Status of the cyric neutron TOF facilities upgrade

We describe progress in the Tohoku neutron time-of-flight (TOF) facilities improved to facilitate high resolution measurements for neutrons in the range 10 ≤ E_n ≲ 60 MeV. Efforts have been concentrated on completing the neutron detection system consisting of twelve neutron detectors, in which 23 1 of NE213 liquid scintillator are encapsulated, and a CAMAC-based data acquisition system. A systematic study of the neutron detection efficiency has been performed by Monte Carlo calculation for monochromatic neutrons with E_n ≲ 34 MeV. Results have been tested by counting neutrons from the ⁷Li(p, n)⁷Be reaction and comparing the yield with the absolute neutron fluence determined by activation.     

Response of a BaF2 scintillation detector to quasi-monoenergetic fast neutrons in the range of 45 to 198 MeV

We have studied the neutron response of a scintillation detector consisting of a 14 cm long, hexagonal-shaped BaF₂-crystal with an inner diameter of 8.75 cm coupled to an EMI9821QB photomultiplier tube. The detector was exposed to calibrated quasi-monoenergetic neutron fields obtained from ⁷Li(p,n)⁷Be reactions. The measurements were performed at neutron energies of 45, 60, 96, 147 and 198 MeV as given by the energies of the incident protons. The experimental pulse-height spectra of the BaF₂-detector are compared with Monte Carlo simulations using the FLUKA code. The detection efficiency of the BaF₂-detector in the energy range of 45–198 MeV was determined as a function of the discriminator threshold and compared to the literature data. At neutron energies above 100 MeV the detection efficiency of the BaF₂-detector was found to be a factor of two higher than that of an NE213-detector of comparable size.     

Crack tip stress effect on delayed hydride cracking velocity of Zr–2.5Nb tubes

Using our new delayed hydride cracking (DHC) model, we scrutinized Pan's DHC test results of a cold-worked Zr–2.5Nb tube after neutron irradiation. DHC velocity at 240 °C of the Zr–2.5Nb tube increased gradually at neutron fluences below 5 × 10²⁵ n/m² (E > 1 MeV) and leveled off to a constant value at higher neutron fluences over it. This neutron fluence dependence of the DHC velocity was found to be similar to that of the Nb concentration in the β-Zr, not that of its tensile stress, which is hard to understand in view of the old DHC models. Normalization of the DHC velocity by hydrogen diffusivity or D_H is found to lessen the apparent yield stress effect on the DHC velocity of Zr–2.5Nb tubes. Consequently, it is demonstrated that the DHC velocity of the Zr–2.5Nb tubes is governed not by the crack tip stresses but by hydrogen diffusion, corroborating the validity of the new DHC model.     

Neutron detection efficiency of a thick lithium glass detector

The neutron detection efficiency of a 9.55 mm thick NE-912 lithium glass scintillator has been determined for the energy range from 25 keV to 2 MeV. It was measured relative to a thin, 0.835 mm thick NE-908 glass, which was determined by Monte Carlo calculations. The measured efficiency curve shows the marked effect of the ¹⁶O resonance at 442 keV, and a strong increase with energy for E_n > 1.2 MeV, due to (n,n′γ) reactions.     

The 9Be(d,n)10B-reaction as intense neutron source with continuous energy spectrum

Neutron energy spectra produced by deuterons of 3 to 8 MeV in a thick ⁹Be-target were measured at various scattering angles. Significant angle dependences were observed. Angular distributions of the most energetic neutrons produced in thin ⁹Be targets can be described quantitatively in DWBA, which is an indication for a direct reaction mechanism. As a consequence all but 0°-neutrons are polarized to a certain extent. Also presented is the neutron energy spectrum of ⁷Li(d,n)⁸Be at 0° produced in a thick ⁷Li-target. The potential of these intense 0°-neutron beams with continuous energy distributions is demonstrated by a measurement of the neutron absorption cross section of natural carbon.     

Measurement of neutron energy spectra from 15 to 150 MeV using stacked liquid scintillators

A multiple liquid scintillator system for measuring the energy spectrum of a neutron beam in the range 15–150 MeV is described. Two or more slabs of NE213 scintillator (13×13×7 cm³) are stacked behind one-another and only events in which a neutron interacts in the upstream scintillator are analysed. The system is designed to minimise the escape of forward recoil protons from the detecting media. Test measurements and Monte Carlo simulations of the detector response to quasi-monoenergetic neutron beams of energies 62.5 and 97.5 MeV are presented.     

Beryllium neutron activation detector for pulsed DD fusion sources

A compact fast neutron detector based on beryllium activation has been developed to perform accurate neutron fluence measurements on pulsed DD fusion sources. It is especially well suited to moderate repetition-rate (<0.2 Hz) devices, such as the plasma focus or Z-pinch. The detector comprises a beryllium metal sheet sandwiched between two large-area xenon-filled proportional counters. A methodology for calculating the absolute response function of the detector using a “first principles” approach is described. This calibration methodology is based on the ⁹Be(n,α)⁶He cross-section, energy calibration of the proportional counters, and numerical simulations of neutron interactions and beta-particle paths using MCNP5. The response function R(E_n) is determined over the neutron energy range 2-4 MeV. The count rate capability of the detector has been studied and the corrections required for high neutron fluence measurements are discussed. For pulsed DD neutron fluencies >3×10⁴ cm⁻², the statistical uncertainty in the fluence measurement is better than 1%. A small plasma focus device has been employed as a pulsed neutron source to test two of these new detectors, and their responses are found to be practically identical. Also the level of interfering activation is found to be sufficiently low as to be negligible.     

Evaluation of large deuterated scintillators for fast neutron detection (E=0.5-20 MeV) using the D(d,n)He, C(d,n) and Al(d,n) reactions

The response of large deuterated liquid scintillators (up to 10 cm diameter by 15 cm) to neutrons 0.5-20 MeV has been studied using the 2.5 MeV neutron generator at the University of Michigan, and the d(d,n), ¹³C(d,n), ²⁷Al(d,n) and other reactions at the University of Notre Dame FN tandem accelerator. The latter utilize 9 and 16 MeV deuteron beams including a pulsed beam, which permitted time-of-flight measurements. Combining pulse-shape discrimination and time-of-flight allows gating on specific neutron energy groups to determine the detector response to specific neutron energies. This will permit accurate simulation of the detector response functions for applications of these detectors in nuclear research and homeland security applications.     

Improved response function calculations for scintillation detectors using an extended version of the MCNP code

The analysis of (e,e′n) experiments at the Darmstadt superconducting electron linear accelerator S-DALINAC required the calculation of neutron response functions for the NE213 liquid scintillation detectors used. In an open geometry, these response functions can be obtained using the Monte Carlo codes NRESP7 and NEFF7. However, for more complex geometries, an extended version of the Monte Carlo code MCNP exists. This extended version of the MCNP code was improved upon by adding individual light-output functions for charged particles. In addition, more than one volume can be defined as a scintillator, thus allowing the simultaneous calculation of the response for multiple detector setups. With the implementation of ¹²C(n,n′3α) reactions, all relevant reactions for neutron energies E_n<20 MeV are now taken into consideration. The results of these calculations were compared to experimental data using monoenergetic neutrons in an open geometry and a ²⁵²Cf neutron source in the complex Darmstadt setup, where in both cases excellent agreement was found.     

A detection system for energetic light heavy ions

A light heavy ion detection system which consists of a gas-filled ionization chamber (IC) connected to a scattering chamber via a time-of-flight (TOF) system has been constructed. The entrance window of the IC has an area of 14 × 40 cm², the active depth is 115 cm. Filled with CF₄ at a pressure of 350 Torr, the energy range for ¹²C and ⁴⁰Ar is 5–20 MeV/A and 6–30 MeV/A, respectively. The TOF system consists of two parallel plate avalanche counters with a flightpath of 70 cm in between. The IC has been tested with ¹²C ions at an energy of 39 MeV. The energy resolution of the IC (1.1%) is mainly determined by the energy straggling in the foils of the TOF system and the ionization chamber. The energy-loss resolution is 3.5%, the horizontal position resolution varies between 6 and 20 mm and the vertical position resolution is 2 mm. The time resolution of the TOF system ranges from 800 ps for ⁴He at 5.0 MeV, to 280 ps for ²⁸Si at 55 MeV.     

14 MeV neutron irradiation effects on cryostability of a composite superconductor for a fusion reactor

The neutron irradiation effects on cryostability of composite superconductors for fusion reactors are studied based on Maddock's condition. In particular, to estimate the effects of 14 MeV neutrons we assumed that the irradiation-induced degradation of critical temperature, critical current density and conductivity of stabilizer are determined by the damage energy depending on the neutron energy spectrum. The cryostability is found to decrease sensitively with increasing the fraction α of fusion neutrons with energy of 10 ～ 14 MeV to the total neutrons, ie, the Cu/superconductor ratio R_ns, to stabilize the conductor, must be increased remarkably with increasing α as well as the total dose of the neutron fluence. For the small R_ns (～4) the stabilized overall current density decreases by several ten percents even at the fluence when T_c and J_c change by only a few percent. This effect is dominated by the severe increase of ρ.     

Efficiency measurements of deuterated liquid scintillators using d(d,n)He coincidence events

Deuterated liquid scintillators are promising from the perspective of n/γ separation and neutron energy measurement without time-of-flight. In order to turn these into precision instruments for nuclear physics, however, the absolute efficiency must be known. The efficiency is typically a function of energy and is strongly dependent on the electronics thresholds used with the photomultiplier detectors. This study involves the extraction of this efficiency using the d(d,n)³He reaction (E_d=16.0 MeV) and n-³He coincidence events. The detection and identification of the ions are very efficient and allows one to project pure monoenergetic neutron spectra all the way down to the electronics threshold. Timing information was also recorded in these measurements to provide further constraints and a consistency check.     

The response function of organic scintillator NE213 to 59.5 MeV neutrons

The response function of a 12.7 cm diameter × 12.7 cm cell of an organic liquid scintillator NE213 has been studied at E_n = 59.5 MeV. In the pulse shape spectra bands due to deuterons, besides protons and alpha particles produced by neutron interactions are identified. In order to discriminate the contribution from the ¹²C (n, np) reaction, a coincidence experiment between the protons in the scintillator and the escaping neutrons from the scintillator was made. To confirm the deuteron band further, a liquid scintillator cell was directly bombarded by a faint deuteron beam with an energy of 56 MeV. The response functions for individual bands are very different from the calculated response using the Monte Carlo method.     

Development of a quasi-monoenergetic neutron field using the 7Li(p,n)7Be reaction in the energy range from 250 to 390 MeV at RCNP

A quasi-monoenergetic neutron field using the (7)Li(p,n)(7)Be reaction has been developed at the ring cyclotron facility at the Research Center for Nuclear Physics (RCNP), Osaka University. Neutrons were generated from a 10-mm-thick Li target injected by 250, 350 and 392 MeV protons and neutrons produced at 0 degrees were extracted into the time-of-flight (TOF) room of 100-m length through the concrete collimator of 10 x 12 cm aperture and 150 cm thickness. The neutron energy spectra were measured by a 12.7-cm diam x 12.7-cm long NE213 organic liquid scintillator using the TOF method. The peak neutron fluence was 1.94 x 10(10), 1.07 x 10(10) and 1.50 x 10(10) n sr(-1) per muC of 250, 350 and 392 MeV protons, respectively. The neutron spectra generated from various thick (stopping length) targets of carbon, aluminium, iron and lead, bombarded by 250 and 350 MeV protons, were also measured with the TOF method. Although these measurements were performed to obtain thick target neutron yields, they are also used as a continuous energy neutron field. These neutron fields are very useful for characterising neutron detectors, measuring neutron cross sections, testing irradiation effects for various materials and performing neutron shielding experiments.     

Evaluation of spectrum measurement devices for operational use

Several neutron spectrometers manufactured by Bubble Technology Industries (BTI) were tested and evaluated in a variety of neutron fields. Findings and conclusions are presented for the following BTI instruments: a modification of the Rotational Spectrometer (ROSPEC) that includes a thermal and epithermal capability, the Simple Scintillation Spectrometer that is used in conjunction with the ROSPEC to extend its high-energy range, and the MICROSPEC N-Probe which is capable of providing a crude spectrum over the energy range from thermal to 18 MeV. The main objective of these measurements was to determine the accuracy of both the energy spectrum and dose equivalent information generated by these devices. In addition, the dose response of the Wide-Energy Neutron Detection Instrument (WENDI-II) was measured in all neutron fields relative to a bare ²⁵²Cf calibration. The performance of the WENDI-II rem meter was compared to the dose information generated by the neutron spectrometers. The instruments were irradiated to bare ²⁵²Cf and ²⁴¹AmBe sources, and in a series of moderated ²⁵²Cf fields using a standard D₂O sphere and a set of polyethylene spheres. The measured spectra were benchmarked with a set of detailed Monte Carlo calculations with the same energy bin structure as that of the instruments under test. These calculations allowed an absolute comparison to be made with the measurements on a bin by bin basis. The simulations included the effects of room return and source anisotropy.     

Neutron Depth Profiling: Overview and Description of NIST Facilities

The Cold Neutron Depth Profiling (CNDP) instrument at the NIST Cold Neutron Research Facility (CNRF) is now operational. The neutron beam originates from a 16 L D₂O ice cold source and passes through a filter of 135 mm of single crystal sapphire. The neutron energy spectrum may be described by a 65 K Maxwellian distribution. The sample chamber configuration allows for remote controlled scanning of 150 × 150 mm sample areas including the varying of both sample and detector angle. The improved sensitivity over the current thermal depth profiling instrument has permitted the first nondestructive measurements of ¹⁷O profiles. This paper describes the CNDP instrument, illustrates the neutron depth profiling (NDP) technique with examples, and gives a separate bibliography of NDP publications.     

A neutron polarimeter for the (p,n) reaction at Ep=50–80 MeV

A conventional type of neutron polarimeter consisting of three 12.5 × 12.5 cm cylindrical liquid scintillators and two 12.5 × 12.5 cm cylindrical plastic scintillators has been used to measure the transverse polarization transfer coefficient K_y^y in the (p, n) reaction in the energy range 50 < E_{p < 80 MeV}. The effective analyzing powers of this polarimeter have been obtained empirically by observing neutrons from the ⁶Li(p, n)⁶Be reaction at θ = 0° and E_p = 50,65 and 80 MeV whose polarizations are deduced from the K_y^y values of the ⁶Li(p,p′)⁶Be¹ (0⁺ reaction by utilizing the analogue relation. Observed effective analyzing powers are significantly reduced particularly at high energy, most likely due to the ¹²C(n,np)¹¹B reaction process. The double scattering efficiencies are 6 × 10⁻⁴ and 3 × 10⁻⁴ with effective analyzing powers of 0.14 and 0.11 at E_{n = 45} and 75 MeV, respectively.     

Measurement of the γ-anisotropy in

The study of the radiative neutron capture by protons, n+p→d+γ, provides valuable information about the nucleon–nucleon interaction. So far, no experimental value has existed for the γ-anisotropy which may appear if neutrons and protons both are polarised. A non-vanishing γ-anisotropy η is a clear-cut signal for the existence of transitions ³S₁→ ³d₁ from the triplet initial state to the ground state of the deuteron. We report the first measurement of this observable. The result is η=(1.0±2.5)×10⁻⁴ at 50.5% polarisation of neutrons and protons.     

High-Precision Determination of the Neutron Coherent Scattering Length

The neutron coherent scattering length b_c has been determined interferometrically to an uncertainty of about 5 × 10⁻⁵ by measuring the nondispersive phase. We propose improving the uncertainty to about 10⁻⁶ by optimizing various parameters of the interferometric experiment. Any uncertainty in the b_c determination arising from possible variations in the constitution of the ambient air can be eliminated by performing the experiment in vacuum. When such uncertainty is attained, it becomes necessary to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correct b_c from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimate b_c. The refractive index for neutrons can thus be determined to a phenomenal uncertainty of about 10⁻¹².     

Superheated emulsions in neutron spectrometry by varying ambient pressure

The principle of present work lies on the dependence of the threshold neutron energy on the dimensionless quantity “degree of metastability (ss)” of superheated liquids. The response of the superheated emulsions consists of the drops of superheated liquid (C₂Cl₂F₄, b.p. 3.77 °C) has been measured at different ‘ss’ by varying ambient pressure at different temperatures, in the presence of neutrons generated in Pb by a (γ,n) reaction from 45 MeV electron LINAC of Hokkaido University. To unfold the neutron energy spectrum, a relationship has been developed between the ‘ss’ of superheated liquids and the threshold neutron energy. The spectrum at the detector position has been calculated by the MCNP code and a comparison has been made with the experimental spectrum. The utilisation of ‘ss’ is more flexible as this relation can be applied to both positive and negative ambient pressures as well as at different ambient temperatures.