Fast Chopper Research Articles

Levels inU237studied by resonant neutron capture

Primary and secondary $\ensuremath{\gamma}$ rays in $^{237}\mathrm{U}$ from resonant neutron capture in $^{236}\mathrm{U}$ have been measured with the fast chopper facility, the neutron monochromator, and 2- and 24-keV neutron beam filters. A level scheme was constructed including 27 levels with spin and parity assignments. Rotational band members of the Nilsson configurations [631\ensuremath{\downarrow}], [622\ensuremath{\uparrow}], [624\ensuremath{\downarrow}], [631\ensuremath{\uparrow}], [633\ensuremath{\downarrow}], and [501\ensuremath{\downarrow}] and of the vibrational bands {[622\ensuremath{\uparrow}] - ${2}^{+}$} and {[631\ensuremath{\downarrow}] + ${0}^{+}$} were identified. There are more levels below 800 keV, in particular, $K=\frac{1}{{2}^{\ensuremath{-}}}$ bands, than expected from comparison with $^{235}\mathrm{U}$ and $^{239}\mathrm{U}$. The neutron binding energy in $^{237}\mathrm{U}$ was determined to be 5125.9\ifmmode\pm\else\textpm\fi{}0.5 keV.NUCLEAR REACTIONS $^{236}\mathrm{U}$($n$,$\ensuremath{\gamma}$), ${E}_{n}=5\ensuremath{-}125$ eV, 2 keV, 24 keV measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, deduced $^{237}\mathrm{U}$ levels, transitions, $J$, $\ensuremath{\pi}$, photon strengths, Ge(Li) detector, three crystal spectrometer, enriched targets, nuclear structure, Nilsson model, vibrational excitations.

A fast electrostatic chopper of low power consumption

An electrostatic chopper for the continuous beams of a 7.5 MV Van de Graaff accelerator is described. The electrostatic deflector uses complementary transistors, driven by optoelectronic couplers, as voltage switches. The power consumption of the high voltage system at 30 kHz repetition frequency is ∼ 3 W. Rise and fall times are symmetric and of the order of 0.4 μs. Experimental time spetra of prompt and delayed γ-rays are presented.

A fast interferometric chopper for neutrons and X rays

FAST neutron choppers have been considerably developed in the past few years. The commonly used choppers are either rotating massive objects through which appropriately shaped neutron channels are cut1 or crystals whose diffracting power may be modulated by magnetic fields2 or dynamic strains3. All these devices suffer from practical limitations of speed, poor duty cycle and the consequent loss of intensity. We describe here our use of a scanning Bragg reflecting crystal interferometer as a fast chopper. Although they have been used with X rays, crystal interferometers work equally well with neutrons4. The interferometer is used as a phase sensitive neutron switch, in which a crystal translation of 0.96 A is sufficient to redirect the beam. At larger translational amplitudes the interferometer has a corresponding multiplex advantage over other neutron choppers. Two of these interferometric neutron choppers could be used to construct a Fourier transform neutron spectrometer with 1,000 times the resolution of competing spectrometers. With a pulsed source a single interferometric chopper could be used to construct a time of flight spectrometer which would provide high resolution even with fast neutrons.

Thermal neutron standard field with a Maxwellian distribution using the KUR heavy water facility

A heavy water facility attached to the KUR (Kyoto University Reactor, swimming pool type, 5 MW) yields pure themal neutrons with a Maxwellian distribution. The facility is placed next to the core of KUR and contains about 2 t of heavy water. The width of the heavy water layer is about 140 cm. The neutron spectrum was measured with the time-of-flight technique using a fast chopper. The measured spectrum was in good agreement with a Maxwellian distribution in the whole energy region for thermal neutrons. The neutron temperature was slightly higher than the heavy water temperature. The contamination of epithermal and fast neutrons caused by photo-neutrons from the γ-n reaction in heavy water is very small. The maximum intensity of thermal neutrons is 3 × 10 11 n/cm·s. When a bismuth scatterer is attached, the gamma ray contamination is decreased to a ratio of 0.05 of gamma rays to neutrons in Rem. This standard neutron field has been used for such experiments as thermal neutron cross section measurement, diffusion length measurement, detector calibration, activation analysis and for biomedical purposes.

Level structure andγtransitions inHg202studied by the (n, γ) reaction

Several ($n, \ensuremath{\gamma}$) techniques at thermal and resonant neutron energies have been used to investigate the level scheme of $^{202}\mathrm{Hg}$. Low and high energy ($n, \ensuremath{\gamma}$) spectra have been measured at the Brookhaven National Laboratory high flux beam reactor using Ge(Li) detectors. Two parameter coincidence spectra and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ angular correlation data were taken. Primary transitions from resonance capture in $^{201}\mathrm{Hg}$ were studied with the fast chopper facility at BNL. The combination of these data led to the construction of a detailed level scheme up to about 2 MeV. Twenty-two excited states were found, most of which could be given spins or spin limits. As in $^{200}\mathrm{Hg}$ a systematic behavior of the level population as a function of energy and spin was observed and could be used for spin assignments. Most of the levels, and in particular their $\ensuremath{\gamma}$ deexcitations, were not accurately known previously. Of special interest is the observation that the lowest excited ${0}^{+}$ state is at 1564 keV. Another ${0}^{+}$ state was discovered at 1643 keV. These findings shed new light on the "extra" ${0}^{+}$ state in $^{200}\mathrm{Hg}$ at 1029 keV. The systematics of level energies and branching ratios in the Hg isotopes is treated and is found to be suggestive of significant nuclear structure differences as a function of neutron number. In particular, a comparison with model calculations for core-coupled states revealed extensive disagreements for $^{202}\mathrm{Hg}$, in contrast to the rather satisfactory comparison found in $^{200}\mathrm{Hg}$.NUCLEAR REACTIONS $^{201}\mathrm{Hg}(n, \ensuremath{\gamma})$, $E=\mathrm{thermal}, 43, 70.9, \mathrm{and} 210$ eV. Measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ $\mathrm{coin}(\ensuremath{\bigominus})$ in $^{202}\mathrm{Hg}$. $^{202}\mathrm{Hg}$ deduced levels, transitions, $J$, $\ensuremath{\pi}$.

Beta decay ofB14

A target of BeO, enriched to 94% in $^{10}\mathrm{Be}$, has been bombarded with 31-MeV $^{6}\mathrm{Li}$ ions, and delayed activities were measured by means of a fast beam chopper and a NaI(Tl)-plastic $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\beta}$ coincidence detecting system. $\ensuremath{\gamma}$ rays corresponding to the known activities $^{11}\mathrm{Be}$, $^{12}\mathrm{B}$, $^{13}\mathrm{B}$, $^{16}\mathrm{N}$, $^{20}\mathrm{F}$, and $^{20}\mathrm{Na}$ were observed in coincidence with $\ensuremath{\beta}$ rays. In addition, the spectrum contained $\ensuremath{\gamma}$ rays of 6.1 and 6.7 MeV decaying with ${T}_{\frac{1}{2}}=16.1\ifmmode\pm\else\textpm\fi{}1.2$ msec. The 6.1-MeV $\ensuremath{\gamma}$ rays are in coincidence with $\ensuremath{\beta}$ rays having ${E}_{max}=14.0\ifmmode\pm\else\textpm\fi{}0.7$ MeV. This activity is assigned to $^{14}\mathrm{B}$ produced in the $^{10}\mathrm{Be}(^{6}\mathrm{Li},2p)^{14}\mathrm{B}$ reaction. Based on the observed $\ensuremath{\gamma}$-ray intensities the $^{14}\mathrm{B}$ $\ensuremath{\beta}$-ray branches to $^{14}\mathrm{C}$ states at 6.09 (${1}^{\ensuremath{-}}$), 6.73 (${3}^{\ensuremath{-}}$), and 7.34 (${2}^{\ensuremath{-}}$) MeV are 81 \ifmmode\pm\else\textpm\fi{} 9%, ${8.6}_{\ensuremath{-}4.0}^{+1.7}$%, and 11%, respectively, where an allowance of 5% has been made for a probable ground-state $\ensuremath{\beta}$-ray branch. It is deduced that $^{14}\mathrm{B}$ has a spin-parity of ${J}^{\ensuremath{\pi}}={2}^{\ensuremath{-}}$ consistent with recent shell-model calculations. The experimental $log\mathrm{ft}$ values for the \ensuremath{\beta} decay of $^{14}\mathrm{B}$ are also in good agreement with theory.RADIOACTIVITY $^{14}\mathrm{B}$: measured ${T}_{\frac{1}{2}}$, ${E}_{\ensuremath{\gamma}}$, ${E}_{\ensuremath{\beta}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\beta}$ coin; deduced ${J}^{\ensuremath{\pi}}$ of $^{14}\mathrm{B}$, $\ensuremath{\beta}$ branches, $log\mathrm{ft}$ values; compared with theory.

Radiative decay of neutron resonances inGe73(n,γ)Ge74

$\ensuremath{\gamma}$ rays following neutron capture in natural germanium have been studied up to ${E}_{n}\ensuremath{\approx}500$ eV with the fast chopper at the Brookhaven high flux beam reactor. Up to this energy only resonances in $^{73}\mathrm{Ge}$ contribute. From the high-energy $\ensuremath{\gamma}$ rays observed, levels in $^{74}\mathrm{Ge}$ are identified to an excitation energy of about 3 MeV. From the low-energy $\ensuremath{\gamma}$ rays depopulating the $^{74}\mathrm{Ge}$ levels, spin assignments for several $^{73}\mathrm{Ge}$ resonances are made; namely 102 eV, ${J}^{\ensuremath{\pi}}={4}^{+}$; 204 eV, ${J}^{\ensuremath{\pi}}={5}^{+}$; and 224 eV, ${J}^{\ensuremath{\pi}}={4}^{+}$. The results of the resonance capture experiments are compared to previous "average-resonance" and thermal-capture work, and in general are found to be in good agreement. Average radiative strengths for $M1$ and $E2$ transitions are calculated and compared to model estimates.[NUCLEAR REACTIONS $^{73}\mathrm{Ge}(n,\ensuremath{\gamma})^{74}\mathrm{Ge}$, in resonances to 400 eV; energy levels in $^{74}\mathrm{Ge}$, ${J}^{\ensuremath{\pi}}$, resonance spins, ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\lambda}f}$; photon strength functions.]

Low-spin states inHg200studied with the (n,γ) reaction

The level scheme of $^{200}\mathrm{Hg}$ has been studied by thermal and resonance neutron capture. The low-energy region has been measured with the high-resolution bent-crystal spectrometer at Ris\o{}, Denmark. The medium- and high-energy $\ensuremath{\gamma}$ rays, coincidence spectra, and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ angular correlations were measured at the Brookhaven National Laboratory (BNL) high flux beam reactor. Primary transitions from resonance capture in $^{199}\mathrm{Hg}$ were studied with the fast chopper facility at BNL. Out of \ensuremath{\sim}520 observed transitions, \ensuremath{\sim}330 were placed in a level scheme containing 60 levels below 3.3 MeV. Many of these states are new and a great number of new ${I}^{\ensuremath{\pi}}$ assignments was made. In particular ${I}^{\ensuremath{\pi}}={0}^{+}$ was assigned to a level at 1515 keV. Four ${1}^{\ensuremath{-}}$ states between 2.4 and 3 MeV are suggested. Possible explanations of their low excitation energy are offered. The simplest require and therefore suggest oblate shapes in $^{200}\mathrm{Hg}$, at least for excitations involving the unique parity orbitals. A very systematic behavior of the level feeding by nonprimary transitions was found in this nucleus and was compared to simple statistical predictions. A detailed comparison was made with model calculations for core-coupled proton states. The most likely candidates for these states are: ${2}_{1}^{+}$ 368 keV, ${2}_{2}^{+}$ 1254 keV, ${2}_{3}^{+}$ 1573 keV, ${0}_{2}^{+}$ 1515 keV, ${1}_{1}^{+}$ 1570 keV, ${3}_{1}^{+}$ 1659 keV. The possibility of a reduced central density of $^{200}\mathrm{Hg}$ is discussed.[NUCLEAR REACTIONS $^{199}\mathrm{Hg}(n,\ensuremath{\gamma})$, $E=\mathrm{thermal},33.5,129.7,\mathrm{and} 175.1$ eV measured ${I}_{\ensuremath{\gamma}}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma} \mathrm{coin}(\ensuremath{\theta})$ in $^{200}\mathrm{Hg}$. $^{200}\mathrm{Hg}$ deduced levels, transitions, $J$, $\ensuremath{\pi}$, ICC, multipolarities, $\ensuremath{\delta}$.]

Total Neutron Cross Section and Resonance Parameters of Promethium-147

The total neutron cross section of 147Pm has been measured from 0.01 to 1000 eV, using compacted oxide samples in the Materials Testing Reactor (MTR) fast chopper. After correcting the data for decay product 147Sm content and water contaminant bias, the total cross section of 147Pm at 0.0253 eV was found to be b. Resonance parameters were determined up to 250 eV by shape-fitting and area analysis, and a negative energy resonance has been postulated to fit the data at thermal. From these parameters a resonance absorption integral of 2192 ± 100 b and a neutron s-wave strength function have been obtained.

Energy levels of 56Mn

The low-energy spectrum of the 55Mn(n, γ) 56Mn reaction has been studied with a γ-diffraction spectrometer. These data allowed the construction of a level scheme for 56Mn with two previously unobserved doublets. High-energy γ-transitions to the low-energy states have been measured for different neutron resonances at the BNL fast neutron chopper. For most states a unique spin and parity could be proposed. A shell-model calculation of the structure of the low-lying states of 56Mn has been carried out with the Oak Ridge-Rochester computer programme; there are plausible theoretical analogues for 11 of the 12 experimentally observed states. Calculated spectroscopic factors and M1 transition probabilities are in qualitative agreement with the experiment.

Total Neutron Cross Section of 242Pu

The total neutron cross section of 242Pu has been measured from 0.0015 to 8000 eV, using PuO2 powder samples in the Materials Testing Reactor (MTR) fast chopper. The data give 26.9 ± 2.0 and 18.5 ±...

A study on the arrangement of slits for fast chopper

The arrangement of slits for fast chopper has been improved in order to simplify the time dependency of its background and also to stengthen the slits against the induced centrifugal force.

The growth of laser oscillations at high excitation

The buildup of single-mode laser oscillations is investigated for the 0.633-μ transition of a helium-neon laser at levels of relative excitation significantly above threshold ( 1.5 \leq n \leq 4 ) using a fast intra-cavity chopper. The time response agrees with theory. The evolution of the signal provides information on the saturation mechanism and allows a very simple determination of resonator quality, relative excitation, unsaturated gain, and degree of saturation of a laser. The saturation parameter is found to depend on the discharge current. This is attributed to a variation of the gas density in the capillary discharge tube caused by gas heating, which affects the collision-dominated atomic lifetimes.

A lithium glass detector bank for neutron time-of-flight measurements

A lithium glass detector bank for the fast chopper facility at the Geesthacht research reactor (FRG-1) is described.

Neutron capture in 238U

The γ-ray spectrum following slow neutron capture in 238U has been measured as a function of incident neutron energy at the Brookhaven HFBR fast neutron chopper. The decay scheme proposed by previous (d, p) and high resolution thermal (n, γ) measurements has been substantially verified. Partial radiation widths for s-wave resonances up to 117 eV have been deduced. By using a simple multi-level analysis, the spectra between resonances from 21 eV down to thermal have been fitted. The behavior of the strong transitions to low-lying l = 1 states is explained using a direct capture model, while the transitions to low-lying collective states can be explained by level-level interference. The distribution of El transition strengths is not consistent with a χ 2 distribution of one degree of freedom.

Pulsed LINAC Neutron Diffraction

WE have successfully tested a new method of obtaining neutron diffraction patterns from polycrystalline specimens using a pulsed linear accelerator (LINAC) as a neutron source in conjunction with a time-of-flight (TOF) detection system. The TOF technique for neutron diffraction has been used by various investigators since the pioneering studies by Buras et al.1–4, and its advantages for studies at high pressure have been demonstrated recently by Brugger et al.5,6. All the previous applications of the TOF technique to neutron diffraction have used a reactor as a neutron source, and generally the pulsing of neutrons has been achieved by means of a fast chopper. In our application, however, the normal pulsed operation of the accelerator eliminates the need for an elaborate and expensive chopper. Furthermore, it has been found that the flux of usable neutrons provided by an accelerator such as the Rensselaer LINAC (Troy, New York) is quite high, as judged by the satisfactory diffraction pattern for an NiO specimen (Fig. 1) obtained in only 4 h in less than optimum conditions (the main features of the pattern could be discerned in the first 2 min of operation). Accelerator neutron sources7 have been used for a variety of other purposes, but there has been no report of their application to neutron diffraction studies of polycrystalline substances.

Pulsed neutron measurements for beryllium in the range of large bucklings

Using the fast chopper as a pulsed neutron source the behaviour of thermal neutron flux decay in beryllium has been studied in the buckling range of 0·0838−0·285 cm −2. The measurements have been performed for cubic or near-cubic geometry. The neutron diffusion parameters have been obtained. Some problems connected with pulsed measurements in beryllium are discussed.

The Total Neutron Cross Section of 115-Day Tantalum-182 from 0.01 to 1000 eV

The total neutron cross section of 182Ta has been measured from 0.01 to 1000 eV using the MTR fast chopper. The observed total neutron cross section at 0.0253 eV is 8200 ± 600 b. Parameters of indi...

About the determination of slow-neutron resonance parameters

Methods of analysis of neutron transmission data in time-of-flight spectrometry are examined, with particular attention to the experimental conditions which should be satisfied for a correct estimation of resonance parameters and errors. To improve the applicability of the shape-analysis method in the work performed with the fast neutron chopper of Ispra, the resolution function of this spectrometer was determined in a detailed way. The main features of the analysis methods applied to the experimental data obtained at Ispra are presented and illustrated by examples.

Total Neutron Cross Section of 232U

The total neutron cross section of 232U has been measured from 0.01 to 10 000 eV using the Materials Testing Reactor (MTR) fast chopper. A 2200 m/sec total neutron cross section of 163 ± 10 b was d...