H.G. Börner

Gamma ray energies and 36Cl level scheme from the reaction 35Cl(n, γ)

Abstract The γ-ray spectrum emitted after thermal neutron capture in 35Cl has been studied by use of the crystal and pair spectrometers installed at the ILL high flux reactor. We identified about 400 transitions in this reaction 326 of which were placed into the 36C1 level scheme; several new states were found. The level energies up to 3.5 MeV were measured with a precision of 5–20 eV relative to the 412 keV 198Au standard, those above 3.5 MeV with a precision of 10ppm. The neutron binding energy was determined to be EB = 8579.68(9) keV.

Identification of all intrinsic excitations below 2 MeV in 168Er

The levels and gamma decay in168Er have been investigated following thermal neutron capture on enriched targets of 167Er2O3. Precision measurements of the gamma spectrum with curved-crystal spectrometers revealed over 700 transitions up to an energy of 2.5 MeV, with intensities spanning five orders of magnitude. A list of primary gamma transitions populating levels in 168Er up to an excitation energy of 3.1 MeV resulted from measurements with the pair spectrometer. Information on the gamma spectrum in the energy region 1400<E<2600 keV was obtained using a Ge(Li) spectrometer. The conversion electron spectrum up to 2.2 MeV electron energy was measured with a beta spectrometer, whence multipolarities of many of the transitions were derived. Average resonance capture measurements at neutron energies of 2 keV and 24 keV were made which ensured that the set of spin 2-5 levels below about 2.2 MeV was complete. From all these data, employing the Ritz combination principle, a level scheme was developed and the levels were arranged into a complete set of 20 low-K rotational bands. This is the most complete and extensive test of the limits of applicability of models for low-lying collective excitations.

The curved crystal gamma ray spectrometers “GAMS 1, GAMS 2, GAMS 3” for high resolution (n, γ) measurements at the high flux reactor in Grenoble

Abstract DuMond-type curved-crystal γ-ray spectrometers have been constructed and installed at the High Flux Reactor in Grenoble. They are used for the measurement of low energy γ-rays up to about 1500 keV emitted by an in-pile target after neutron capture. The target is located in the center of the tangential throughtube where the flux of thermal neutrons is ∼ 5.5 × 10 14 /cm 2 s. Two separate spectrometer systems, one of focal length 5.76 m (GAMS 1) and the other of focal length 24.0 m (GAMS 2/3) view the source from the opposite ends of the beam tube. The GAMS 1 spectrometer is designed to operate in the γ-ray energy interval 20 ⪅ E γ ⪅ 400 keV. Angular resolution as small as 1.1 arcsec have been achieved with this instrument. The GAMS 2 3 sysmt em consists of two spectrometers on top of each other, operating at +θ Bragg and −θ Bragg , respectively. The GAMS 2 3 diffractome ter is designed to cover the energy interval 200 ⪅ E γ ⪅ 1500 keV. Minimum line widths of ∼0.8 arcsec have been achieved. In routine measurements the detection sensitivity for neutron capture γ-rays is ∼1 mb, for GAMS 1 between 80 keV and 300 keV and for GAMS 2 3 from 300 keV to 800 keV. Both spectrometer systems allow for source migrations due to heating effects in the beam tube. The reflection angles in the spectrometers are measured with Michelson-type angle interferometers. The spectrometers serve mainly for nuclear structure studies.

The enigma of 114Cd. A classical case of ambiguities in quantum mechanical state mixing

Abstract Lifetime measurements of levels near 2 MeV in 114 Cd give quantitative evidence for collective 3-phonon vibrational states. An analysis of the level scheme reveals a remarkable ambiguity with origins in the basic properties of quantum mechanical mixing.

The level structure of 114Cd from (n, γ) and (d, p) studies☆

Abstract Gamma rays and conversion electrons have been measured following thermal neutron capture in 113 Cd using the crystal spectrometers GAMS and the β-spectrometer BILL at the High Flux Reactor of the ILL at Grenoble. Primary γ-rays following thermal and average resonance neutron capture at E n = 2 keV and 24 keV were recorded at the High Flux Beam Reactor at the Brookhaven National Laboratory. The 113 Cd(d, p) 114 Cd reaction was studied with the Q3D spectrograph at the Munich tandem accelerator. Combining all these experimental results an almost complete level scheme of 114 Cd was constructed up to 3.3 MeV including 48 excited levels with spin and parity information. The level scheme is discussed in terms of particle-hole excitations across the Z = 50 closed shell coupled to collective states, as well as in an interacting boson configuration mixing scheme.

Determination of short lifetimes with ultra high resolution (n, γ) spectroscopy

Abstract We show how ultra high resolution (n, γ ) spectroscopy can be used to determine lifetimes of nuclear excited levels through the observation of Doppler broadening of deexciting transitions. This Doppler broadening is induced by the recoil imparted by feeding transitions and allows the study of lifetimes −11 s.

Dopper shift attenuation lifetime measurement in 54Cr following thermal neutron capture

Abstract The double crystal spectrometer GAMS4 in combination with the ILL high flux reactor has been used to determine the lifetimes of the 2620 KeV 2 + 2 , 3074 KeV 2 + 3 and 3720 KeV (1, 2) + states in 54 Cr. The initial recoil energy of about 0.5 KeV imparted by the primary γ-radiation after thermal neutron capture in 53 Cr produces Doppler broadened line shapes of the secondary transitions. The large 2 + 3 →2 + 1 M1 strength of B (M1)=0.39(6) μ 2 N suggests the 2 + 3 state to be mixed symmetry character within the interacting boson model IBM-2.

A study of the low-lying states in 178Hf through the (n, γ) reaction☆

The nucleus 178Hf was studied through thermal neutron and averaged resonance neutron capture reactions. The γ-ray and conversion electrons were measured with high resolution spectrometers. A level scheme up to an excitation energy of ∼2.1 MeV was constructed. It includes ∼65 levels, most of which are ordered into 18 rotational bands. The level scheme is complete up to about 1800keV for spins between 2 and 5. The neutron binding energy was established to be at 7626.3 (3) keV. The consistent Q form of the IBA-1 (CQF) was used to describe the low-lying collective γ and Kπ = 0+ bands. The agreement with the data was found to be excellent for the energies and B(E2) ratios of the ground and γ bands, whereas the agreement was poor for the Kπ = 0+ bands.

The rotational structure of 227Ra

Abstract The level structure of 227Ra has been studied using the (n, γ), (d, p) and ( t , d) reactions and the β− decay of 227Fr. A model-independent level scheme was established including 28 levels below 1500 keV. Cross sections and excitation energies have been measured for 72 levels below 2.5 MeV and analysing powers have been measured for 25 levels below 1.4 MeV. The level structure is interpreted in terms of the Nilsson model. The ground-state configuration is 3 2 + [631↑] and the first excited band [633↓] starts at 1.7 keV. Several octupole vibrations coupled to single-particle configurations are tentatively assigned above 280 keV. The neutron binding energy was determined to be 4561.41 ± 0.27 keV. The half-life of 227Fr was measured to be 148 ± 2 s.

The level structure of 196Pt

Abstract The 195 Pt(n, γ) 196 Pt reaction has been investigated with the use of numerous experimental techniques. A level scheme has been established consisting of 58 levels below 2.5 MeV, most of which are assigned J π values of 0 + , 1 + , or 2 + . Average resonance capture measurements have led to the identification of all 0 + , 1 + , or 2 + states below 2.5 MeV and the location of the pairing energy gap at ≈ 1.8 MeV. All positive-parity states in 196 Pt below the energy gap can be understood in terms of the O(6) limit of the interacting boson approximation (IBA) model. In addition to the positive-parity states, a number of probable negative-parity states have been identified. These states can be understood in terms of a model involving partially decoupled bands.