gamma ray spectroscopy with ultra-high precision

Radiation Physics and Chemistry 61 (2001) 465–467

Gamma ray spectroscopy with ultra-high precision

J. Jolie*

Institute of Physics, Physics Department, University of Fribourg, Perolles, CH-1700 Fribourg, Switzerland

Abstract

The energy of gamma rays emitted after thermal neutron capture can nowadays be measured with parts-per-millionprecision. This precision allows one to measure tiny Doppler effects caused by recoil due to the preceding emission of

gamma rays or neutrinos. The study of the Doppler profiles of gamma rays has given rise to new applications in nuclearand solid state physics r 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Gamma-ray spectroscopy; Crystal spectrometers; Nuclear lifetimes; Interatomic potentials

1. The GAMS4 spectrometer

Gamma-ray spectroscopy with part per millionprecision has become feasible with the development of

the GAMS4 spectrometer (Deslattes et al., 1980; Deweyet al., 1989). This double flat crystal spectrometer usestwo hyperpure Si or Ge crystals which can successively

Laue diffract gamma rays coming from an in-pile sourceat the high-flux beam reactor of the Institut LaueLangevin (Grenoble, France). The source is placed in a

neutron flux of 4.5� 1014 n cm�2 s�1 and provides thevery strong gamma ray source needed to compensate forthe low efficiency of the spectrometer.

To determine the Bragg angles, the spectrometer usestwo-frequency Michelson interferometers which allowone to measure the crystal angles with sub milliarcseconds precision. An important feature of the

spectrometer is that it possesses two geometries. Whenboth crystals are positioned parallel to each other thegeometry is non-dispersive yielding the possibility to

measure experimentally the instrumental response. Byturning the second crystal over twice the Bragg anglewith respect to the incident gamma rays a dispersive

geometry is obtained which allows the measurement ofthe energy distribution of the incident gamma rays.

The spectrometer is used for two classes of measure-ments: absolute energy determination and lineshapedetermination. For absolute measurements the energy ofa gamma ray is determined from the angle between the

peak positions in the dispersive and non-dispersivegeometry. The goal of the absolute measurements isthe precise determination of the mass of the neutron

(Kessler et al., 1999; Dewey and Kessler, 2000). Thelineshape analysis uses one geometry and determines theintensity distribution of the gamma ray relative to the

peak position. Here we will concentrate on the relativelineshape measurements.

2. Lineshape analysis

The analysis of the lineshape is very dependent on theinstrumental response which can be measured exactly inthe non-dispersive geometry. This allows one to test

dynamical diffraction theory at very high energies. Anexcellent agreement is obtained between theory andexperiment, if the theory is folded with a small gaussian.The ability to measure exactly the instrumental response

permits the accurate determination of very small energybroadenings in the dispersive geometry. Those arise dueto Doppler effects related to atomic motion. The motion

can have several origins: thermal motion, recoil aftergamma ray emission or neutrino emission (B .orner andJolie, 1993). As an illustrative example, thermal velo-

cities down to the velocity of sound have been extractedfrom the data. Once the lineshapes determined the

*IKP, University of Cologne, Z .ulpickerstra BL 77, D-50937,

Cologne, Germany. Tel.:+49-221-470-3456; fax: +49-221-470-

5168.

E-mail address: jolie@ikp.uni-koeln.de (J. Jolie).

0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 3 0 2 - 4

measured Doppler effects yield useful information aboutthe physical processes causing the Doppler broadening.

3. Gamma ray induced Doppler broadening (GRID)

Besides thermal broadening, Doppler effects onsecondary transitions due to nuclear recoil can bemeasured. The recoil is due to primary gamma rays

emitted after neutron capture It leads to lineshapeswhich depend on the slowing-down process in solids andon the nuclear lifetime of the intermediate level. Bycomparing the nuclear clock, the radioactive decay law,

to the atomic clock, the slowing down time, informationabout either process can be obtained. The gamma rayInduced Doppler broadening technique such allows to

measure lifetimes in the picosecond to femtosecondregion which are unattainable by other experimentalmethods. These experiments yielded several important

results on the nuclear collectivity of multiphonon statesin heavy even–even nuclei (see Jolie, 1999, for a recent

review). Another application of the GRID methodconsists in using the knowledge on the lifetimes to study

the interatomic potentials causing the slowing-downprocess in the energy domain below 0.5 keV. This can bebest done with single crystal targets that cause an

orientation dependent fine structure on the lineshapes(Jentschel, 1997; Stritt et al., 2000). The slowing down isthen simulated with Molecular Dynamics simulations

using different interatomic potentials. The so obtainedindividual recoil trajectories are used to construct thetheoretical lineshapes as a function of the nuclearlifetime, which is then fitted to the data. For each

potential the extracted lifetime and the fit quality yieldthen measures of the quality of the potential.

4. Neutrino induced Doppler broadening (NID)

Besides GRID other recoils can be used. An

important example forms the 3 eV recoil induced bythe emission of a neutrino after electron capture in

Fig. 1. Doppler broadened gamma ray line observed for Sm recoiling in EuO after emission of a neutrino. The dashed line shows the

measured instrumental response for this third-order reflection.

J. Jolie / Radiation Physics and Chemistry 61 (2001) 465–467466

152Eu. As shown in Fig. 1, the line is clearly broadenedby the factor v/c=6.54� 10�6. The very low recoil

energy allows the study of oscillations of the atomsaround their equilibrium position by molecular dy-namics simulations. The obtained lineshape then yields

information on the form of the interatomic potential(Stritt et al., 1998), but also might learn us somethingabout the helicity of the neutrino (Jolie and Stritt, 2000).

5. Conclusion

Ultra-high precision gamma-ray spectroscopy extends

the range of applications of radiation towards newdomains of research. We hope to have given to thoseunacquainted with this new field, a flavour of its many

new possibilities.

Acknowledgements

This work was supported by the Swiss National

Science Foundation and the Institut Laue Langevin. Theauthor is grateful to Hans B .orner, Michael Jentscheland Nicolas Stritt for their contributions to the

development of new methods using ultra-high precisiongamma-ray spectroscopy and to R.D. Deslattes, M.S.Dewey and E.G. Kessler. For the continuous improve-

ments of the GAMS4 spectrometer.

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