Lifetimes by Doppler shift using solid and gaseous stopping media

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Submitted on 1 Jan 1969

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Lifetimes by Doppler shift using solid and gaseous

stopping media

R.W. Kavanagh, N. Schulz, J.C. Merdinger

To cite this version:

215.

LIFETIMES BY DOPPLER SHIFT

USING

SOLID

AND

GASEOUS STOPPING MEDIA

R. W.

_KAVANAGH,

N. SCHULZ and

_J.

C.

_MERDINGER,

Institut de Recherches _{Nucléaires, Strasbourg-Cronenbourg.}

Résumé. - Des valeurs ou limites de la vie _moyenne d’états excités dans _{33S, 38Ar,} 40K

et 43Ca ont été obtenues _{par l’observation} des

_{déplacements}

en

_énergie

des

rayonnements

03B3

par l’effet

Doppler.

Des milieux ralentisseurs solides

(Au,

Cu,

C)

et _gazeux

_(Xe,

_Ar)

à

diffé-rentes

_{pressions (P ~}

30

_kg/cm2)

ont été utilisés.

Abstract. - Lifetime values _or limits for

low-lying

levels in _33S, 38Ar and 40K have been found from _03B3-ray

_Doppler

shift attenuation

_(DSA)

in Au, Cu, and C solids, and for levels in 33S and 43Ca

_by

DSA in Xe and Ar gases at various _{pressures up} to 30

_kg/cm2.

Revu£ DE PHYSIQUE ~PPI,IQU~~ TOSIE 4, JUIX 1969,

I.

Introduction.

- Since the advent of

high

resolu-tion

_Ge(Li)

detectors,

Doppler

shift attenuation

measu-rements to obtain

_lifetimes

of nuclear levels have become

_readily

accessible

and

_widely

used. In this

method,

the nuclear lifetimes have to be

_compared

to

the

_slowing

down time of the

_recoiling

excited nuclei.

When the ions are

_stopped

in

solids,

lifetimes in the

10-1~

to

10-12

second range are observable.

By using

gaseous media to

_stop

the excited nuclei and

_varying

the gas pressure, the

slowing

down times can be varied so that lifetimes in the lO-12 to 10-8 second _range can be measured.

_Using

both of these methods we have determined lifetime values or limits for

low-lying

levels in

335, 38Ar,

40K and 43Ca.

II.

_{Expérimental procédure.}

- The levels in the

nuclei

_335,

4oK and 43Ca were

_{populated by}

means

of

_{(a, n)}

_reactions,

and those of 38Ar

_by

the

_{(oc, p).}

The oc

_energies

were chosen not too far from thresholds in order to have the nuclei recoil in the forward

direc-tion with little

_spread

in

_angle

and initial

_velocity.

Neutron

_producing

reactions

_{generally give good yield}

even close to threshold. Recoil velocities obtained

_by

bombarding

with oc

particles

were in the range v

0.004 ~ - ~

0.008.

c

For the measurements with solid

_stopping

media the

targets

consist of thin

_evaporated

films

_(typically

20

_{~Zg Jcm2)}

on thick

gold,

_{copper and} carbon

backings.

The measurements in 43Ca were

_performed

_by

use of

argon as a common

_target

and

stopping

_gas. For the

other measurements _{with gas}

_stopping

_media,

thin

targets

were

_deposited

on the gas side of the

pressure-retaining platinum

or Havar foils.

A 22 cm3 coaxial

_Ge(Li)

counter was

_placed

at

distances between 2 and 4 cm from the

_target.

The detector has a resolution of 3.5 keV for 1.33 MeV

y-rays. Pulses were

_amplified

_by

a model 118 A Ortec

preamplifier

and a model TC 200 Tennelec

amplifier.

The y-ray

spectra

were recorded in 2 048 channels

of an

_{Intertechnique}

_{pulse-height analyzer.}

Radio-active sources were used

[to

calibrate the

_system

in

energy and to monitor its

_stability.

FIG. 1. - Gamma rays from the 593 _- 0 keB~ and

the 993 - 374 keV transitions observed in the

4:> ~rB.r( cx,

n)43Ca

reaction _{for dïfferent gas pressures.}

216

For each level the unshifted y-ray energy at 900 with

_respect

to the

_incoming

beam and the shifted

y-ray energy at 00 were measured. For the

measure-ments _{with gaseous}

_stopping

_media,

_{the energy of}

the y-rays emitted in the recoil direction was measured

at different _{pressures up} to 30

_kgjcm2.

Measurements

with recoils into vacuum or low _pressure were

perfor-med in order to obtain the

_fully

_{shifted y-ray}

_energies.

These were

_always

found to be in

_good

_agreement

with the calculated

energies.

An illustrative

_example

of

the results obtained for two _{gamma-rays in the}

4OAr(oc, n)43Ca experiment

is

_given

in the

_figure

1. III.

_Analysis

of the data. - The measured

Doppler

shift

_F,

as a fraction of the full

shift,

was used to derive the nuclear lifetimes. The centroids of the

_peaks

define the

_energies

of the _{gamma-rays. The}

_F(T~

cur-ves were obtained from the theoretical formalism

given

by

Blaugrund [1].

It takes into account the electronic

and nuclear contributions to the

_slowing

down and the nuclear

_scattering

of the recoil ions. The

elec-tronic contribution is corrected

_{by including}

the

perio-dic variations of the

_experimental

electronic

_stopping

cross-sections with the atomic number of the recoil ion. Also an

_{adjustable parameter fn}

for the nuclear

contri-bution is introduced. For the ions considered

_here,

no

_experimental

_{range-energy data} are available from which one could deduce the correct value of

_fn.

However,

this latter can also be found when states with

well known lifetimes are

_present,

and has been

tentati-vely

taken to be about 0.3 of the Fermi-Thomas

value

_[2].

~.

REFERENCES

[1]

BLAUGRUND

(A.

E.),

Nucl.

_Phys.,

_1966, 88, 501.

[2]

LINDHARD

(J.),

SCHARFF

_(M.)

and SCHIOTT

_{(H. E.),}

Kgl.

Danske Vidensk. _Selskab., Mat.

Fys.

Medd., 1963, 33, 14.

MESURES DE VIES MOYENNES PAR EFFET DOPPLER

A.

_GALLMANN,

F.

_HAAS,

B. HEUSCH et M.

_TOULEMONDE,

Centre de Recherches Nucléaires de _{Strasbourg-Cronenbourg.}

Résumé. 2014 Des vies moyennes de niveaux nucléaires ont été _{déterminées par} la méthode

de l’effet

_Doppler.

Les

rayonnements

gamma de désexcitation ont été détectés dans des

compteurs

Ge(Li).

Des vies _moyennes ont été _{obtenues pour} les niveaux 0,953 MeV de 12B

et 6,44 MeV de 14N.

Abstract. - Lifetimes of nuclear states have been obtained

by

the

_Doppler

shift atte-nuation method. _{The 03B3-rays} were detected in lithium-drifted

_germanium

counters. Lifetimes of the 12B 0.953 MeV and the 14N 6.44 MeV levels are

_reported.

REVUE DE PHYSIQUE APPLIQUÉE TOME 4, JUIN 1969,

La

_possibilité

d’utiliser des

_compteurs

_Ge(Li)

de très bonne résolution

_permet

actuellement la mesure

de _{vies moyennes} nucléaires

_comprises

entre 10-12

et 5 X 10-1~ s. La mesure des

_{déplacements}

en

_énergie

des raies y est bien

_plus

_{précise qu’avec}

les détecteurs NaI utilisés

_{précédemment.}

D’autre

_part,

la résolution des

_Ge(Li)

est

_{généralement}

de l’ordre de

_grandeur

ou inférieure aux

_{déplacements Doppler;}

il est _alors

pos-sible d’étudier les distributions

_{d’énergies}

_des

rayon-nements _gamma observés.

Les

_rayonnements

_{gamma des réactions}

11B( d,

p)12B(1)

et

_12C(3He,

_{p) 14N (2)}

à des

_énergies

incidentes de

_2,1

et

_4,0

_MeV

respec-tivement,

ont été détectés dans un

_compteur

_Ge(Li)

de 22 cm3.

_{L’exploitation}

des

_spectres

_gamma

enre-gistrés

à

_Oy

=

70,

900 et

_1730,

se faisait de la

_façon

suivante : les

_{déplacements}

en

_énergie

des

rayonne-ments _{gamma dus à l’effet}

_Doppler

ont été

_obtenus,

après

soustraction d’un bruit de fond

_exponentiel

sous les

_pics

étudiés,

_{par la} détermination des centres

de

_gravité

des

_pics

aux trois

_angles.

Les

_{déplacements}

des raies y ont été

_analysés

en tenant

_compte

de la

perte

d’énergie

des ions de recul dans les milieux