A coincidence Mössbauer experiment with 119mSn

HAL Id: jpa-00209645

https://hal.archives-ouvertes.fr/jpa-00209645

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A coincidence Mössbauer experiment with 119mSn

E.I. Vapirev, P.S. Kamenov, D.L. Balabanski, S.I. Ormandjiev, K. Yanakiev

To cite this version:

E.I. Vapirev, P.S. Kamenov, D.L. Balabanski, S.I. Ormandjiev, K. Yanakiev. A coinci- dence Mössbauer experiment with 119mSn. Journal de Physique, 1983, 44 (6), pp.675-677.

�10.1051/jphys:01983004406067500�. �jpa-00209645�

675

A coincidence Mössbauer experiment ^{with 119mSn}

E. I. Vapirev, ^{P. S.} Kamenov, ^{D. L.} Balabanski, ^{S. I.} Ormandjiev and K. Yanakiev Department ^{of Atomic} Physics, University ^of Sofia, ^Sofia 1126, Bulgaria

(Reçu le 7 octobre 1982, révisé le 3 janvier ^1983, accepté ^{le 24} février 1983)

Résumé.

La distribution en temps ^{de la} radiation Mössbauer filtrée est étudiée expérimentalement. ^{Les résultats}

obtenus sont en très bon accord quantitatif ^avec la théorie développée par Lynch, ^{Holland et} Hammermesh en

1960. On en déduit que les désaccords observés dans l’expérience ^{de la} dépendance ^en temps de la radiation diffusée sont dus à certaine raison n’étant pas prise ^en considération et ne sont pas dus à l’approche théorique.

Abstract.

The time distribution of resonantly filtered Mössbauer radiation is investigated experimentally. ^The

results are in very good quantitative agreement ^{with the} theory developed by Lynch, Holland and Hammermesh in 1960 [1]. ^{It is concluded} that the observed discrepancies ^{in the} experiments ^{on time} dependence ^of resonantly

scattered radiation [3] ^{are due to some} unaccounted reason and not to the theoretical approach.

J. Physique ⁴⁴ (1983) ^{675-677 JuIN} 1983,

Classification

Physics ^Abstracts

23.20

1. Introduction.

In 1960 Lynch, Holland and Hammermesh [1]

published ^a paper in which they showed that a reso- nant absorber placed ^{between a Mossbauer source}

and a detector modifies the decay exponent ^{so that} eventually ^the modified exponent goes ^over the

normal, unmodified one

i.e. in some periods ^after

the formation of the excited state more counts are

recorded with an absorber than without it. Our

experiment ^{aims to observe and} compare with

theory ^{the «} negative absorption >> ^{as Wu et al.} [2]

called it. The comparison ^{should be accurate since}

some unexplained deviations still exist in scattering experiments [3]. ^Those scattering experiments ^are

very precise ^{and the observed} discrepancies (at ^the place of the first minimum, theory ^and experiment

differ 4-10 times) question ^the applicability ^{of the}

theoretical approach. ^{Also we shall} try ^{to understand} better what we observe.

The present work ^{describes an} experiment ^similar

to that first one [1] but with another source and better time resolution.

2. Experiment.

The experiment is carried out with a source 119msn.

Figure ^{1 shows the} experimental arrangement.

The start signal ^is given by the 25.3 keV X-ray following ^the totally converted 65.7 keV transition which feeds the Mossbauer level. The 23.9 keV

Fig.1. -Experimental arrangement: ^{1. Source} Bal19mSn03’

-

50 000 cps; 2. Absorber CaSno3, ²⁰ mgJcm2, ^natural

abundance; 3. Stilbene crystals coupled ^with photomulti- pliers RCA8575; 4. Pd filter for the 25.3 X-ray; ^{CFD -}

constant fraction discriminator, ^{TAC -} time-to-amplitude

converter, ^{MCA -} multichannel analyser.

gamma-ray from the decay of the metastable state serves as a stop signal.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01983004406067500

676

The time resolution of the set up is approximately

2.5 ns. It was measured by Compton scattering ^of

the 60 keV _gamma radiation from 241Am _source, photomultipliers facing each other. A similar figure

was obtained by ^a computer ^{fit of an unfiltered} exponent. This time resolution was achieved with stilbene crystals instead of plastic scintillators. The stilbene crystals provide less dynamic range for the CF discriminator (higher effective atomic number and therefore more photoeffect for low energy pho- tons).

For a typical ^{« run » of 5-15} days ^two spectra ^were recorded with periodic commutation - a Ir spectrum

(energy ^{shift of} several natural linewidths r between

source and absorber) ^{and a} 100 spectrum (off resonance spectrum, Doppler velocity V

oo). After random coincidences and prompt peak subtraction the ratio of the two spectra ^is calculated, ^thus normalizing

and « rectifying ^{» the} exponent

^this procedure

is equivalent ^{to a} multiplication of the time dependent spectrum by ^{a factor} exp(T/r), ^{here r is the mean}

lifetime of the resonance level. Also when dividing

the two spectra any possible apparatus non-linearities

are avoided. The _energy shifts between source and absorber and the thickness of the absorber are chosen

so as to observe clearly ^{the most} interesting ^case

- the excess of counts in certain periods ^{after the}

formation of the excited state.

3. Discussion.

The experiments definitely confirmed the phenomenon

of ^« negative absorption ^{». Moreover, the} experiment

is in very good agreement ^{with the} theory

^the

solid lines ^on figure ^{2 are} the theoretical calculations

using ^the Hammermesh [1] ^{formula. No numerical} integration ^was done since the resolution time is much less than the ^mean lifetime. The conclusion.

is that the reason for the observed deviations in

scattering experiments [3] ^most probably ^{is not in}

the theory developed by Thieberger et ^al. [4] ^which theory follows the same approach ^{as in} [1]. ^The

calculations in both _papers are based on classical

optics

^{the resonant} media is characterized by ^a complex ^{index of} refraction. Of course there is no

« negative absorption ^{», the} only possible ^{reason for}

the excess of counts in certain periods ^{is that some} photons ^are delayed ^{in the} absorber - this is consistent with the idea for complex index of refrac- tion.

However when using ^{such an} approach (later ^this approach ^was confirmed quantum mechanically [5])

it should be remembered that there are two important

differences in comparison ^to conventional optics : A) ^the delay ^{in time is due} to resonant centres dispers-

ed randomly ^{in the absorber and it is} independent

Fig. ^2.

^{Ratio of} I, ^and 100 spectra. Ir

^time dependent spectrum ^for energy shift between ^source and absorber several natural linewidths r, here 3.6 F and 5.6 r ; ^{D - thick-}

ness of the absorber, ^D

nuf; n

^{number of resonant} nuclei per CM2; a - ^resonant cross-section ; f - probability

for recoilless absorption.

of the _average distance between them, ^and B) ^the delay ^{is not due to a} change ^{of the} velocity ^of propa-

gation ^{of the} photon, ^{it is due to a} very interesting phenomenon - ^the photon ^is delayed by ^{a resonant}

centre without exciting ^it

^« capturing ^{effect »} [6].

A) ^and B) ^{are not} in contradiction with classical

optics ^{since the} quantity ^{index of} refraction has been

generalized ^for randomly dispersed centres at average distance much greater ^{than the} wavelength ^{of the}

incident photon ^or particle (see e.g. [7]). ^{The index}

of refraction -is expressed by the forward scattering amplitude, ^the density ^{of the} scattering ^{centres and}

the wavelength of the incident photon ^or particle.

The forward scattering amplitude ^{is a} result of the interaction of the incident wave with a single scattering

centre.

These arguments may help ^to understand better

why the classical formalism works so well.

The delay phenomenon in forward direction and the radioactive decay ^{are related to some extent}

- an unstable state exists for ^a certain time. The

delay ^{in forward direction is} energy dependent, ^it depends ^on the energy difference ^{E -} E’o, ^{E is the}

energy of the incident photon, Eo ^{is the resonance}

energy of the scatterer. This implies the idea that the decay process may also be energy dependent Acknowledgments.

The authors are grateful ^to Dr. L. T. Tzankov for

helpful discussions.

677

References

[1] ^{LYNCH, F.} J., HOLLAND, ^R. E., HAMMERMESH, M., Phys. ^{Rev. 120} (1960) ^513.

[2] WU, ^C. S., LEE, ^Y. K., BENCSER-KOLLER, N., SIMMS, P., Phys. Rev. Lett. 5 (1960) ^432.

[3] DROST, H., LOJEVSKY, H., PALOW, K., WALLENSTEIN, R., WEYER, G., ^{5th Int.} Conf. ^{on Mössbauer} Spec- troscopy, Bratislava, 1973, p. 713.

[4] THIEBERGER, P., MORAGUES, ^J. A., SUNYAR, ^A. W., Phys. ^{Rev. 171} (1968) ^425.

[5] HARRIS, ^S. M., Phys. ^{Rev. 124} (1961) ^1178.

[6] BARISHEVSKY, ^V. G., Iadernaja optica poliarizovannih sred, ^Minsk (Buelorussian ^State University) 1976, p.132.

[7] LAX, M., ^{Rev. Mod.} Phys. ²³ (1951) 287;

GOLDBERGER, ^M. L., WATSON, ^K. M., ^Collision Theory

(Wiley, ^N. Y.) ^1964.