Photofission Experiments Research Articles

Performance of a twin position-sensitive Frisch-grid ionization chamber for photofission experiments

A position-sensitive twin Frisch-grid ionization chamber has been constructed for future photofission experiments using nearly monochromatic, linearly polarized gamma-ray beams. By exchanging the anode plates in the standard ionization chamber on both sides by an array of grid- and strip-anodes, which are rotated by 90° relative to each other and read out by means of resistive charge division, a position sensitivity is achieved that allows the azimuthal fragment emission angle and hence the fission axis orientation to be determined. The performance of this gaseous detector has been studied using the well-known 252Cf spontaneous fission process. The fission axis orientation could be determined relative to an arbitrary axis in space with a resolution better than 7° FWHM. Measured pre-neutron mass and total kinetic energy distributions are consistent with literature, which ensures that the mass and energy resolution for fission fragments is not affected by the position-sensitive structure.

Photofission method for quantification of fissile material in large drums

Abstract Photofission is one of the active non-destructive analysis (ANDA) techniques used for quantification of fissile materials in large containers. Sets of photofission experiments were conducted at an industrial 10 MeV electron LINAC facility with the aim to develop an active monitoring system. A mock-up drum of 50 cm diameter and 100 cm length was placed horizontally and irradiated with high-energy photons axially. Two banks of high sensitivity Helium-3 proportional counters, with six detectors in each bank were used to measure delayed neutrons after photofission. Average absolute efficiency of the neutron detection system with Cf-252 source was measured as 9.2 %. Photofission experiments were conducted using a 15.6 gram thorium dioxide sample. Minimum detection limit was estimated at eight angular positions along periphery of the drum. The minimum detection limit from the measurement of delayed neutrons for the farthest position of the sample was estimated as 976 ± 57 mg for 1 kW electron beam in scan mode which actually impinged around 40 W beam power on the thorium dioxide sample. The average photofission delayed neutron yield/100 fissions for Th-232 is estimated from the experiments as 3.6 ± 1.1.

Gamma-beam photofission experiments at ELI-NP: The future is emerging

At the Extreme Light Infrastructure - Nuclear Physics facility (ELI-NP), high-power laser systems together with high-brilliance gamma beams are the main research tools. The status of the construction of the facility is reported. The emerging photofission experimental program with brilliant gamma beams at ELI-NP is presented with emphasis on the prepared day-one experiments. The physics cases of the flagship experiments at ELI-NP are discussed, as well as the related instruments which are under construction for their realization.

Tests of ionization chambers for future photofission experiments

A recent set-up of a multi-cathode Frisch-grid ionization chamber and experimental results of a proof-of-principle experiment are presented. Preliminary data on the obtained polar angular and mass distributions as well as total kinetic energy of fission fragments produced in neutron-induced fission of 232Th and 238U are discussed that show that the chamber has been operated successfully. For the additional measurement of the azimuthal angular distribution, a design of segmented anodes has been tested. Preliminary data do not allow a suffcient reconstruction of the azimuthal angle.

Simulation of photofission experiments at the ELI-NP facility

An extensive experimental program for the study of photofission will take place at the Extreme Light Infrastructure – Nuclear Physics (ELI-NP) facility, where different actinide targets will be exposed to a brilliant gamma beam to produce fission fragments. We report on the implementation within the Geant4 simulation toolkit of the photofission process, of related background processes, and of extended ionic charge parameterization. These developments are used to evaluate the production rates of photofission fragments and their release efficiency from the actinide targets.

Perspectives for photonuclear research at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility

The perspectives for photonuclear experiments at the new Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility are discussed in view of the need to accumulate novel and more precise nuclear data. The parameters of the ELI-NP gamma beam system are presented. The emerging experimental program, which will be realized at ELI-NP, is presented. Examples of day-one experiments with the nuclear resonance fluorescence technique, photonuclear reaction measurements, photofission experiments and studies of nuclear collective excitation modes and competition between various decay channels are discussed. The advantages which ELI-NP provides for all these experiments compared to the existing facilities are discussed.

Photofission experiments at the ELI-NP facility

At ELI-NP, high-power lasers together with a very brilliant γ beam are the main research tools. The high-power laser system (HPLS) and the γ beam system (GBS) of ELI-NP are presented. The expected performance of the electron accelerator and production lasers of the GBS, and the targeted operational parameters of the γ beam are described. Possible laser-induced fission and γ beam photofission experiments which are under preparation at ELI-NP, and the different set-ups and instrumentation, are designed for these experiments, are presented.

Photofission of 238U Nuclei

The spectrum of the 238U(γ, f) photofission reaction which is obtained using bremsstrahlung photon beams from a clinical e-linac is analyzed. The following nuclei 94Y, 134Te, 136Pr, 142Ba, 138Xe, 104Tc in the spectrum of our experiment have been identified. The gamma ray energies of the produced nuclei are compared with the literature values. This work is a proof-of-concept experiment which is the first of in the series of our future planned photofission experiments using a clinical e-linac operated with different endpoint energies.

Quantification of fissionable materials in pure and mixed form using photofission

Photofission experiments have been conducted at the industrial 10MeV LINAC of Bhabha Atomic Research Centre, to develop a method for quantification of actinides. A number of natural uranium and thorium samples were fabricated in pure and mixed forms and irradiated for photofission. Delayed neutron and delayed gamma decay data produced after photofission were analyzed by the formation of decay groups, for 10MeV end point energy. A method for quantification of relative proportions of actinides is developed and it is observed that under constant experimental conditions, even with an industrial accelerator, quantification of actinide in mixed state can be carried out with a reasonable accuracy and with a maximum error in estimation of less than 40%.

Semiempіrical description of Ge(Li)- and HPGe- detectors efficiency for photofission experiments

Semiempіrical description of Ge(Li)- and HPGe- detectors efficiency for photofission experiments

Parallel-plate avalanche detectors with anode wire grids

A fission-fragment detection system was designed and built at The George Washington University, to be used in photofission experiments at the Saskatchewan Accelerator Laboratory and the Jefferson Laboratory. The fission fragments were detected using parallel-plate avalanche detectors with anode wire grids. An array of several target–detector pairs was mounted inside a low-pressure reaction chamber. The results of calibrations of the detectors using a 252Cf source and their performance with a bremsstrahlung photon beam during the experiments are presented.

Clustering-related aspects in the compound nucleus formation

A new approach to describe the compound nucleus formation process, where clustering aspects of the nucleus are taken into account, is worked out. It is shown that quantitative differences between the compound nucleus cross-section of232Th and238U, as observed in recent photofission experiments, could be due to the different cluster preformation probabilities of these nuclei.

Silicon strip detectors for fission

Abstract The performance of a silicon strip detector especially designed for photofission experiments and a particular arrangement for low count rate photofission measurements are presented. Other applications for the same detection system are discussed.

Measurement of fission yields using target materials in contact with solid state track detectors

Abstract A method to evaluate fission yields in experiments of induced and spontaneous fission which use fissionable target materials in the configuration of contact with dielectric track detectors has been developed. The effect of energy absorption of the fission fragments by the target sample, and the optical resolution power for observation of etched fission tracks are taken into account in deducing the total efficiency factors resulting from the use of targets of different thicknesses. With this method, the conditions to give the maximum observable density of etched tracks can be obtained, which is important to experiments of low fission yields. A number of useful formulae applicable to mica and plastic track detectors for evaluation of fission yields are reported. The method has been successfully applied to recent photo-fission experiments of low fission yields, and it can also be useful in the study of other fission cases as well as in applications to fission-related problems.

A low-pressure MWPC and its applications in the high energy electrofission and photofission experiments

Abstract A low-pressure MWPC (LPMWPC) is described, and its characteristics obtained under laboratory conditions as well as with the help of photon and electron beams are given. The LPMWPC is designated for registration of the fission fragments of heavy nuclei. The “active” uranium targets were constructed on the basis of this LPMWPC and were used in electrofission and photofission experiments in the GeV energy region.

A 4π double obelisk of position sensitive PPACs for new vistas in photofission experiments

An arrangement of two-dimensional position sensitive PPACs (parallel plate avalanche counters) covering a large solid angle (> 50% of 4 π) for use in a new type of photofission experiments is described. For the first time simultaneous measurements of both the photofission fragment angular and mass distributions are enabled by this device. The mass information is obtained by the double time of flight technique (with modest accuracy) whereas an excellent angular resolution of < 1° is achieved. The detector performances are outlined. First applications in high intensity bremsstrahlung and tagged photon experiments are discussed.

Arrangements of parallel plate avalanche detectors for electrofission and photofission experiments

The construction and performance of several arrangements of parallel plate avalanche detectors are described. The set ups can be used for the measurement of: fragment time of flight, photofission cross sections, out of plane angular distributions, inclusive and exclusive electrofission cross sections. First results are discussed, demonstrating the wide field of applicabilities of these detectors.

Automatic scanning of solid state nuclear track detectors at low track density

A device for scanning large areas of Kimfol fission track detectors is described. It is being used in photofission experiments where ∼ 10 4 cm 2 of film per run have to handled. The tracks are first enlarged by etching and sparking. The film is then scanned by a high resolution Vidicon camera and the x and y coordinates of each track are stored in a computer. A typical scanning speed is 10 cm 2/s. The counting efficiency is close to 100%.

Study of fission channels in even-even actinide nuclei using a monochromatic polarized photon beam

We discuss the advantages that can be attained in performing photofission experiments by using a linearly polarized photon beam. The features of the longitudinal and transversal fragment angular distributions in fission induced by polarized photons are shown. A procedure to extract information on the fission channels is worked out in details. An experimental arrangement for the measurements of the photofission angular distributions is finally suggested, also in connection with the main characteristics of a polarized monochromatic photon beam now available at the Frascati National Laboratories (Italy).

The preparation of large area uranium target layers for a multiple parallel plate fission detector

Abstract The preparation of large area uranium layers (diameter 65 mm) with a thickness of 2–3 mg/cm 2 by electrolysis on Al foils is described. The layers are used in a multiple parallel plate avalanche detector with 30 counter gaps for photofission experiments. The performances and applications of this detector set-up are discussed.