields will be given and the development of target–ion-sources and of the fission targets for MAFF will be characterized.

Low energy radioactive ion beams in Dubna

The Dubna radioactive beam factory (DRIBs) [1] will make use of two possibilities for producing secondary beams of radioactive nuclei. During the first phase of the project (Phase I) the possibility for obtaining radioactive nuclei in fragmentation reactions of stable nuclei, accelerated by the cyclotron U400M to intermediate energies (∼ 50 MeVA−1), will be realized. The fragmentation products, obtained in a special ion source, will be converted into single-charged ions, which after transportation to the second cyclotron (U400) will be accelerated up to energies of ∼ 20–25 MeVA−1. This method will allow obtaining mainly beams of light radioactive nuclei with Z 30 with intensities up to 108 pps (e.g., nuclei such as 6He). In the second phase of the project (Phase II) it is supposed to produce and accelerate radioactive neutron-rich nuclei in the mass region 80 A 140. This mass region corresponds to the fragments of low-energy fission of heavy nuclei. For this reason use will be made of the photofission of 238U. The γ -quanta will be produced by the electron accelerator (the microtron MT-25), where the electron beam with an intensity of about 20 μA has an energy of 25 MeV. With the help of a special converter the electron beam is transformed into a beam of γ -quanta with up to 25 MeV energy and a flux of 1014 s−1. This beam, focused into a narrow angle, will fall onto a 238U-target weighing 100 g. It is wellknown that the photofission cross section has a maximum corresponding to the giant dipole resonance at an energy of the γ -quanta equal to Eγ = 13.5–14 MeV [2]. This brings forth an increase of the photofission probability. The yield of the fission fragments will increase as a result of the interactions of the secondary neutrons (γ n and fission neutrons) with the U-target. When the mentioned above parameters of the beam and target are realized, one can get as much as 1011 fission fragments/second. Taking into account the widths of the mass and charge distributions of the fragments, the yields of definite fission fragments can be estimated. When the efficiency of the ion source and the transport system amounts to some 20–30% it is possible to obtain beams of fission fragments in the region of Kr and Xe with an intensity of up to 109 s−1. Simultaneously with the fragments situated close to the maxima of the mass distribution (A = 90 and 130), asymmetric fission fragments are formed with a rather high yield. For the isotopes situated at the “tails” of the fragment

Exotic beams produced by fast neutrons

First results from the research and development program PARRNE (Production d'Atomes Radioactifs Riches en NEutrons) are presented. Its aim is the investigation of the optimum conditions for the production of neutron-rich fission fragment beams extracted from thick targets irradiated by fast neutrons.

PIAFE: Experiments with accelerated fission fragments

Abstract Even very experienced experimental nuclear physicists are surprised when they hear for the first time that fission fragments, produced in a special source at a reactor, should be sent to an accelerator in order to irradiate targets with neutron rich radioactive heavy-ion beams. Of course, there are many technical problems which have to be solved. However, a close look at this exotic project shows that the fission fragment beams have rather high intensities and that there is a wealth of new and very interesting physics which can be done with such a facility. This project is presently being discussed in Grenoble, where the High Flux Reactor of the Institut Laue-Langevin (ILL) is only about 400 m from the accelerator complex SARA(Systeme Accelerateur Rhone Alpes) of the Institut des Sciences Nucleaires (ISN). The name of the project is PIAFE (Projet d' Ionisation et d' AcctlCration de Faisceaux Exotiques), which sounds (to French ears) like le piaf, the sparrow. A workshop and collaboration meeting ...

A new method to identify nuclear charges of fission fragments

For a mass and velocity selected beam of fission fragments, the elemental components of the beam have been determined by measuring the difference between the time the fragments enter an axial ionization chamber (with the electrical field lines parallel to the particle trajectory) and the time the anode pulse crosses a given level. The nuclear charge resolution achieved for typical fission fragments out of the light mass group in thermal neutron induced fission of 235U is Z/ ΔZ = 43 for a nuclear charge Z = 39.

The influence of plasma effects on the timing of fission fragments with semiconductor detectors

For fission fragments incident on surface-barrier detectors of low resistivity (ϱ ≈ 400Ω cm) the dependence of the fast current signal on field strength and local resistivity variations of the basic material has been studied. It is well known that with the fast signal as timing reference a time shift is observed when varying the bias voltage. This is due to a change in plasma time. The additional variation of plasma time with mass and/or energy of the heavy ions necessitates a time-walk correction in fission-fragment time-of-flight studies, which has been determined by two independent methods to be about ± 200 ps for the mean light and heavy fragment of binary fission from 252Cf. Local resistivity changes of the silicon wafer the detectors are made from, give rise to local plasma-time shifts, which spoil the time resolution of large-area detectors. For a narrow well-collimated beam of fission fragments an optimum time resolution of 215 ps has been obtained.

Fission fragment energy spectrum determination with solid state nuclear track detectors

Attempts are described to optimize the spectrometric response of a number of solid state nuclear track detectors (SSNTD), including fused silica, soda-lime glass and phosphate glass. Diameters of the etch pits resulting from normally incident beams of fission fragments from a 252Cf source in vacuum have been measured, by both optical and scanning-electron microscopy, as functions of the period of etching and of the thickness of Al absorber introduced in the path of the beam. The best resolution of the heavy and light fission fragment groups, as inferred from the diameter distributions, is obtained with phosphate glass SSTND covered with ≈5 μm Al foil during irradiation.

Fission fragment damage in tungsten

Abstract The study of fission fragment damage in tungsten by field-ion microscopy described in an earlier paper (Bowkett et al. 1965) has been continued. Prepared specimens have been bombarded with a collimated beam of fission fragments at a measured inclination to the specimen axis, giving results which show that in small specimens a column of material can be pushed out along the fission fragment path. These findings lend support to the explanation advanced for the light contrast tracks observed by electron microscopy of thin foils that have been irradiated with fission fragments (e.g. Bowden and Chadderton 1962).

Method of Forming Fine Holes of Near Atomic Dimensions

A method of forming small holes in certain materials is described. The material is first exposed to massive, high energy particles such as fission fragments and is then immersed in a chemical etching solution which dissolves the damaged material along the paths of the particles. With this technique it is possible to form holes which are as small as 25 Å in diameter and are up to ∼10 μ long. The hole diameters can be increased at will from the minimum size by continued etching. A density of up to ∼1011 holes/cm2 can be attained by a short irradiation in a typical reactor. The holes can be accurately aligned by collimating a beam of fission fragments. Fine holes have been formed in this way in a number of nonmetallic materials including glass, mica, and other silicate minerals.