.
-
ç
.
TOFL.
ORNL P
1307
.
ar!
.
.
.:
.
.
9
;
FEEEEEEE
11:25 14
16
MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963


9
ES
VER
1
•
.
LEGAL NOTICE
This report was prepared as an account of
Government sponsored work. Neither the
United States, nor the Commission, nor any
person acting on behalf of the Commission:
A. Makes any warranty or representa-
tion, expressed or implied, with respect to
the accuracy, completeness, or usefulness
of the information contained in this report,
or that the use of any information, appa-
ratus, method, or process disclosed in this
report may not infringe privately owned
rights; or
B. Assumes any liabilities with respect
to the use of, or for damages resulting from
the use of any information, apparatus,
.. method, or process disclosed in this report.
As used in the above, "person acting on
behalf of the Commission" includes any em-
ployee or contractor of the Commission, or
employee of such contractor, to the extent
that such employee or contractor of the
Commission, or employee of such contractor
prepares, disseminates, or provides access
to, any information pursuant to his employ-
ment or contract with the Commission, or
his employment with such contractor.
*
ORNY P43.07
CONF-150706-70 MASTER
064
Ayurvu NOISE
Neutron Resonance Capture in 26-la Shell Nucles
I. Bergqvist, J. A. Biggerstall, J. H. Gibbons, and W. M. Good
Oak Ridge National Laboratory
Oak Ridge, Tennessee
LEGAL NOTICE -

This report was prepared u HA ACOCA of Govenoral sponsored work, Nelebor the United
saws, nor the Commission, nor wy porno acuos os behalf of the Commoloa:
A. Kun ny warraty or prinutos, exproud or implied, will respect to the accu-
ricy, completeness, or wwwfaldast of the laformation contatood is the report, or that we we
ol say Information, appunto, pothod, or proceso decloud in de report may not latringe
prinuly omad richte; or
B. Ashmon nay liabiuues no respect to the un of, or for dupa o resulting from the
um ol may taformelloa, appundus, method, or procow di cloud la to report.
As used in the abon, "parece acting as bakall of the Commission includne way ou.
ploge or contractor of Use Cocaminton, or employs of rock coatracor, to the extent that
toch employee or contractor of the Comaloola, or employee of much coolrictor preparu,
dlasemiasins, or provide accu to, da laformation purnuant to kiu esployment or coatrict
viu ute Coutadaslon, or this employment with such contractor.
A neutron resonance capture experiment on nuclei in the 28-là shell,
1.e. in the mass region 19 AS 40 has been performed. The apparatus
has been described in detail (1). Neutrons were produced by the Lil'(0,n)Be
reaction and the time-of-flight technique was employed with a flight path
of about 40 cm. The garma-ray spectrometer was a well-sluielded 9 in. x 12 in.
Naf(TI) crystal.
In Fig. 2, the gamma-ray spectra from the capture in two resonances
in Mg 24 (,) are shown. The 84-keV resonance 18 well established to have
Jose [2]. Strong transitions are observed to the ground state ),
the first excited state at 0.58 Mev $*) and to the levels at 1.96 Mevca
and 2.
o transitions to negative parity states have been
.
·
Research sponsored by the U. S. Atomic Energy Commission wder contract
with the Union Carbide Corporation.
act
. Visiting scientist from Research Institute of National Defense,
Stockholm, Sweden.
PATENT CLEARANCE OBTAINED. RELEASB TO
THE PUBLIC IS APPROVED. PROCEDURES
ARE ON FILE IN THE RECEIVING SECTION,
-
Identified, which confirms the assignment to the resonance (see discus-
sion below). The 50-keV resonance has not previously been reported from
neutron measurements but appeared very distinctly in our measurements.
This level has been seen in the (d,p) reaction (3]. An bv=2 stripping
angular distribution seems to fit these experimental results, which means
that the level could be either or . However, the gamma-ray spectrum
shows strong transitions to ground and to the first excited state, similar
to the 84-kev () resonance. Transitions to negative parity states are
weak. Thus, our results favor an assignment of to this resonance as
well as to the 84mseV resonance.


Ха, ум
Enes 84 nov; v
Мого, мога
ERES 80 nov:
|
34
COUNTS/AE (arbitrary units)
COUNTS/SE (orbitrory units)
ENERGY (MO)
ENERGY (Mor)
(**)(727 (42) (42")
Ya V \
.
.
(**) (**(**) (**)
**
PREDOMINANTLY
--- ..... ------ ----..
.
.
.
.
.
Fig. 1. Neutron capture gamma-ray spectra of Mg Prom the
84-keV and the 50-keV resonance states. The bar graphs
represent the primary gamma-ray lines from thermal neutron
capture with dashed bars to indicate that the corresponding
gamma ray has a precursor.
-
-
-
--
.
The kev-range neutron resonance structure of nuclei in the 25-1d
shell region has been rather thoroughly studied by transmission methods,
with the assignment of spins, parities, and total width in a number of cases.
For such resonances, we endeavor to measure other resonance parameters
(T., and ) to determine partial radiation widths for various multipole
transitions. In the present pote we will comment on a few characteristics
of the systematics thus far observed of the ratio M/EL of the reduced
widths ( ES). The Weisskopf estimate of this ratio is about
boy ~ 3 x 10-2 (for r. = 1.2 L) for nuclei with 195 A $ 33. For nuclei
with A < 20 it has been found that M. transitions in terms of Weisskopf units
are on the average a factor of 3 stronger than 1. If this is true also for
nuclei in the 25-1d shell, then M/F1 ~ 10-7, Compilations of radiative
widths from charged particle reactions 14,5) in nuclei in the 25-1d shell
give a factor as large as 30-50 in favor of M transitions in terms of
Weisskopf units, i.e. M/A ~ 1. Such a large strength of M transitions is
understandable if one assumes the presence of a giant resonances" due to
"spinflip" of 25/2 nucleons. The effect would have its maximm 1f the ld5/2
level were full and the idzlo level empty [6]. A similar effect has also
been proposed in heavy nuclei [7].
The 2s-là shell compound nuclei formed after neutron bombardment of
elements in the mass range 19 S A433 are, because of the nature of the
relative abundance, preponderantly of two types: (a),(2, N = 2 + I) for
Z even and (b) (2, N = 2 + 2) for z odd. It will be observed [8] that the
thermal. neutron capture spectra from these two classes of nuclei differ in
a systematic fashion which can be described by saying that, "in the even-oda
nuclei (i.e. Mg, Si, s) electromagnetic transitions are predominantly .
A (M/E1. ~ 20), whereas in the odd-odd nuclei (Na, c1, Al, P) M and .
transition strengths are comparable (M/E1. . 2004).
Since thermal capture is s-wave, the first question 18 whether the
$-wave resonant
scecta snow the same tegnes AA PA A
OA harmon
-3-
capture. We show one case to illustrate this point for the odd-odd and one
case for the even-oää сlasses of nuclei (Fig. 2), viz, the 35 keV resonance
(3+) in 'Al target nucleus and the u kev resonance ) In 32s. A striking
agreement is seen in the case of aluminum for both thermal and resonance capture
M/E ~ 1. M transitions in the sulfur spectrum from resonance capture to the
ground and first excited states are much stronger than from thermal capture.
However, the ratio M/El is about 2 x 104 and 100%, respectively, ...e.
M/F << 1.
+
-
-
-
-
.

ART (a, y) ARO
fra ww; veje
832(0, y1s33
(na 111 mov; V**
COUNTSAF (orbi ory win)
courtstas (arbitrary wits)

L
! ! ;
illiette
; ; !:
ód
iT
DUONGY (A
?
ONEROY (Wola
(**)
*
Mengu
l
in its afin
(**)
6. Mencanny
Fig. 2. Neutron capture gamma-ray spectra from the 35-keV
resonance state in °Al and the 11-keV resonance state in
33s. (See text of Fig. 1)
-
-
- -
.
The next question is whether p-wave resonant capture spectra exhibit the
-
systematics of thermal capture. The MI/ratio in Mg C+(a,x) (Fig. 1) from
tú a 84-kev resonance is of the order of 1002 (or lees, since no M. transitions
have definitely been identified). This means that the intensity of lines to
negative parity levels such as those at 3.41 MeV and 4.27 MeV is less than a
few percent. The spectrum from the 50-keV resonance 18 statistically less
accurate but shows that M. transitions are weak. Examples of p-wave capture
-4
spectra in an odd-Z nucleus (40F) are given in reference 1. It 18 shows
that transitions to the even pazity states at low excitation energies,
the same levels to which strong M transitions occur in thermal capture, are.
attenuated.
. The large difference of the average M1/EK ratios from nucleus to nucleus
in the 28-1d shell nuclei found in thermal neutron capture are, at least
partly, reflected in resonance capture. The presence of a glant resonance
states" might explain the enhancement of Ma transitions (over el) found in
these several nuclei.
References
1. J. R. Bird, J. A. Biggerstaff, J. #. Gibbons, and W. M. Good, Phys. Rev.
138, B20-25 (1965).
2. R. Č. Block, W. Haeberli, and H. W.Newson, Phys. Rev. 109, 1620 (1958).
3. R. Midaleton and S. Hinds, Nuclear Physics 34,404 (1962).
4. C. van der Lew, Int. Conf. of Nuclear Physics, Paris, France (1964).
5. E. Spring and A. Varkki, Soc. Scient. Fennica 29, 7 (1964).
6. D. Kurath, Phys. Rev. 130, 1525 (1963).
7. I. Bergqvist, B. Lundberg, and N. Starfelt, Int. Cont. on Nuclear Paysics
with Reactor Neutrons, Argonne (1963), ANL-6797, p. 220.
8. L. V. Groshev, A. M. Demidov, v. N. Lutsenko, and v. I. Pelehov,
Atlas of y-ray Spectra from Radiative Capture of Thermal Neutrons
(Pergamon Press, London, 1959).
END
4
DATE FILMED
18 / 27 /65