AECL-5560 ATOMIC ENERGY OF CANADA LIMITED L'ENERGIE ATOMIQUE DU CANADA LIMITEE BET/A-DELAYED PROTON EMISSION: A NEW SERIES OF PRECURSORS AND THE MEASUREMENT OF 19 16 S NUCLEAR LIFETIMES by J.C. HARDY, J.A. MacDONALD, H. SCHMEING, T, FAESTERMANN, H.R. ANDREWS, J.S. GEIGER, R.L. GRAHAM and K.P. JACKSON Text of an invited talk presented by J.C. Hardy to the Washington Meeting of the American Physical Society, April 1976 Chalk River Nuclear Laboratories Chalk River, Ontario August 1976
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AECL-5560
ATOMIC ENERGYOF CANADA LIMITED
L'ENERGIE ATOMIQUEDU CANADA LIMITEE
BET/A-DELAYED PROTON EMISSION: A NEW SERIES OF PRECURSORSAND THE MEASUREMENT OF 19 16 S NUCLEAR LIFETIMES
by
J.C. HARDY, J.A. MacDONALD, H. SCHMEING, T, FAESTERMANN,
H.R. ANDREWS, J.S. GEIGER, R.L. GRAHAM and K.P. JACKSON
Text of an invited talk presented by J.C. Hardy to the Washington Meeting
of the American Physical Society, April 1976
Chalk River Nuclear Laboratories
Chalk River, Ontario
August 1976
ATOMIC ENERGY OF CANADA LIMITED
BETA-DELAYED PROTON EMISSION: A NEW SERIES OF PRECURSORS
AND THE MEASUREMENT OF 1 0 ~ 1 6 s NUCLEAR LIFETIMES
by
J.C. Hardy, J.A. Macdonald, H. Schmeing, T. Faestermann,H.R. Andrews, J.S. Geiger, R.L. Graham & K.P. Jackson*
Text of an invited talk presented by J.C. Hardy to the WashingtonMeeting of the American Physical Society, April 1976.
* Physics Department, University of Toronto, Toronto, Canada.
Chalk River Nuclear LaboratoriesChalk River, Ontario
August 1976
AECL-5560
Emission de protons retardés par la désintégration bêta:
nouvelle série de précurseurs et mesure
des durées de vie nucléaire Ç^IO s) ,
par
J.C. Hardy, J.A. Macdonald, H. Schmeing, T. Faestermann,H.R. Andrews, J.S. Geiger, R.L. Graham & K.P. Jackson*
Résumé
L'étude de l'émission de protons retardes par la désintégration bêtapermet d'obtenir des données concernant aussi bien la décroissance bêta duprécurseur radioactif que les propriétés des états excités dans l'émetteur.Parmi les éléments légers, où quelques fortes transitions dominent généralement,la série de pair-Z, Tz
= -3/2 noyaux a permis d'avoir un aperçu préalable deT = 3/2 états analogiques et des transitions suprapermises qui les peuplent.On a obtenu des résultats dans une nouvelle série de précurseurs pair-Z avecT z - +1/2. Comme tous les lourds précurseurs connus, les noyaux identifiésjusqu'à présent - 65Ge, 69Se, 73Kr, 77Sr, 81Zr et provisoirement 85Mo - seprésentent comme de larges continuums de protons. Cependant, la disponibilitéd'une telle série de noyaux permet d'extraire des spectres observés une imagesystématique de la fonction de l'énergie de désintégration bêta ainsi que desdensités de niveaux, des largeurs et des énergies de désintégration des spectresobservés. En ayant recours a une nouvelle technique expérimentale, les auteursont pu déterminer les valeurs absolues des largeurs par une mesure directe dela durée de vie moyenne des états peuplant l'émetteur. Cette technique permetde mesurer les rayons X en coïncidence avec les protons. Etant donné que lesétats émetteurs de protons sont fréquemment peuplés par la capture d'électronsde couche K, leurs durées de vie peuvent être mesurées par rapport à celle dela lacune de la couche K (^10~l"s dans cette région de masse), simplement ennotant les nombres relatifs de rayons X coïncidents qui correspondent auxnoyaux d'émission et de filiation. Ces données permettent de mettre à dure •épreuve les calculs de modèle et les formules de masse dans une région denoyaux avec N ^ Z très éloignée de la vallée de stabilité.
*Department of Physics, University of Toronto, Toronto, Canada
L'Energie Atomique du Canada, LimitéeLaboratoires Nucléaires de Chalk River
Chalk River, Ontario
Août 1976
AECL-5560
ATOMIC ENERGY OF CANADA LIMITED
BETA-DELAYED PROTON EMISSION: A NEW SERIES OF PRECURSORS
AND THE MEASUREMENT OF 1O~1 6 s NUCLEAR LIFETIMES
by
J.C. Hardy, J.A. Macdonald, H. Schmeing, T. Faestermann,H.R. Andrews, J.S. Geiger, R.L. Graham & K.P. Jackson*
ABSTRACT
The study of beta-delayed proton emission yields information bothon the. beta-decay of the precursor nucleus and on the properties of excitedstates in the emitter. Among light elements, where a few strong transitionsusually dominate, the series of even-Z, T z = -3/2 nuclei provided an earlyinsight into T = 3/2 analogue states and the superallowed transitions thatpopulate them. We have now obtained results on a new series of even-Zprecursors with Tz = +1/2. Like all known heavy precursors, the nuclei sofar identified - 65Ge, 69Se, 73Kr, 77Sr, 81Zr and provisionally 85Mo - exhibitbroad proton continua. However, the availability of such a series of nucleimakes it possible to extract a systematic picture of the beta-decay strengthfunction as well as level densities, widths and decay energies from theobserved spectra. By the addition of a new experimental technique we havealso been able to determine the absolute values of the widths through directmeasurement of the average lifetime of states populated in the emitter.The method involves the measurement of X-rays in coincidence with protons.Since the proton-emitting states are frequently populated through K-shellelectron capture, their lifetimes can be measured relative to that of theK-shell vacancy (y 10~16 s ±n this mass region) simply by observing therelative numbers of coincident X-rays corresponding to the emitting anddaughter nuclei. These data all provide stringent tests of model calculationsand mass formulae in a region of nuclei with N ^ Z, far removed from thevalley of stability.
* Department of Physics, University of Toronto, Toronto, Canada.
Nuclear Physics BranchChalk River Nuclear Laboratories
Chalk River, Ontario
August 1976
AECL-5560
- 1 -
Beta-delayed particle decay from an excited state was first
observed in 1916 by Rutherford and Wood when they noted the presence of a few
high energy alpha-particles in addition to the familiar groups from thorium C
?1 2(~ Bi). It wasn't until 1930, though, that Gamow was able to explain the
mechanism by which these high energy alphas could be emitted. His explanation,
read 45 years later, has a quaint ring to it but it is succinct and accurate.
212Me noted first that thorium C can beta-decay to thorium C' ( Po), and he
then goes on to say that if the latter nucleus "is left in an excited state...
either the alpha-particle will cross the potential barrier surrounding the
nucleus and will fly away with the total energy of the excited level... or it
will fall down to the lowest level, emitting...gamma-rays, and will later fly
away as an ordinary alpha-particle"
That seemed to settle the issue, and except for the 1939 discovery
of beta-delayed neutrons from uranium fission, very little interest was generated
by the phenomenon until 1963. At that time, with the first detection of beta-
delayed protons, and more recently with the construction of high resolution
neutron detectors the spectroscopic usefulness of these beta-delayed processes
2)came to be recognized .
A more contemporary illustration of beta-delayed proton decay
appears in fig. 1. Its spectroscopic value can be seen from the one-to-one
relationship that exists between the proton decay of a state in the emitter
and the beta-transition feeding that state; the observed intensities of protons
thus reflect the intensities of the preceeding beta-decays and, through theu.,
the properties of excited states in the emitter. For the process to occur at
all, though, it is necessary to have a precursor nucleus with a relatively
high beta-decay (or electron capture) Q-value feeding an emitter with a
relatively low proton separation energy (B ). This combination is more likely
to occur in neutron-deficient nuclei well removed from the valley of stability,
- 2 -
so immediately one sees that delayed protons provid. a means of studying exotic
nuclei that might o *erwise be intractable.
Among light nuclei, where states in the emitter are well separated,
individual transitions can be clearly resolved and over the years a variety
of complex (3-decay schemes have been examined in this manner. In fact, the
series of even-Z, T = -3/2 nuclei, which now spans fourteen precursorsz
between C and Ge, has provided the most extended region for the systematic
observation of 3~delayed proton emission, yielding a variety of spectroscopic
information on matters as diverse as Gamow-Teller giant resonances, analogue
states and isospin mixing.
Emitters among the heavier elements, though as numerous, have not
fitted so neatly into a regular pattern. Their analysis too is not so
transparent, since a high density of states in the emitter results in
proton spectra that appear as unresolved continua. Thus, instead of
individual transitions, one usually must deal with the average behaviour of
many transitions described within the framework of a statistical model.
In general terms, the intensity I of an individual proton
transition should depend on two factors: (a) the intensity of the g-decay
(plus electron capture) branch from the "precursor" to the relevant proton-
unstable state (denoted i) in the "emitter"; and (b) the branching ratio for
subsequent proton emission from that level to the state f in the "daughter"
nucleus. Thus
ipi f « [/<o.>2]x [^"/(i-p1 + iy1)] (i)
where f is the statistical rate function and <a>, the Gamow-Teller matrix
element for the 8-decay. In the event that individual transitions are not
resolved, the proton spectrum shape can be written
I (E ) = l< I ± f >„P P f p Ep
- 3 -
where <> indicates that a statistical average has been taken (at the
p
appropriate proton energy E ) over the values of T and the 3-decay matrix
element, which are both assumed to scatter with a Porter-Thomas distribution.
Qualitatively, these simple equations provide an immediate picture
of a delayed proton spectrum (fig. 2). The low energy part of the spectrum
reflects the effect of the Coulomb barrier in rapidly changing the magnitude
of F relative to V while at higher energies, where T »T , the reduction in
intensity with increasing proton energy is simply a consequence of decreases
in the statistical rate function as the energy available for 3-decay goes to
zero. Thus, without specifying the details any more than this, we could use
a delayed proton spectrum to extract a value for (Q -B ) and possibly to
study the relative variations in T near the top of the Coulomb barrier.
To proceed much farther, though, we need some independent description
of the general behaviour of T , T and <o > as a function of excitationp y
energy. Such descriptions exist of course but their reliability in this
context remains a matter for conjecture. The least controversial is probably
F whose variation with energy can be obtained from an optical model pre-
scription based on scattering data . Photoexcitation and (n,y) results4)
have yielded strength functions for calculating T , while 3-decay strength
functions have been derived from global fits to known 3-decay lifetimes .
These methods will be returned to later but for the moment it should be
emphasized that their value lies mostly in reproducing the energy dependence
of a particular quantity not its absolute magnitude. Given the energy dependence,
though, the delayed proton data can be used to yield exactly what is lacking:
the absolute magnitudes.
- 4 -
It is here that the systematic study of a series of emitters becomes
important, together with a new experimental technique. The half life of a
precursor and its branching ratio for proton emission gives the absolute value
of the g-decay strength, while the low energy behaviour of the singles
spectrum relates T to T . Both these results can and have already ' been
derived from delayed proton data, but never before in a series of similar
nuclei where consistency can be checked. More important, however, is the
development of a technique whereby the average lifetimes of levels in the
emitter - lifetimes in the 10 s. region - can be directly measured to
obtain the absolute magnitude of V and, through it, the magnitude of F •
The essence of the technique for measuring these short lifetimes
is shown in fig. 3. It involves comparison of the decay time of a nuclear
state with the filling time of a vacancy in the atomic K shell. Any nucleus
(with atomic number Z) that decays by electron capture to excited states in
the daughter (Z-l) produces simultaneously a vacancy in the atomic K shell.
If, as in the case of g-delayed proton decay, those excited states are
unstable to proton emission, then the energy of the X-ray emitted with the
filling of the atomic vacancy will depend upon whether the proton has already
been emitted (in which case the X-ray would be characteristic of a Z-2
element) or has not yet been emitted (a Z-l element). If the nuclear and
atomic lifetimes are comparable, then the K X-rays observed in coincidence
with protons will lie in two peaks whose relative intensities uniquely relate
one lifetime with the other.
The nuclei we have concentrated our attention on (see fig. 4) all
have even-Z and T = +1/2. The precursors positively identified so far are
65Ge, 69Se, 73Kr, 77Sr and 81Zr with half-lives ranging from 31 s for 65Ge
- 5 -
to 6 s for Zr. The next nucleus in the series, Mo, has already been
provisionally identified, and more members should be observable up to ^ Sn.
Only Kr was previously known as a delayed proton precursor although
6-delayed y~rays had also been observed ' from Ge and Se. No evidence
whatever has been reported for activities with the same half-lives from
77 81Sr, Zr and the heavier members.
All precursors were produced through target bombardments with heavy
ion beams from the Chalk River upgraded MP tandem. The reactions involved