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

(and the corresponding bombarding energies) were: Ca( Si, 2pn) Ge, 90 MeV;

40Ca(32S, 2pn)69Se, 100 MeV; 6°Ni(16O, 3n)73Kr, 75 MeV; 4°Ca(4°Ca, 2pn)77Sr,

130 MeV; 52Cr(32S, 3n)81Zr, 110 MeV; and 56Fe(32S, 3n)85Mo, 120 MeV. Each

has been schematically represented in fig. 4 by an arrow with its tail on the

compound nucleus and its head pointing to the precursor produced.

As illustrated in fig. 5 the targets - in each case a self-supporting2

foil >v. 1.2 mg/cm thick - were attached to a vertical shaft and mounted with

their surfaces inclined at 15° with respect to the beam direction. Following

bombardment., the beam was magnetically interrupted and the target pneumatically

driven in "V 2 s from the irradiation position to a counting position 25 cm

below, where detectors were arranged to observe it in extremely close geometry.

A surface barrier counter telescope with an 11 vim AE detector was mounted

% 1.5 mm from the front of the target subtending a solid angle of about 20% of

24TT Sr. A 200 mm X-ray detector was placed the same distance from the back

of the target, and a large Ge(Li) Y~ r a v counter was ̂ 2 cm away, viewing the

target through the counter telescope. Protons were distinguished from the

high flux of positrons by feeding the AE and E signals to an analogue power

law identifier.

- 6 -

During the counting period, singles events for protons, X-rays

and y~rays were routed separately into four or eight sequential spectra

each in order to extract lifetime information. In addition, data

for p-X, p-y and X-y coincidences were stored, event by event on magnetic

tape, for subsequent playback with selecced gating conditions. A total of

11 ADC's were employed simultaneously.

More numerous p-y data were also obtained by replacing the Ge(Li)

detector with NaI(T£), and y-P, coincidences were, recorded in a third con-

figuration, which utilized a NE102 plastic sci .tillator for beta detection.

Energy calibrations were all established both with sources and on-line by

producing known activities.

Some of the singles proton energy spectra are shown in fig. 6.

The origin of each activity has been established through a combination of:

1) the energy of coincident X-rays; 2) measured excitation functions; 3) energy

systematics; and 4) agreement between the observed proton half lives and

those of g-delayed y~rays populating known states in the daughters. The

smooth curves in the figure are the results of detailed calculations based

upon the principles already described. Only the ratio of f /T was allowed

to vary in order to match the experimental data at low energies, and (Q -B )

was varied for agreement at high energies. It is evident that overall

agreement with the average behaviour of the data is quite satisfactory,

Porter-Thomas fluctuations apparently causing deviations usually much less

than a factor of two. This observation, together with the insensitivity of

the calculated behaviour at high energy to the other details of the calculations,

leads one to believe that such measurements can indeed yield accurate

decay energies.

- 7 -

As an independent check, though, we have measured the decay energy

of Se by two other means. Obviously at the same time as these spectra were

being accumulated, Y~rav data were also being stored, and with the aid of

measured decay rates and coincident X-rays, these y-rays could be placed

69into detailed decay schemes. The scheme for Se appears in fig. 7. It is

irrelevant to the present discussion except insofar as it illustrates the

69alternate methods of measuring the decay energy of Se. The first approach

was to measure the positron spectrum in coincidence with 691.4 keV y-rays,

which de-excite the 789.4 keV state in As. The second was to measure the

proton spectrum in coincidence with annihilation radiation, i.e. those protons

emitted following $ -decay rather than electron capture. The ratio of those

coincidences, N D,to the total proton counts, N , is completely independentPfcs P

of the proton spectrum and reflects only the energy dependence of the ratio

of positron emission to electron capture. The experimental results appear

in fig. 8.

The smooth curve in fig. 8a is a standard spectrum shape obtained

experimentally from the known decays of Co and Sc , which have comparable

end-point energies. It has been scaled, as has the calculated curve in

fig. 8b, in order to obtain experimental decay energies from both sets of

experimental data. The results are compared with one another and with the

proton spectrum analysis in table I. The comparison makes it evident that

the determination of the (Q -B ) value from the proton singles spectrum is aec. p

reliable technique apparently not seriously affected by Porter-Thomas

fluctuations nor by anomalies in the beta strength function. It can thus be

used in cases where the proton and gamma-ray yields make the other methods

impracticable..

Having now dealt with the proton singles spectrum and its usefulness

in determining both the decay energies and the ratio of T /T , I should

like to turn to the absolute determination of F through direct measurement

of level lifetimes. The method, which involves comparison of nuclear level

decay times with the atomic K-vacancy filling time, has already been illustrated

in fig. 1. The experimental spectrum of X-rays recorded in coincidence with

protons from Se is shown as the histogram in fig. 9. The two peaks

correspond, as expected, to X-rays emitted before (higher energy) and after

(lower) proton emission.

The X-rays in coincidence with y~rays were also recorded at the

same time, so "standard" peak shapes and energies for each relevant element

could be established by plotting coincidences with specific known Y~rays*

These X-ray peaks have much better statistics than the p~X data and are

plotted as smooth curves in fig. 9. With these standard curves, the x-^ay

peak ratios can be determined unambiguously and in fact plotted as a function

of proton energy.

Before showing the results, though, I should like to emphasize again

that we are not here measuring the lifetimes of individual states but rather

the average of a group of states whose lifetimes are described by a Porter

Thomas distribution. The time decay of such a group is not exponential, as is

the K-vacancy filling, but rather is given by

<N(t)> <* (2Xt+i)"1"5 (3)

where A is the average decay rate, and the partial widths for y emission are

assumed negligible. This effect, which is illustrated in fig. 10, is a con-

sequence of the enhancement of very short and very long half-lives in

a Porter-Thomas distribution. Apart from its conceptual interest, it leads

-9-

only to minor complications when calculating the relationship betx^een level

lifetimes and the observed X-ray ratios.

The observed X-ray ratios plotted as a function of coincident proton

energy appear at the bottom of fig. 11 where the singles spectrum appears again

at the top for comparison.. The smooth curves are the results of the same calcu-

lations mentioned earlier, except that the absolute "normalization" of T has

been allowed to vary, T /T being held constant, to optimize agreement with the

X-ray data. The agreement with the data under these conditions is good, although

the indication of upward fluctuations from the average at low energies in both

spectra, suggesting strongly favoured proton decay in this region, is an intri-

guing possibility that will need to be studied in future experiments with im-

proved counting statistics.

For the time being, though, we can examine in fig. 12 what these

results produce for the average partial widths in As. The error bars give

some indication of the uncertainties involved in fitting the data. The proton

partial widths T are related to optical model transmission coefficients T

through the equation

rp = T(27rp)"1 (4)

where p is the level density. For p we have used the formulas of Gilbert and

Cameron who combine a Fermi gas expression at high excitations with a con-

stant temperature representation at low. Actually it was the level density

parameter a that was varied in order to adjust the "normalization" of F . The

~ P

value obtained for this parameter compares favourably with the prescriptions

of reference 7 and with results from neighbouring nuclei. The values for V

are somewhat higher than predictions using simple El strength functions, but

such values are not inconsistent with known widths in the same mass region.

- 10 -

We have already obtained some lifetime data on the other delayed

proton precursors in the series, so it is evident that this new measurement

technique shows promise of quite broad applicability. In this context,

it is the variation of K-vacancy lifetimes with atomic number that determines

the technique's usefulness. In fig. 13 the variation of Tv is shown for the

range of Z covered by the T = 1/2 series of precursors. Also shown are

curve •. giving estimates for the maximum width of proton emitting states that

can be populated by the precursors' electron capture decays. Where these

maximum values are comparable with or greater than Tv the p-x coincidence

technique can be usefully applied. Evidently this is the case when proton

decay can occur with L=0 but the situation is less favourable for higher

L-values and higher Z.

This dependence on L-value, however, is not without its virtues.

The measured level lifetimes could in some cases be used to determine the

L-value of proton emission, and this in turn could establish, or at least

restrict, the spin of the precursor itself.

The conventional methods for measuring lifetimes directly - delayed

coincidence and Doppler shift techniques - cover a wide range of times greater

than about 10 seconds. For shorter lifetimes only the blocking technique,

which has so far been restricted to a few favourable mate'ials, has been

available till now for making direct measurements. It can be seen from

fig. 13 that the range of nuclear lifetimes made available in principle by

the p-X coincidence method abuts the region conventionally accessible and

extends it downward by more than two orders of magnitude. It is already

apparent that this will lead, in conjunction with other aspects of g-dolayed

proton spectra, to a detailed understanding of many nuclei far removed from

the valley of stability.

- 11 -

REFERENCES

1) G. Gamow, Nature Ylb_ (1930) 397.

2) J.C. Hardy, in Nuclear Spectroscopy & Reactions, Part C, edited by

J. Cerny (Academic Press, New York, 1974) pg. 417.

3) G.S. Mani, M.A. Melkanoff & I. Iori. Commissariat à l'Energie Atomique,

CEA-2379 (1963).

4) G.A. Bartholomew, E.D. Earle, A.J. Ferguson, J.W. Knowles & M.A. Lone

in Advances in Nuclear Physics (edited by M. Baranger & E. Vogt)

Volume 7, pp 229-324. Plenum Press, New York, 1973.

5) K. Takahashi, M. Yamada & T. Kondok, Atomic Data & Nuclear Data Tables

12 (1973) 101.

6) P. Hornshtfj, K. Wilsky, P.G. Hansen, B. Jonson & O.B. Nielson, Nucl. Phys.

A187 (1972) 599 and 609.

7) P. Hornsh^j , K. Wilsky, P.G. Hansen & B. Jonson, Nucl. Phys. A187

(1972) 637.

8) R.L. Auble, Nuclear Data Sheets JJS (1975) 351.

9) E. Nolte, Y. Shida, W. Kutschera, R. Prestele & K. Morinaga, Z. Physik

268 (1974) 267.

10) A. Gilbert & A.G.W. Cameron, Can. J. Phys. 4J3 (1965) 1446; J.W. Truran,

A.G.W. Cameron & E. Hilf, CERN report 70-30 (1970) 275.

11) K.D. Sevier, Low Energy Electron Spectrometry, pp 220-241, Wiley-

Interscience, New York, 1972.

12) 0. Keski-Rahkonen & M.O. Krause, Atomic Data & Nuclear Data Tables

14 (1974) 139.

- 12 -

TABLE I: Measurements of Q -P for Seec p

Measurement Q -Bec p

technique (keV)

g+ end point (fig. 8a) 3422 ± 90

N O/N (fig. 8b) 3364 ± 75PP P

Proton spectrum (fig. 6) 3400 ± 100

- 13 -

DAUGHTER / P

PRECURSOR

E-C/pZ.N

EMITTER( Z - l , N+l )

TBP

'E.C.

FOR INDIVIDUAL PROTON TRANSITION i - » f :

FOR UNRESOLVED PROTON SPECTRUM:

Ip(EP) = S < I p > E

Fig. 1: Decay scheme for a typical delayed-proton precursor, illustrating

some symbols and terms used in the text.

- 14 -

UN

ITS

(AR

B.

tzto

10

5

LL)

PROTONSPECTRUMIP(EP) X

1.0 2.0 3.0PROTON ENERGY, Ep(MeV)

Fig. 2: Typical proton spectrum calculated for a medium mass nucleus,

illustrating the contributions from beta-decay and proton widths.

- 15 -

X-RAY ENERGY

Fig. 3: Pictorial representation of the p-X coincidence technique for

lifetime measurements on proton unstable states produced by

electron capture.

- 16 -

50 -

Fig. 4: A portion of the chart of nuclides showing the T = 1/2 precursorsz

already positively identified (solid boxes with mass number within)

and tentatively identified (dashed box).

- 17 -

SHAFT FROMPNEUMATIC —CYLINDER

IRRADIATIONPOSITION

/ €\ TARGET

BEAM

X-RAY —COUNTER

COUNTINGPOSITION

IL'Ji\_y COUNTER

ot)- PROTONCOUNTERTELESCOPE

Fig. 5: Schematic diagram of the experimental arrangement.

- 18 -

fSr

30 j

201-

oo

5

50 I-

1.0 2.0 3.0 4 0

PROTON ENERGY (MeV)

Fig. 6: Spectra of protons observed following the decays of Ge, Se, Kr77 73

and Sr. The latter has been corrected for -\, 25% contribution from Kr pro-

tons, whose presence was discerned from the coincident X-ray spectrum. The

smooth curves are the results of statistical model calculations with the dashed

lines at high energy representing ± 100 keV changes in (Q -B ) values. A

general decay scheme is also shown.

P/1.4%/

2* 4417,

, P/98.6%

0+ 34Ooj68 -

Ge + p

- 19 -

/

-6700 E C / r

-4300 J

S

67909Se

0.07%

?O <o'

$8?• !?

i >>i

< <$

^ to to

A

•

Of Q ") O

F * — - • • • - - j

E I/3*+tr l°9 "(keV) ** (O/^

C

2346.6 1.0 6.1

2149.9 1.4 6.1

1864.3 2.5 6.01690.7 2.1 6.1

1076.2 2.0

789.4 22.5

497.3 3.5

20.220.024.8

6.4

5.5

6.4

5.85.5.n

69'As

Fig. 7: Decay scheme for Se.

- 20 -

to

o

150

100-

5 0 -

1 2 3 4NOMINAL BETA ENERGY (MeV)

JITS

Q:

tz

AR

BI

5

4

3

2

PROTON ENERGY (MeV)

Fig. 8: (a) Spectrum of positrons in zoincidence with 691.4 keV gamma-rays

from the decay of Se; the smooth curve is the scaled up experimental spectrum

shape. (b) Experimental N R/N spectrum compared with a curve calculated frompp p 2

the known positron to electron-capture ratio. The normalized x f° r the fitsshown in (a) and (b) are both slightly less than 1.0.

- 21 -

10 II 12X-RAY ENERGY (keV)

13

Fig. 9: The histogram gives the spectrum of X-rays observed in coincidence

with all delayed protons. The smooth curves are X-rays measured simultaneously,

with the same detector, in coincidence with specific known gamma-rays; they are

normalized in height only to fit the histogram.

22 _

Fig. 10: Decay curve of a group of states whose decay rates scatter with a

Porter-Thomas distribution about the average value A, compared with the expon-

ential decay of a single state with decay rate X.

- 23 -

500

PROTON ENERGY ( MeV)

Fig. 11: (a) Spectrum of protons observed following the decay of Se; the

experimental resolution (FWHM) was ̂ 90 keV. A simplified decay scheme is also

shown; energies are given in MeV relative to the As ground state. (b) Ratio

of Ge X-rays (measured in coincidence with protons) relative to those from As,

plotted as a function of coincident proton energy. The smooth curves in (a)

and (b) are the results of calculations described in the text.

- 24 -

>

I

<

g

10

1.0

0.1

0.01

: LEVELS

I" Ty = / E 3

1

IN

fE./

/

/

1

69As

>,/p.,

/

^ — —

rp=T(27r/>)"

1 r

L_zJ i

i I

i i

i

r5.0 6.0

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Fig. 12: The average values of F and V as a function of excitation energy69 P Y

in As. The error bars at the right correspond to the uncertainty in deter-

mining the absolute values from the delayed proton spectra and lifetime measurements.

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Fig. 13: Plot of K-vacancy width as a function of atomic number '

(solid curve). Also shown as dashed curves are the approximate maximum

widths for levels that could be produced by electron capture from precursors

in the T = 1/2 series; these curves are given for levels that decay byz

L = 0,1,2 proton emission to the daughter ground state.

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