A new neutron monitor with silver activation

A moderator-type neutron monitor has been developed, which registers delayed beta rays from neutron-induced silver activation and which is able to measure dose equivalent in pulsed fields with peak dose rates of several thousand Sv h^?1. The monitor uses four silicon diodes in the centre of a polyethylene moderator, 30 cm in diameter. Two of the diodes are covered by natural silver foils and two of them by tin foils. The latter are used to subtract photon-induced pulses. For registering signals, a pulse height threshold is set at 662 keV, which minimizes the effect of ¹³⁷Cs and lower energy radiation and – in addition – enhances the detection of beta rays from the shorter half-life silver isotope ¹¹⁰Ag (25 s) as compared to the longer half-life isotope ¹⁰⁸Ag (144 s). The results of measurements in neutron and photon calibration fields, of MCNPX neutron response calculations and of first measurements in a high-intensity pulsed field at the PSI accelerator are shown.     

The measurements of thermal neutron flux distribution in a paraffin phantom

The term ‘thermal flux’ implies a Maxwellian distribution of velocity and energy corresponding to the most probable velocity of 2200 m s^???1 at 293.4 K. In order to measure the thermal neutron flux density, the foil activation method was used. Thermal neutron flux determination in paraffin phantom by counting the emitted rays of indium foils with two different detectors (Geiger–Muller counter and NaI(Tl)) was the aim of this project. The relative differences of the outcome of the experiments were between 2.5% and 5%. The final results were compared with MCNP4C outputs and the best agreement was generated using NaI(Tl) by a minimum discrepancy of about 0.6% for the foil placed 8.5 cm from the neutron source.     

Monte-Carlo calculations of the activation of the accelerator target holder and shadow cone during the calibration of the ITER diagnostic devices with monoenergetic neutrons

The ITER International Fusion Energy Organization has solicited IRSN Laboratory for Neutron Metrology and Dosimetry to study the possibility to calibrate, in monoenergetic neutron fields at 14 and 2.45 MeV, the neutron detectors to be placed inside the future fusion reactor. In addition to the estimate of the necessary irradiation times, the dose equivalent rates from some of the neutron activated beam line elements had been calculated to consider the cooling time mandatory before access. Neutron activation calculations have been performed with the Fluka Monte-Carlo code. The resulting dose equivalent rates depend strongly of the neutron beam intensity as well as the neutron energy. In the worst case, for 14 MeV neutrons at an emission rate of 10¹² s⁻¹, a cooling time of 24 h would be needed for a close access to the shadow cone. Several days would be mandatory in the case of the target holder.     

Collection and behaviour of radon progenies on thin Mylar foils

Thin Mylar foils are often used to protect detectors from contamination. However, these foils can be electrostatically charged, possibly leading to their contamination with airborne radon progenies. In the present work, the collection and behaviour of radon progenies on Mylar foils was investigated in detail using alpha spectrometry. The radon progenies collection rate of a small Mylar foil (3 cm²) is equivalent to an air sampler with a flow rate of approximately 0.1 m³/h. It was demonstrated that such contamination may jeopardise the validity of the entire analysis if not interpreted correctly.     

Messung von thermischen Aktivierungs-Wirkungsquerschnitten mit hoher Genauigkeit

Activation cross sections have been determined for some isotopes which are important for flux monitoring or which have rarely been measured sofar. The following reactions were investigated: Sc⁴⁵(n, γ)Sc⁴⁶, In¹¹⁵(n,γ)In^116m , La¹³⁹(n,γ)La¹⁴⁰, Hf¹⁷⁹(n,γ)Hf^180m and Hf¹⁸⁰(n, γ)Hf¹⁸¹. The activations were performed in a purely thermal neutron spectrum (Cd ratio > 1100) using one of the bent neutron guide tubes at the FRM. The isotopes were irridiated in the form of foils sandwiched with gold foils as flux monitor. The flux ratio between measuring and monitor foil was corrected for self-absorption, self-scattering and scattering from neighbouring foils. The activities were measured in a 5×5″ well type NaJ-detector by integral counting ofγ-pulses exceeding 22 kev. The NaJ-detector was calibrated for singleγ-energies to better than 0.3%, the accuracy being even better in the case ofγ-γ-cascades. Because of the high accuracy in the activation measurement, the total error of the cross sections measured is smaller by a factor of 5 to 10 compared to previous data. An analysis of all contributions to the total error is given in detail.     

Calibration of PADC-based neutron area dosemeters in the neutron field produced in the treatment room of a medical LINAC

PADC-based nuclear track detectors have been widely used as convenient ambient dosemeters in many working places. However, due to the large energy dependence of their response in terms of ambient dose equivalent (H¹(10)) and to the diversity of workplace fields in terms of energy distribution, the appropriate calibration of these dosemeters is a delicate task. These are among the reasons why ISO has introduced the 12789 Series of Standards, where the simulated workplace neutron fields are introduced and their use to calibrate neutron dosemeters is recommended. This approach was applied in the present work to the UAB PADC-based nuclear track detectors. As a suitable workplace, the treatment room of a 15 MV Varian CLINAC DHX medical accelerator, located in the Ospedale S. Chiara (Pisa), was chosen. Here the neutron spectra in two points of tests (1.5 m and 2 m from the isocenter) were determined with the INFN-LNF Bonner Sphere Spectrometer equipped with Dysprosium activation foils (Dy-BSS), and the values of H¹(10) were derived on this basis. The PADC dosemeters were exposed in these points. Their workplace specific H*(10) responses were determined and compared with those previously obtained in different simulated workplace or reference (ISO 8529) neutron fields.     

The spatial distribution of thermal and epithermal neutrons in a graphite moderated spallation neutron field

The Gamma-3 assembly is located at the Joint Institute for Nuclear Research, Dubna, Russia. It consists of a cylindrical lead target (ø = 8 cm, L = 58.8 cm) surrounded by reactor grade graphite (110 × 110 × 60 cm). The target was irradiated with a beam of 1.6 GeV deuterons from the Nuclotron accelerator and CR-39 track detectors coupled to LR-115 2B film were used to measure the slow neutron distribution on the surface of the graphite. The detection efficiency of the CR-39 in the CR-39/LR-115 2B system was measured using a custom made calibration setup and found to be (1.12 ± 0.05) × 10⁻³ and (6.1 ± 1.2) × 10⁻⁴ tracks per neutron, for thermal and epithermal neutrons respectively, under the etching and counting procedures described in this work. The irradiation of the Gamma-3 was also simulated using MCNPX 2.7 Monte Carlo code and good agreement between the experimental and calculated track densities was found. This serves as a good validation for the computational models used to simulate spallation neutron production, transport and moderation.     

Methods for producing 195m Pt isomer

Among nuclides, ^195m Pt isomer is characterized by a number of properties that make it suitable for use in nuclear medicine. Effective ways of optimizing the Pt isomer yield at low impurity contents must be sought. The method based on double neutron capture by the ¹⁹³Ir target nucleus with subsequent populating of ^195m Pt through β decay allows chemical isolation of the isomer. In this work, the problem is analyzed, test experiments on Ir activation with neutrons are conducted, and theoretical estimates of the ^195m Pt yield are presented. Full-scale model experiments on the IBR-2 reactor in Dubna should confirm the effectiveness of the method.     

Intercomparison of the response of different photon and neutron detectors around a spent fuel cask

An intercomparison of the response of different photon and neutron detectors was performed in several measurement positions around a spent fuel cask (type TN 12/2B) filled with 4 MOX and 8 UO₂ 15 × 15 PWR fuel assemblies at the nuclear power plant Gösgen (KKG) in Switzerland. The instruments used in the study were both active and passive, photon and neutron detectors calibrated either for ambient or personal dose equivalent.The aim of the measurement campaign was to compare the responses of the radiation instruments to routinely used detectors.It has been shown that especially the indications of the neutron detectors are strongly dependent on the neutron spectra around the cask due to their different energy responses. However, routinely used active photon and neutron detectors were shown to be reliable instruments.     

Neutron interactions as seen by a segmented germanium detector

The GERmanium Detector Array (GERDA) is designed for the search for “neutrinoless double-beta decay” ( 0ν2β with germanium detectors enriched in ⁷⁶Ge . An 18-fold-segmented prototype detector for GERDA Phase II was exposed to an AmBe neutron source to improve the understanding of neutron-induced backgrounds. Neutron interactions with the germanium isotopes themselves and in the surrounding materials were studied. Segment information is used to identify neutron-induced peaks in the recorded energy spectra. The Geant4-based simulation package MaGe is used to simulate the experiment. Though many photon peaks from germanium isotopes excited by neutrons are correctly described by Geant4, some physics processes were identified as being incorrectly treated or even missing.     

Isomerism,total decay energies,and absolute γ-ray intensities of the heavy Pd and Ag isotopes

The decays of^{113, 114, 115g, 115m, 116}Pd have been studied with an emphasis on the determinations of total decay energies and absoluteγ-ray intensities. The experiments also provided improved data on these quantities for the decays of^{114, 115g, 115m, 116m}Ag. A search for isomeric transitions using a high resolution electron detector resulted in the identification of isomericE3 transitions in¹¹⁵Pd and^{113, 115, 116, 117}Ag. The isomeric branching was determined in all cases. TheQ _β-values of^113–116Pd and^113–116Ag have been measured using aβγ-coincidence method.     

Cross sections for 197Au(n, γ)198Au and 115In(n, γ)116mIn in the neutron energy region 2.0–7.7 MeV

The cross sections for the reactions ¹⁹⁷Au(n, γ)¹⁹⁸Au and ¹¹⁵In(n, γ)^116mIn have been measured with the activation method in the neutron energy region 2.0–7.7 MeV. The influence of background neutrons on the results was studied in considerable detail. The main problems are caused by low-energy neutrons produced by charged-particle reactions in the target material and secondary neutrons from nonelastic reactions in the sample and surrounding materials.The measured capture cross sections are generally lower than previous results and the deviation tends to increase with increasing neutron energy. The data are also compared with calculations based on the compound-nucleus model and quite good agreement is obtained.     

Franco-Russian comparison of mixed neutron and gamma radiation field dosimeters at the Silène reactor

This paper gives the results of dosimetry measurements carried out in the Silène reactor at Valduc (France) with neutron and photon dosimeters in mixed neutron and gamma radiation fields, in the frame of a Franco-Russian comparison of dosimeters. Neutron dosimetry was supplied by passive semiconductors, activation detectors and nuclear track detectors. For photon dosimetry, thermoluminescent and passive semiconductor detectors were used. The experiments were located at 3 m from the reactor core, in free air and also at the front and back of a tissue-equivalent phantom. The pulse operating mode of the reactor was used to simulate a criticality accident with solid fissile material, while the free evolution mode simulated a criticality accident in a fissile solution. The photon absorbed dose showed a slight increase on entering the phantom compared to measurements in free air, probably due to backscattering by the phantom. At the rear of the phantom, the neutron kerma was four times lower than on the front, whereas the photon dose was only two times lower. The heterogeneity of dose inside the phantom was far greater for neutrons than for photons.     

Coexisting structures in 115Sn and 116Sn

Excited states up to I ≈ 20 in ¹¹⁵Sn and ¹¹⁶Sn, populated via the (¹⁸O, αxn) reactions, have been studied using the DORIS Ge detector array in conjunction with charged particle detectors. In both nuclei, spherical as well as regular, deformed level structures were found. The spherical states are interpreted to arise from pure neutron configurations, while the deformed, intruder bands obviously involve proton 2p-2h excitations across the Z = 50 shell gap.     

Photon contributions from the 252Cf and 241Am–Be neutron sources at the PSI Calibration Laboratory

At the accredited PSI Calibration Laboratory neutron reference fields traceable to the national standards of the Physikalisch-Technische Bundesanstalt (PTB) in Germany are available for the calibration of ambient and personal dose equivalent (rate) meters and passive dosimeters. The photon contribution to the ambient dose equivalent in the neutron fields of the ²⁵²Cf and ²⁴¹Am–Be sources was measured using various photon dose rate meters and active and passive dosimeters. Measuring photons from a neutron source usually involves considerable uncertainties due to the presence of neutron induced photons in the room, due to a non-zero neutron sensitivity of the photon detector, and last but not least due to the energy response of the photon detectors. Therefore eight independent detectors and methods were used to obtain a reliable estimate for the photon contribution of the two sources as an average of the individual methods. For the ²⁴¹Am–Be source a photon contribution of approximately 4.9% was determined and for the ²⁵²Cf source a contribution of 3.6%.     

Measurements of natCd(γ, x) reaction cross sections and isomer ratio of 115m,gCd with the bremsstrahlung end-point energies of 50 and 60 MeV

The flux-weighted average cross sections of ^natCd(γ, xn)^{115g,m,111m,109,107,105,104}Cd and ^natCd(γ, x)^{113g,112,111g,110m}Ag reactions were measured at the bremsstrahlung end-point energies of 50 and 60 MeV. The activation and off-line γ-ray spectrometric technique was carried out using the 100 MeV electron linear accelerator at the Pohang Accelerator Laboratory, Korea. The ^natCd(γ, xn) reaction cross sections as a function of photon energy were theoretically calculated using the TALYS-1.95 and the EMPIRE-3.2 Malta codes. Then, the flux-weighted average cross sections were obtained from the theoretical values of mono-energetic photons. These values were compared with the flux-weighted values from the present study and were found to be in general agreement. The measured experimental reaction cross-sections and integral yields were described for cadmium and silver isotopes in the ^natCd(γ, xn)^{115g,m,111m,109,107,105,104}Cd and ^natCd(γ, x)^{113g,112,111g,110m}Ag reactions. The isomeric yield ratio (IR) of ^115g,mCd in the ^natCd(γ, xn) reaction was determined for the two bremsstrahlung end-point energies. The measured isomeric yield ratios of ^115g,mCd in the ^natCd(γ, xn) reaction were also compared with the theoretical values of the nuclear model codes and previously published literature data of the ¹¹⁶Cd(γ, n) and ¹¹⁶Cd(n, 2n) reactions. It was found that the IR value increases with increasing projectile energy, which demonstrates the characteristic of excitation energy. However, the higher IR value of ^115g,mCd in the ¹¹⁶Cd(n, 2n) reaction compared to that in the ¹¹⁶Cd(γ, n) reaction indicates the role of compound nuclear spin alongside excitation energy.     

Induced photonuclear interaction by Rhodotron-TT200 10 MeV electron beam

In this paper the photonuclear interaction induced by 10 MeV electron beam generating high-intensity neutrons is studied. Since the results depend on the target material, the calculations are performed for Pb, Ta and W targets which have high Z, in a simple geometry. MCNPX code has been used to simulate the whole process. Also, the results of photon generation has been compared with the experimental results to evaluate the reliability of the calculation. The results show that the obtained neutron flux can reach up to 10¹² n/cm²/s with average energies of 0.9 MeV, 0.4 MeV and 0.9 MeV for these three elements respectively with the maximum heat deposited as 3000 W/c³, 4500 W/c³ and 6000 W/c³.     

CdZnTe <Emphasis Type="BoldItalic">γ</Emphasis> detector for deep inelastic neutron scattering on the VESUVIO spectrometer

In this paper it is shown that solid-state cadmium-zinc-telluride (CZT) is a promising photon detector for neutron spectroscopy in a wide energy interval, ranging from thermal (25 meV) to epithermal (70 eV) neutron energies. In the present study two CZT detectors were tested as part of the inverse-geometry neutron spectrometer VESUVIO operating at the ISIS pulsed neutron source. The response of the CZT detector to photon emission from radiative neutron capture in ²³⁸U was determined by biparametric measurements of neutron time of flight and photon energy. The scattering response function F(y) from a Pb sample has been derived using both CZT and conventional ⁶Li-glass scintillator detectors. The former showed both an improved signal to background ratio and higher efficiency as compared to ⁶Li glass, allowing us to measure F(y) up to the fourth ²³⁸U absorption energy (E_r=66.02 eV). Due to the small size of CZT detectors, their use is envisaged in arrays, with high spatial resolution, for neutron-scattering studies at high energy (>1 eV) and low wavevector (q <10 Å^-1) transfers. PACS 29.30.Hs; 29.30.Kv; 29.40.Kw; 61.12.Ek     

Determination of the Products of Irradiation of Natural Molybenum By Deuterons with Energy up to 13 MeV by Means of Semiconductor Gamma- and X-Ray Spectrometry

Introduction

Among the products of irradiation of natural molýbdenum by deuterons with energies up to 13 MeV, by means γ and X-ray speclrometry were identified these radlonudldes: ⁹⁹Nb, ^92mNb, ^95m+gNb ⁹⁶Nb, ⁹⁹Mo, ¹⁰¹Mo, ⁹²Tc, ^93m+gTc, ^94m+g^>Tc, ^95m+gTc, ⁹⁶Tc, ^97mTc, ^99mTc,¹⁰¹Tc, and ⁸⁹Zr. With the more intensive lines of γ transitions of ⁹⁰Nb, ⁹⁶Nb, ^93m+gTc, ^94m+gTc, ^95m+gTc, ⁹⁹Tc and ^97mTc accurate photon energies have been established and for the lines 1475.7 and 1520.5 keV of ⁹³Tc, 820.2 keV of ^95mTc, and 96.4 keV of ^97mTc also the absolute intensities of the emitted γ were determined. By means of aluminium foils, by which the set of Mo foils had been interlaid, the atomic recoil in forward and backward direction ivas established for the 17.82 mg/cm² Mo foils and deuteron energy up to 12.7 MeV and the ion current of 3.4 μA.     

On the decay of116gIn

The decay of the 1⁺, 14 s ground state of¹¹⁶In produced by neutron capture has been reinvestigated using Ge(Li) detectors. It is found that besides the ground state of¹¹⁶Sn, excited levels at 1293.6 (2⁺), 1756.9 (0⁺), 2225.3 (2⁺), 2545.7 (0⁺, 1⁺, 2⁺), 2649.8(2⁺), 2790.7(2⁺) and 2844.6(2⁺) keV are populated, while no evidence could be obtained for the population of the 0⁺ and 2⁺ levels at 2030 and 2112.4 keV, respectively. The activation cross-section for^116m2In (2.16s,I ^π = 8^?) has been deduced as 83±8 b.