The ⁸Be ground state formation in ¹²C(γ,3α)-reaction
The energy dependence of the total cross section the ⁸Be ground state formation at the energies up to 40 MeV is measured. Angular distributions at three energy intervals are obtained. Results are compared with predictions of a direct knockout α-cluster mechanism.
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| Опубліковано в: : | Вопросы атомной науки и техники |
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| Дата: | 2001 |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2001
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| Цитувати: | The ⁸Be ground state formation in ¹²C(γ,3α)-reaction / S.N. Afanas’ev, A.F. Khodyachikh // Вопросы атомной науки и техники. — 2001. — № 1. — С. 56-58. — Бібліогр.: 8 назв. — англ. |
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Afanas’ev, S.N. Khodyachikh, A.F. 2015-03-17T12:10:17Z 2015-03-17T12:10:17Z 2001 The ⁸Be ground state formation in ¹²C(γ,3α)-reaction / S.N. Afanas’ev, A.F. Khodyachikh // Вопросы атомной науки и техники. — 2001. — № 1. — С. 56-58. — Бібліогр.: 8 назв. — англ. 1562-6016 PACS: 13.75Gx. https://nasplib.isofts.kiev.ua/handle/123456789/78446 The energy dependence of the total cross section the ⁸Be ground state formation at the energies up to 40 MeV is measured. Angular distributions at three energy intervals are obtained. Results are compared with predictions of a direct knockout α-cluster mechanism. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Nuclear reactions The ⁸Be ground state formation in ¹²C(γ,3α)-reaction Образование основного состояния ⁸Be в реакции ¹²C(γ,3α) Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| title |
The ⁸Be ground state formation in ¹²C(γ,3α)-reaction |
| spellingShingle |
The ⁸Be ground state formation in ¹²C(γ,3α)-reaction Afanas’ev, S.N. Khodyachikh, A.F. Nuclear reactions |
| title_short |
The ⁸Be ground state formation in ¹²C(γ,3α)-reaction |
| title_full |
The ⁸Be ground state formation in ¹²C(γ,3α)-reaction |
| title_fullStr |
The ⁸Be ground state formation in ¹²C(γ,3α)-reaction |
| title_full_unstemmed |
The ⁸Be ground state formation in ¹²C(γ,3α)-reaction |
| title_sort |
⁸be ground state formation in ¹²c(γ,3α)-reaction |
| author |
Afanas’ev, S.N. Khodyachikh, A.F. |
| author_facet |
Afanas’ev, S.N. Khodyachikh, A.F. |
| topic |
Nuclear reactions |
| topic_facet |
Nuclear reactions |
| publishDate |
2001 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| format |
Article |
| title_alt |
Образование основного состояния ⁸Be в реакции ¹²C(γ,3α) |
| description |
The energy dependence of the total cross section the ⁸Be ground state formation at the energies up to 40 MeV is measured. Angular distributions at three energy intervals are obtained. Results are compared with predictions of a direct knockout α-cluster mechanism.
|
| issn |
1562-6016 |
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https://nasplib.isofts.kiev.ua/handle/123456789/78446 |
| citation_txt |
The ⁸Be ground state formation in ¹²C(γ,3α)-reaction / S.N. Afanas’ev, A.F. Khodyachikh // Вопросы атомной науки и техники. — 2001. — № 1. — С. 56-58. — Бібліогр.: 8 назв. — англ. |
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2025-11-26T01:24:45Z |
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THE 8BE GROUND STATE FORMATION IN 12C(γ,3α)-REACTION
S.N. Afanas’ev, A.F. Khodyachikh
National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine
The energy dependence of the total cross section the 8Be ground state formation at the energies up to 40 MeV is
measured. Angular distributions at three energy intervals are obtained. Results are compared with predictions of a
direct knockout α-cluster mechanism.
PACS: 13.75Gx.
1. INTRODUCTION
The investigation of the α−disintegration reactions is
of the great interest because it is widely used to prove
the predictions of the α−cluster nuclear model, quasi-α-
particle mechanisms of nuclear reactions and to
investigate the characteristics of the αα−interactions.
When the initial and final nuclei have zero isotopic spin,
the dipole transitions are suppressed. In this case it is
possible to verify the validity of the isospin violation
law.
The 12C(γ,3α)-reaction was investigated with a help
of nuclear emulsions, which were exposed to
monochromatic photons from the reaction of a radiation
capture of proton by the nucleus of Lithium [4,5] and by
the bremsstrahlung photons [5-9]. The quality
agreement between the experimental data in the sense of
the excitation curve form is observed, while the value of
total cross-section has a large discrepancy. This
discrepancy may be caused by the large amount of the
three-particle background reactions on the nuclei of
nuclear emulsions, which are difficult to distinguish only
by the transverse momentum conservation law. The
multiple scattering causes a large inaccuracy in particle
momentum measurements.
The ground state formation of the 8Be was observed
only in the experiments with monochromatic photons
where the 12C(γ,3α)-reaction was surely singled out. The
α1-particle is easy to identify in the channel of the 8Be
ground state production. The analysis of the angular
distributions of α1-particles has shown [4,5] that in this
channel electrical dipole transitions, which are
prohibited by the law of the isospin conservation, are
preferable. Further experiments didn’t show this
channel.
There are some advantages of the 12C(γ,3α)-reactions
investigation by means of the diffusion chamber. In this
case one can obtain an admixture-free target. Owing to
low gas density an inaccuracy in the momentum
measurement greatly diminished because of a multiple
scattering decreasing. An ionization density and its
change along a track length make it possible to identify
a particle charge. The comparison of momentum values
that were obtained by curvature and length
measurements for every track was used as a final
criterion to identify a α-particle.
2. EXPERIMENTAL METHOD
The experiment was performed by the method of a
diffusion chamber in a magnetic field with the strength
of 1.5 T.
The chamber, filled with a mixture of methane
(13%) and helium, was exposed to a beam of
bremsstrahlung photons with end point energy of
150 MeV. Owing to the gas filling, the tracks of slow
residual nuclei had measurable lengths and their images
on a photographic film were sufficiently clear at
pressures close to an atmospheric one. A large angular
coverage and low density of the medium made it
possible to measure the angular distributions of charged
products of multiparticle photonuclear reactions in a
broad energy range. This experimental method makes
possible a charge identification of particles by analysis
of the ionization density and its change along the track.
A primary selection of events of thee-pronged stars was
carried out on the base of the conclusion that all the
tracks were left only by twice charged particles and
those tracks were almost complanar. The inaccuracy in
the momentum measurement was increasing when the
angle between the particle momentum and the median
plane was growing because of the track length
diminishing. An event was not taken into account if the
directing cosine exceeds 0.64 and two or three particles
escaped from the chamber volume. The geometrical
correction for not scored events was calculated by
Monte-Carlo simulation of the reaction.
After a momentum measurement for every track was
done, a sum of momentum projections onto axes was
obtained for each event. Because of a measurement
inaccuracy this sum was not equal to zero. An imbalance
onto axis, along which a γ-quanta was directional, came
to a difference between this sum and a γ-quanta
momentum.
A γ-quantum energy was equal to the sum of kinetic
energies of three α-particles and the reaction threshold.
In Fig.1 dot line shows the absolute value of the
momentum imbalance projection distribution onto a
transverse axis. The distribution at momentum
imbalance projections up to 15 MeV/c was fitted by a
gaussian-shape curve and its half-width (FWHM) was
equal to 5.8 MeV/c. For comparison, the solid line
represents the data of the experiment with
monochromatic photons [4] with a half-width equal
16 MeV/c. The discrepancy may be explained by a large
56 PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY. 2001, № 1.
Series: Nuclear Physics Investigations (37), p. 56-58.
inaccuracy of the angle measurements because of a
multiple scattering in nuclear emulsion experiments.
Only а quarter of selected three-track stars satisfied a
requirements of the momentum-balance analysis and
were scored as a 12C(γ,3α)-reaction. The energy-
momentum conservation law makes it possible to
calculate the energy of the γ-quanta forming a star and
to define more accurately a α-particle momentum. This
was done in a following way: when two particles stop in
the chamber volume the kinematical parameters of the
third particle, which didn’t stop, were calculated. And
when momentum of two particles that were extracted
from a curvature measurement did not have large values
of the directing cosines along the vertical axis then the
third particle momentum that has large directing cosine
was calculated.
0 10 20
0
50
100
nu
m
be
r o
f e
ve
nt
s
δ P, MeV/c
Fig. 1. The module projection imbalance distr-
ibution of events. Dots – this experiment, curve –[4]
This results in a sufficient inaccuracy diminishing.
The width of the excitation energy distribution of the
8Be ground state, which is shown in Fig. 2, is an
apparatus value. The inaccuracy in the momentum
measurement extracted from this distribution is equal to
the 1.4 MeV/c.
The main background reaction is the 12C(γ,n)3He2α
because the experimental method is unable to
distinguish 4He from 3He. The 12C(γ,n)3He2α-reaction
yield is 3.5 times higher then the one of the 12C(γ,3α)-
reaction, but its admixture does not exceed 3 %. The
three-α-prong events are emitted by the 4He(γ,n)3He-
reaction that is accompanied by the 3He scattering on the
4He nuclei. Here three rays are always complanar, but a
vertex of an event was often outside of the beam
volume. When the chamber was filled with pure helium
200 of three-tracks events with the vertex inside of the
beam volume were selected. All these events were
treated as 12C(γ,3α) stars but all of them were later
rejected because of the momentum balance non-
fulfillment.
3. EXPERIMENTAL RESULTS
In Fig. 2 a histogram shows an in- pair relative
energy distribution W for the energy region between 0.0
and 0.5 MeV. The maximum of the distribution is
located near the value of 0.09 MeV. The half-width of
the distribution is about of 0.026 MeV. The dots show
the result [4] obtained by the nuclear emulsion method,
which were irradiated by monochromatic photons [4].
The results were normalized by the equal amount of
events and are in a good agreement
0.00 0.25 0.50
0
25
50
N
um
be
r o
f e
ve
nt
s
W, MeV
Fig. 2. In-pair relative energy distribution. The
histogram – this experiment, dots – [4]
The position of maximum in the energy distribution
for the 8Be ground state decay obtained from the
spectrometric measurements is marked with the arrow.
The distribution [8] has following parameters: the
maximum is located at 0.078 MeV and the half-width is
equal to 6.8±1.7 eV. It means that the concentration of
events near 0.09 MeV may be explained by the 8Be
ground state formation. The width being observed in our
experiment is the apparatus one.
15 30
0
30
60
cr
os
se
ct
io
n,
µ
bn
Eγ , MeV
Fig. 3. The total cross section as a γ-quanta energy
function. Dots – this experiment, the curve – [1]
Fig. 3 shows the total cross section of the 12C(γ,α)
8Be-reaction as a function of the energy. All errors are
statistical. The integral cross sections are equal to 0.38±
0.03 MeV mb. One can see a structure in the excitation
curve. The clearest maximum is located near 18 MeV.
One can notice the irregularities near 12.5 and 26 MeV.
In the previous works of the carbon 3-alfa-particle
brake-up the 8Be ground state was observed only with
the energies up to 20 MeV. The total cross section is
measured here for the first time. The relative yield of
this channel amounts 11 % of total cross section of the
12C(γ,3α)-reaction in the whole energy interval. At the
energies of 10.3 and 19.4 MeV the 12C nuclei has exited
57
states of high width and the isospin T=0 [8]. The decay
of these states into three α-particles is permitted.
Probably, the curve structure up to 20 MeV reflects the
12C ground state generation.
There are three maxims in the total cross section
predicted by the α-cluster direct knockout mechanism
and in the nuclear shell model frames [1]. Coulomb and
nuclear final-state interactions do not change the quality
picture. The model predicts the value of the cross
section in maximum, which is close to the experimental
data. The position of the maximum near 18 MeV is
predicted satisfactory, while the other two are shifted in
comparison with the experiment.
Fig. 4 shows angular distributions of α1-particles for
three energy intervals. Dots represent data [4] that have
been obtained by means of the nuclear emulsion
method. The data are normalized. At the energies below
15.6 MeV the angular distributions are almost
symmetrical relative to 90° and that’s why the statistics
of symmetrical intervals was summed.
0 90
0
1 E
γ
<15.6
0 90 180
0
1
15.6<E
γ
<20
0 90 180
0
1
E
γ
>20
dσ
/d
Ω
,a
rb
.s
ca
le
Θ s.c., deg
Fig. 4. The α1-particle angular distributions. The
histogram – this experiment, dots – [3]. The curves are
explained in the text
If in this reaction a 12C compound nucleus is formed
the angular distributions should have the form of sin2θ
or sin2θ cos2θ for the pure E1- and E2- transitions
respectively. The angular distributions in the energy
interval 9-15.6 MeV are the evidence of the quadruple
transition predominance. The same result one should
expect also in the model of direct α-cluster knockout
mechanism [1]. While the 8Be nuclei transits to 0+ level,
the Coulomb and nuclear final state interactions do not
distort vastly the angular distributions, and the initial
state with l=0 plays the main role in the (γ,α) reaction.
In the compound nuclei formation limit one can draw a
conclusion that electrical dipole transitions dominate in
the other two energy intervals. However, E1-transitions
are prohibited by the rule of selection of the isospin ∆T=
±1 for even-even nucleus. This is also confirmed by the
small value of the reaction cross section being
measured. But the conclusion about the dipole transition
domination is not considered to be definitive. The model
of the direct knockout of the α-particle from the state
with the orbital moment of l=2 and transition of the 8Be
to the 2+ level [1] shows that the angular distributions
are strongly distorted by the Coloumb and nuclear final
state interactions. The result of this calculation for the
quadruple transitions with taking into account only the
Coulomb final state interaction is shown on Fig. 4 by the
dotted line. Calculations predict the sin2θ-like form of
distribution. Final state nuclear interactions increase
substantially the isotropic part of the angular
distributions. The model supposes that simultaneously
with the knockout of the α-particle from the state with
the l=2 the transition of the 8Be from the 2+ to 0+ level is
possible.
In conclusion, the channel of photo-generation of the
8Be nucleus from the reaction C(γ,3α) is identified. The
total cross section of the reaction channel in the
energetic region between 10 and 35 MeV and angular
distributions in three energy intervals were measured.
With the energies of 15 to 35 MeV the angular
distributions have a sin2θ-like form. According to a 12C
compound nuclei formation there is a good agreement
with E1-absorption, which is prohibited in this reaction
by the rule of the isospin selection. The contradiction is
removed by the model of the direct α-particle knock-out
mechanism, which takes into account the Coulomb and
nuclear final state interactions.
REFERENCES
1. R.I. Jibuti, V.I. Mamasakhlisov. On the
photodesintegration theory for the lightest nuclei //
Yadern. Fiz. 1965, v. 2, p. 59-63 (in Russian).
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photodesintegration of 12C nucleus into three
particles in hyperspherical basis // Yadern. Fiz.
1978, v. 28, №1(7), p. 30-38 (in Russian).
3. R.I. Jibuti, R.Ya. Kezerashvili, N.J. Shubitidze.
Photodesintegration of the α-cluster nuclei on the α-
particles // Yadern. Fiz. 1992, v. 55, p. 3233-3240
(R).
4. F.K. Goward, J.J. Wilkins. Cross-section for the
photodisintegration of carbon into three α-particles //
Proc. Roy. Soc. 1953, v. 217, p. 357-358.
5. F.K. Goward, J.J. Wilkins. The 12C(γ,3α) reaction
and energy levels of 8Be and 12C // Proc. Roy. Soc.
1955, v. 228, p. 376-396.
6. M.E. Toms. Properties of 12C(γ,3α) reaction below
21.5 MeV // Nucl. Phys. 1954, v. 50, p. 561-584.
7. R.A. Golubev, V.V. Kirichenko. α-particles angular
distribution in the reactions
γ 12C→3α and
γ 16O→4α
// Yadern. Fiz. 1997, v. 9, p. 1572-1579 (in
Russian).
8. F. Ajzenberg-Selove. Energy levels of light nuclei
A=5-10 // Nucl. Phys. 1974, v. A227, p. 1-243.
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2000, №2.
Серия: Ядерно-физические исследования (36), с. 3-6.
5
S.N. Afanas’ev, A.F. Khodyachikh
National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine
PACS: 13.75Gx.
1. INTRODUCTION
REFERENCES
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