Back-n white neutron facility at CSNS and first-year nuclear data measurements

Back-streaming neutrons through the incoming proton channel at the spallation target station of China Spallation Neutron Source (CSNS) were exploited to build a white neutron beam line (the so-called Back-n). With a proton beam of 100 kW in beam power and 1.6 GeV in kinetic energy and a thick tungsten target and modest moderation by the cooling water through the target slices, the neutron beam is very intense which is in the order of 7.0×106 n/cm2/s at 77 m from the target and has an excellent energy spectrum spanning from 0.5 eV to 200 MeV. In addition, the time resolution related to the time-of-flight measurements is very good for neutron energy determination. Altogether, it makes the CSNS Back-n one of the best white neutron sources in the world and very suitable for nuclear data measurements, especially for neutron-induced cross-section measurements. Since the completion of the Back-n beamline and four physics spectrometers in March 2018, the first batches of experiments on nuclear data measurements have been carried out, which are summarized in this article.

Measurement of the energy spectrum of secondary neutrons in a proton therapy environment

Measurement of the energy spectrum of secondary neutrons were carried out at the OncoRay Proton Therapy facility in Dresden, following an approach originating in neutron metrology which is well suited for both the characterization of secondary neutron fields at proton therapy facilities and the validation of Monte Carlo simulations. For the experiment, a brass target was placed in the proton beam and Bonner spheres measurements were made at a distance of 2 m from the target and at different angles, 15° to 120°, with respect to the incoming proton beam. The measured spectra were compared to Monte Carlo simulations.

Relativistic proton-impact excitation of hydrogen atom in the presence of intense laser field

A theoretical treatment, using the first Born approximation, is presented to analyse the results of relativistic laser-assisted proton – hydrogen atom scattering. Specific calculations are carried out for excitation of hydrogen atoms from 1s1/2 to 2s1/2 states by proton impact. We work in an approximation in which the incoming proton may be described by Dirac–Volkov states in the presence of a laser field. Semi-relativistic Darwin wave functions are used to describe the hydrogen atom in its initial and final states, while relativistic, spin, and laser interaction effects are also accounted for. The results presented in this paper show that the differential cross section for this process depends not only upon the energy of the incident proton, but also upon its interaction with the laser field through intensity and frequency.

AnomaloustqγCouplings inγpCollision at the LHC

We have examined the constraints on the anomaloustqγ (q=u,c)couplings through the processpp→pγp→pWbXat the LHC by considering four forward detector acceptances:0.0015<ξ<0.5,0.0015<ξ<0.15,0.015<ξ<0.15, and0.1<ξ<0.5, whereξ=Eγ/EwithEγandEthe energies of the photon and of the incoming proton, respectively. The sensitivity bounds on the anomalous couplings have been obtained at the 95% confidence level in a model independent effective Lagrangian approach. We have found that the bounds on these couplings can be highly improved compared to current experimental bounds.

Zippers: Base Catalysis

A base, you should recall from Reaction 2, is the second hand clapping to the acid’s first. That is, whereas an acid is a proton donor, a base is its beneficiary as a proton acceptor. The paradigm base is a hydroxide ion, OH–, which can accept a proton and thereby become H2O. However, in the context of catalysis, the topic of this section, its role is rather different: instead of using its electrons to accept the proton, it uses them to behave as a nucleophile (Reaction 15), a searcher out of positive charge. Instead of forming a hydrogen–oxygen bond with an incoming proton, it sets the electronic fox among the electronic geese of a molecule by forming a new carbon–oxygen bond and thereby loosening the bonds to neighbouring atoms so that they can undergo rearrangement. The OH– ion in effect unzips the molecule and renders it open to further attack. Base catalysis has a lot of important applications. An ancient one is the production of soap from animal fat. To set that scene, I shall consider a simple model system, the ‘hydrolysis’ (severing apart by water) of the two components of an ester, 1 (the same compound I used in Reaction 17, a combination of acetic acid and ethanol), and then turn to soap-making itself. You saw in Reaction 17 how esters can be broken down into their components, a carboxylic acid and an alcohol, by an acid; here we see the analogous reaction in the presence of a base. To be specific, the reagent is a solution of sodium hydroxide, which provides the OH– ions that catalyse the reaction. We watch what happens when a solution of sodium hydroxide is added to an ester and the mixture is boiled. The O oxygen atoms of the ester have already ripened the molecule for nucleophilic attack by drawing some of the electron cloud away from the C atom to which they are both attached, leaving it with a partial positive charge, 2. The negatively charged OH– ion sniffs out that positive charge and jostles in to do its business.

Synthesis of the New 2-Alkyl-nido-2,7,10-C3B8H11 Tricarbaborane by Protonation of [7-Alkyl-nido-7,8,10-C3B8H10]-: A Reversible Cage-Carbon Rearrangement

Protonation of the [7-R-nido-7,8,10-C3B8H10]- (where R = PhCH2 (1a) or R = Me (1b)) tricarbollide anion, with concentrated H2SO4 in a two-phase aqueous/CH2Cl2 system, yields the new neutral tricarbaborane: 2-R-nido-2,7,10-C3B8H11 (where R = PhCH2 (2a) or R = Me (2b)). The three cage-carbons of the [7-R-nido-7,8,10-C3B8H10]- anion are located on the open face, but spectroscopic and DFT/GIAO/NMR studies of 2a and 2b show that during the protonation reaction, isomerization of the cage framework occurs to produce the neutral tricarbaborane having a 2,7,10-structure in which only two of the carbons remain on the open face. The third (R-substituted) carbon adopts a five-coordinate vertex off of the open face, thus enabling the incoming proton to adopt a bridging position on the B-B edge of the new C2B3-open face. The skeletal rearrangement is reversible, since deprotonation of 2a or 2b regenerates the anions 1a and 1b, respectively, having the 7,8,10-configuration. In agreement with the experimentally observed structures of the anionic (7,8,10-structure) and neutral (2,7,10-structure) species, DFT calculations at the B3LYP/6-311G*-level show that the [7-Me-nido-7,8,10-C3B8H10]- anion (1b, structure 16) is 28.9 kcal/mol more stable than the [2-Me-nido-2,7,10-C3B8H10]- isomer (3b, structure 18), while for the neutral tricarbaborane, the 2-R-nido-2,7,10-C3B8H11 (2b, structure 14) structure is more stable than any 7,8,10-structure (structures 7-11) which has the added proton in an endo position on the open face. Transition state calculations at the HF/6-31G*-level yielded a simple, low-energy pathway (activation barrier of only 6.5 kcal/mol for the transition state TS18/16) for the rearrangement of [2-Me-nido-2,7,10-C3B8H10]- (3b, structure 18) to [7-Me-nido-7,8,10-C3B8H10]- (1b, structure 16) requiring the movement of only one cage atom, B11, from its original position in the C7-B8-B9-C10-B11 plane of 3b, to the C7-C8-B9-C10-B11 plane of 1b.

Mechanistic and stereochemical studies on 3-oxo steroid Δ4-Δ5-isomerase from human placenta

The mechanism of isomerization of delta 5-3-ox steroids to delta 4-3-oxo steroids was examined by using the membrane-bound 3-oxo steroid delta 4-delta 5-isomerase (EC 5.3.3.1) and the 3 beta-hydroxy steroid dehydrogenase present in the microsomal fraction obtained from full-term human placenta. (1) Methods for the preparation of androst-5-ene-3 beta, 17 beta-diol specifically labelled at the 4 alpha-, 4 beta- or 6-positions are described. (2) Incubations with androst-5-ene-3 beta, 17 beta-diol stereospecifically 3H-labelled either in the 4 alpha- or 4 beta-position showed that the isomerization reaction occurs via a stereospecific elimination of the 4 beta hydrogen atom. In addition, the complete retention of 3H in the delta 4-3-oxo steroids obtained from [4 alpha-3H]androst-5-ene-3 beta, 17 beta-diol indicates that the non-enzymic contribution to these experiments was negligible. (3) To study the stereochemistry of the insertion of the incoming proton at C-6, the [6-3H]androst-4-ene-3, 17-dione obtained from the oxidation isomerization of [6-3H]androst-5-ene-3 beta, 17 beta-diol was enzymically hydroxylated in the 6 beta-position by the fungus Rhizopls stolonifer. Retention of 3H in the 6 alpha-position of the isolated 6 beta-hydroxyandrost-4-ene-3, 17-dione indicates that in the isomerase-catalysed migration of the C(5) = C(6) double bond, the incoming proton from the acidic group on the enzyme must enter C-6 from the beta-face, forcing the existing 3H into the 6 alpha-position.

An Ab Initio Study of the Effect of Substituents on Protonation of the Carbonyl Group

Ab initio calculations have been performed on a series of molecules HCOX, where X=CH3, NH2, OH, F, H, and also on the corresponding oxygen protonated cations. Rotational studies show the incoming proton to be trans to the substituent X except in formyl fluoride, where internal hydrogen bonding stabilizes the cis isomer. The geometries of the fluoro- and amino-substituted cations have been optimized along with that of oxygen protonated ketene. Proton affinities are found to be very dependent on substituents with the order of basicity being [Formula: see text]an order which closely follows the negative charge on the oxygen. Plots of the computed charge on the carbon atoms against the core binding energy are different for the cations and neutral compounds but can be made to fall on the same line by correcting the binding energy using "the charge potential model" for the K-shell electron which is removed.

THE ANGULAR DISTRIBUTION OF GAMMA RAYS IN THE C12(p,p′γ) REACTION

Protons of 7.1 Mev. energy from the MIT cyclotron have been used to investigate the angular distribution of gamma rays from the C12(p,p′γ) reaction with respect to the incoming proton beam. These gamma rays result from transitions between the first excited state of C12 at 4.45 Mev. and the ground state. The resulting distribution can be fitted by the expansion[Formula: see text]which is consistent with an assignment of two for the angular momentum of the first excited state of C12.