J. P. Meier

Application of low temperature calorimeters for precise Lamb shift measurements on hydrogen-like very heavy ions

The precise determination of the 2s12, 2p12, 2p32 → 1s12 X-ray transitions in hydrogen-like very heavy ions such as 208Pb81+ or 238U91+ provides a sensitive test of QED, especially of higher order contributions to the self-energy which are not accessible by alternative methods. Such measurements became possible recently by X-ray spectroscopy using highly charged ions stored and cooled in heavy ion storage rings. A better energy resolution of the X-ray detector is most essential in order to improve the experimental accuracy, which is presently about one order of magnitude worse than the theoretical uncertainty. An energy resolution of ΔE = 30–50 eV for Eγ = 50–100 keV, and a photopeak efficiency above 30% may be provided by a low temperature calorimeter. It will be constructed on the basis of already existing arrays of silicon microcalorimeters, but with larger volume and high Z absorbers. Such a detector will allow a more precise determination of the 1s-Lamb shift and for the first time the direct investigation of the 2s-Lamb shift in hydrogen-like heavy ions. In the present contribution the experimental scenario for QED experiments at storage rings, the present status of experimental and theoretical investigations and the design of an appropriate low temperature detector are discussed.

Application of low temperature calorimeters for the detection of energetic heavy ions

Abstract The energy sensitive detection of energetic heavy ions with calorimetric low temperature detectors is investigated. The temperature readout was done with an aluminum transition edge thermometer operated at T ≈ 1.5 K. For 20Ne-ions with an energy of E = 100 MeV/u from the SIS accelerator at GSI Darmstadt the best energy resolution obtained was ΔE E = 1.9 × 10 −3 . This value corresponds to the energy spread of the ion beam of the SIS. In a first application of such detectors the excitation of the giant resonance in lead nuclei via the reaction natPb(20Ne, 20Ne′)natPb∗ was investigated by separating inelastically from elastically scattered 20Ne-ions in the energy spectrum. At a scattering angle ΘLab = 3° the excitation energy and the strength of the giant resonance were found to be in good agreement with theoretical predictions. In a first test with an extracted cooled 238U-beam with an energy of E = 360 MeV/u from the storage ring ESR with an intrinsic beam energy spread of ΔE E ≤ 2 × 10 −4 an energy resolution of ΔE = 97 MeV ( ΔE E = 1.1 × 10 −3 ) was measured. The baseline noise was ΔE = 17 MeV. Further improvement of the energy resolution seems possible.

High resolution detection of energetic heavy ions with a calorimetric low temperature detector

Abstract A low temperature calorimeter with good energy resolution was developed for detection of energetic heavy ions. It consists of a doped germanium crystal as thermometer and a sapphire crystal as absorber, and is operated in a 4 He window cryostat at temperatures ranging from 1.3 to 2.0 K. For 2.38 GeV 209 Bi ions a relative energy resolution of ΔE E = 1.8 × 10 −3 was observed, most probably still limited by the energy spread of the incident ion beam.

Calorimetric low temperature detectors for low-energetic heavy ions and their application in accelerator mass spectrometry

The energy-sensitive detection of heavy ions with calorimetric low temperature detectors was investigated in the energy range of E=0.1-1 MeV/amu, commonly used for accelerator mass spectrometry (AMS). The detectors used consist of sapphire absorbers and superconducting aluminum transition edge thermometers operated at T approximately 1.5 K. They were irradiated with various ion beams (13C, 197Au, 238U) provided by the VERA tandem accelerator in Vienna, Austria. The relative energy resolution obtained was DeltaE/E=(5-9) x 10(-3), even for the heaviest ions such as 238U. In addition, no evidence for a pulse height defect was observed. This performance allowed for the first time to apply a calorimetric low temperature detector in an AMS experiment. The aim was to precisely determine the isotope ratio of 236U/238U for several samples of natural uranium, 236U being known as a sensitive monitor for neutron fluxes. Replacing a conventionally used detection system at VERA by the calorimetric detector enabled to substantially reduce background from neighboring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of 236U/238U=6.1 x 10(-12) could be obtained, representing the smallest 236U/238U ratio measured at the time. In addition, we contributed to establishing an improved material standard of 236U/238U, which can be used as a reference for future AMS measurements.

Energy sensitive detection of heavy ions with transition edge calorimeters

Status and results of a research project are reported, which aims at developing calorimetric low temperature detectors for heavy ions. The special conditions for the detection of energetic heavy ions are discussed. The response of an aluminium transition edge calorimeter, operated at about1.5K, to the impact of heavy ions was investigated. The observed energy resolutions ΔE/E=3.9×10−3for5.9MeV/u208Pb-ions, ΔE/E=1.6×10−3for4.8MeV/u58Ni-ions and ΔE/E=4.3×10−3for100MeV/u20Ne-ions are most probably not yet limited by the intrinsic detector resolution. The present results already represent an improvement over conventional solid state detectors and ionization chambers.

Precise Lamb Shift Measurements in Hydrogen‐Like Heavy Ions—Status and Perspectives

The precise determination of the energy of the Lyman α1 and α2 lines in hydrogen‐like heavy ions provides a sensitive test of quantum electrodynamics in very strong Coulomb fields. For the first time, a calorimetric low‐temperature detector was applied in an experiment to precisely determine the transition energy of the Lyman lines of lead ions 207pb81+ at the Experimental Storage Ring (ESR) at GSI. The detectors consist of silicon thermistors, provided by the NASA/Goddard Space Flight Center, and Pb or Sn absorbers to obtain high quantum efficiency in the energy range of 40–80 keV, where the Doppler‐shifted Lyman lines are located. The measured energy of the Lyman α1 line, E(Ly‐α1, 207Pb81+) = (77937±12stat±23syst) eV, agrees within errors with theoretical predictions. The systematic error is mainly due to uncertainties in the non‐linear energy calibration of the detectors as well as the relative position of detector and gas‐jet target.

High-precision x-ray spectroscopy of highly charged ions with microcalorimeters

The precise determination of the energy of the Lyman α1 and α2 lines in hydrogen-like heavy ions provides a sensitive test of quantum electrodynamics in very strong Coulomb fields. To improve the experimental precision, the new detector concept of microcalorimeters is now exploited for such measurements. Such detectors consist of compensated-doped silicon thermistors and Pb or Sn absorbers to obtain high quantum efficiency in the energy range of 40–70 keV, where the Doppler-shifted Lyman lines are located. For the first time, a microcalorimeter was applied in an experiment to precisely determine the transition energy of the Lyman lines of lead ions at the experimental storage ring at GSI. The energy of the Ly α1 line E(Ly-α1, 207Pb81+) = (77937 ± 12stat ± 25syst) eV agrees within error bars with theoretical predictions. To improve the experimental precision, a new detector array with more pixels and better energy resolution was equipped and successfully applied in an experiment to determine the Lyman-α lines of gold ions 197Au78+.

Precise Determination of the 1s Lamb Shift in Hydrogen-Like Lead and Gold Using Microcalorimeters

Quantum electrodynamics in very strong Coulomb fields is one scope which has not yet been tested experimentally with sufficient accuracy to really determine whether the perturbative approach is valid. One sensitive test is the determination of the 1s Lamb shift in highly-charged very heavy ions. The 1s Lamb shift of hydrogen-like lead (Pb81+) and gold (Au78+) has been determined using the novel detector concept of silicon microcalorimeters for the detection of hard x-rays. The results of eV for lead and eV for gold are within the error bars in good agreement with theoretical predictions. To our knowledge, for hydrogen-like lead, this represents the most accurate determination of the 1s Lamb shift.

Precise determination of the 1s Lamb Shift in hydrogen-like heavy ions at the ESR storage ring using microcalorimeters

The precise determination of the energy of the Lyman α1 and α2 lines in hydrogen-like heavy ions provides a sensitive test of quantum electrodynamics in very strong Coulomb fields. To improve the precision of such experiments, the new detector concept of microcalorimeters, which detect the temperature change of an absorber after an incoming particle or photon has deposited its energy as heat, is now exploited. The microcalorimeters for x-rays used in these experiments consist of arrays of silicon thermometers and x-ray absorbers made of high-Z material. With such detectors, a relative energy resolution of about 1 per mille is obtained in the energy regime of 50–100 keV. Two successful measurement campaigns to determine the 1s Lamb Shift in Pb81+ and Au78+ have been completed: a prototype array has been applied successfully for the determination of the 1s Lamb Shift of Pb81+ at the ESR storage ring at GSI in a first test experiment. Based on the results of this test, a full array with 32 pixels has been equipped and has recently been applied to determine the 1s Lamb Shift in Au78+ ions. The energy of the Lyman-α1 line agrees within error bars well with theoretical predictions. The obtained accuracy is already comparable to the best accuracy obtained with conventional germanium detectors for hydrogen-like uranium.