H.H. Stroke

Use of a Continuous Wave Laser and Pockels Cell for Sensitive High-Resolution Collinear Resonance Ionization Spectroscopy.

New technical developments have led to a 2 orders of magnitude improvement of the resolution of the collinear resonance ionization spectroscopy (CRIS) experiment at ISOLDE, CERN, without sacrificing the high efficiency of the CRIS technique. Experimental linewidths of 20(1) MHz were obtained on radioactive beams of francium, allowing us for the first time to determine the electric quadrupole moment of the short lived [t_{1/2}=22.0(5) ms] ^{219}Fr Q_{s}=-1.21(2) eb, which would not have been possible without the advantages offered by the new method. This method relies on a continuous-wave laser and an external Pockels cell to produce narrow-band light pulses, required to reach the high resolution in two-step resonance ionization. Exotic nuclei produced at rates of a few hundred ions/s can now be studied with high resolution, allowing detailed studies of the anchor points for nuclear theories.

Odd-even staggering in mercury isotope shifts: Evidence for Coriolis effects in particle-core coupling

Abstract The observed variation with neutron number of the odd-even staggering of the mercury i 13/2 -neutron isomeric nuclei lend support to the odd-particle rotation-aligned coupling scheme. This scheme was previously found to account for the energies of the low-lying mercury states based on these i 13/2 levels.

Bolometric detection of the recoil spectrum in the alpha decay of 210Po

Abstract Bolometers at low temperature are being developed as particle spectrometers and potential detectors of dark matter particles through the measurement of recoil energies. A successful observation of the recoil spectrum from a 210 Po electrodeposited source was made for the first time with the use of a 2 mm 3 composite-composite diamond bolometer operated in a 300 mK cryostat. The measured recoil ratio is 0.98±0.06 recoil/alpha above 50 keV, with the well defined peak in the spectrum at 101±2.5 keV.

Performance and applications of cooled composite bolometers in the field of ionizing radiation metrology

Two major limitations met in the field of ionizing radiation metrology, i.e. energy resolution and detection energy threshold, may be improved by one or two orders of magnitude by using composite bolometers cooled below 300 mK and operating in the pulse mode. The major advantages are: possibility of choosing the absorber (from diamond to BGO: Al2O3, Ge, Si, BaF2, LiF, etc), the absence of statistical noise (phonon quanta are in the 10−4 eV range) and ultimate resolution in the 10−1 eV range for small bolometers. The theoretical and experimental limits are analyzed and solutions for extraneous noise limitations in a complete system are described. The most recent results in alpha-particle and gamma-ray spectrometry using a new design, the composite-composite bolometer, are given. In these, a resolution of 10.5 keV was obtained for 6 MeV alpha particles and < 2.5 keV for241Am 18 keV X rays using a 24 sapphire bolometer. The ideal bolometer concept is also discussed. In this, the simultaneous detection of quanta (due to ionization, fluorescence, ballistic phonons) and thermal energies will make it possible to have lower thresholds by amplification in the target. It should thus be possible to determine the characteristics of the particles (their precise arrival time and the positions of the impacts) and provide improvements in the field of integral counting for the absolute measurement of radioactivity.

Development of the CRIS (Collinear Resonant Ionisation Spectroscopy) beam line

The CRIS (Collinear Resonant Ionisation Spectroscopy) beam line is a new experimental set up at the ISOLDE facility at CERN. CRIS is being constructed for high-resolution laser spectroscopy measurements on radioactive isotopes. These measurements can be used to extract nuclear properties of isotopes far from stability. The CRIS beam line has been under construction since 2009 and testing of its constituent parts have been performed using stable and radioactive ion beams, in preparation for its first on-line run. This paper will present the current status of the CRIS experiment and highlight results from the recent tests.

Nuclear charge radii of potassium isotopes beyond N = 28

We report on the measurement of optical isotope shifts for 38;39;42;44;46-51 K relative to 47 K from which changes in the nuclear mean square charge radii across the N = 28 shell closure are deduced. The investigation was carried out by bunched-beam collinear laser spectroscopy at the CERN-ISOLDE radioactive ion-beam facility. Mean square charge radii are now known from 37 K to 51 K, covering all f7=2-shell as well as all p3=2-shell nuclei. These measurements, in conjunction with those of Ca, Cr, Mn and Fe, provide a first insight into the Z dependence of the evolution of nuclear size above the shell closure at N = 28.

Advances towards fast thermal detectors of intermediate mass with high resolution and large dynamic range

Abstract The limiting characteristics of intermediate mass bolometers cooled to low temperature for the detection of single events are analyzed over a large energy range with the contributions of inherent noise at low energy, of statistical effects at intermediate energy, and of nonlinear effects at high energy. The optimal design of a fast (time constant τ m ΔE FWHM E (max) 5 MeV) is described from functional block diagram to the calculation of parameters for each element. This has been applied to the construction and successful tests of a 100 mg silicon bolometer. We obtained resolutions better than 1 keV on γ-rays from 13 to 130 keV with a base line resolution of 300 eV and a measured time constant of less than 400 μs. The same bolometer was also used for α particle spectrometry in the 5 to 8 MeV range with a resolving power greater than 200. From measurements of the short lived 214 Po atoms recoiled into the bolometer we deduced a half-life of 160 ± 12 μs, which extends the excellent time resolution possible with such bolometers.

Nuclear magnetic moment of 203Hg by optical pumping

Abstract Optical-pumping techniques have been used to obtain a direct measurement of the nuclear magnetic moment of the 47-d radio-isotope 203 Hg. The result is ( 203 μ) uncorrected 0.83549 (13) μ N . The effect of distributed nuclear magnetization gives an hfs anomaly 203 Δ 201 = 0.0064 (2).

Further investigations of thermal cryogenic detectors in the field of ionizing radiation metrology

Abstract Thermal cryogenic detectors (bolometers, microcalorimeters) have been operated in the pulse mode to quantify ionizing radiation. These detectors exhibit considerable promise for applications in high resolution spectrometry and integral counting. A compact cryogenic system has been developed that can easily be operated for 15 h at 300 mK. This temperature is probably low enough for α-particle spectrometry. We have recently installed a composite-composite bolometer into this cryostat. The detector consists of two cubic crystals (a sapphire target and a diamond scatterer) coupled to a chemically doped Ge temperature sensor. The time constants of the bolometric signal are such that processing and digital filtering of the pulses is required. Careful examination and processing of the recorded pulses permits the elimination of some causes of the loss in energy resolution (pile-up, microphonics, collection defect). Moreover, the very strong dependence of the gain on the temperature has led us to define stability criteria and to select, a posteriori, measurement periods showing sufficient stability.

Laser assisted decay spectroscopy at the CRIS beam line at ISOLDE

The new collinear resonant ionization spectroscopy (Cris) experiment at Isolde, Cern uses laser radiation to stepwise excite and ionize an atomic beam for the purpose of ultra-sensitive detection of rare isotopes and hyperfine structure measurements. The technique also offers the ability to purify an ion beam that is contaminated with radioactive isobars, including the ground state of an isotope from its isomer. A new program using the Cris technique to select only nuclear isomeric states for decay spectroscopy commenced last year. The isomeric ion beam is selected using a resonance within its hyperfine structure and subsequently deflected to a decay spectroscopy station. This consists of a rotating wheel implantation system for alpha and beta decay spectroscopy, and up to three high purity germanium detectors for gamma-ray detection. This paper gives an introduction to the Cris technique, the current status of the laser assisted decay spectroscopy set-up and recent results from the experiment in November 2011.