close

Вход

Забыли?

вход по аккаунту

?

Презентация

код для вставкиСкачать
On the question about energy
229
of 3.5 eV state of Th
S.L. Sakharov
Petersburg Nuclear Physics Institute,
Gatchina, Russia
The 229Th level scheme
Left side: the level scheme (8 excited states, 15 transitions) from ref. [1].
Right side: the levels introduced in the present work using the transition energies from ref. [1].
Energies are given in keV.
The energy of the first isomeric state in 229Th was determined to be пЃ„=3.5п‚±1.0 eV, using the following combinations of
level energies from the level scheme:
пЃ„= 97-25-71
пЃ„= [148-146]-[118-117]
пЃ„= 97-69-29
пЃ„= [148-146]-[76-74]
where пЃ„ is the isomer energy and in square brackets the differences of transitions energies (not the energies themselves,
as in the first two relationships) are taken.
Placement of some transitions in
229Th level scheme
Placement of transition, to 3.5 eV isomer or to the g.s.
Energy
of
transition,
keV
Reference, year of publication
1994 [1] 1999 [8]
Helmer Firestone
2002 [9]
Gulda
2003 [2]
Barci
2005 [4]
Helene
2006 [7]
Ruchowska
2007 [5]
Beck
29
isomer
isomer
isomer
g.s.,
isomer
g.s.,
isomer
isomer
g.s.,
isomer
71
isomer
isomer
isomer
g.s.,
isomer
g.s.,
isomer
isomer
g.s.,
isomer
146
isomer
isomer
g.s.
isomer
g.s.,
isomer
g.s.
The energy of the isomeric transition пЃ„ depending on the energy of the
transition excluded from the 229Th level scheme from ref.[1].
Fig.2.
Lower part: dashed line is for the level scheme ( 8 excited states and 15 transitions), each point is for the same
scheme but with 14 transitions, with each transition in turn being removed from the scheme.
Upper part: dashed line is for the enlarged level scheme ( 14 excited states and 31 transitions), each point is for
the same scheme but with 30 transitions, with each transition in turn being removed from the scheme.
The close level spacing between the two bandheads raises the question of
determining the correct intensities of the transitions between the lower states.
Fig. 6
The low energy multiplets.
229
90Th139
Fig 1. Level scheme and g-ray transitions of 229Th from ref.[4]. (Continuous arrows) Undisputed gray transitions; (dashed and dotted lines) not well-established g -ray transitions; (spins and parity)
left side, Ref. [2] Barci; right side, Ref. [9] Gulda
Table I. Different hypotheses about some g – ray transitions in 229Th
g-ray
Initial level
energy
(keV)
(keV)
29
29
Final level (keV)
Ref.[Beck]
92%
8%
3.5 eV
7.6(5)eV
Ref.[Barci]
3
71
пЂ«
49%
3
G.S.25%
51%
2
2
164
146
164
пЂ«
9.3 (6)eV 14.3 (10)eV
2
пЂ«
60%
G.S.40%
146
Ref.[Gulda]
75%
3
71
Ref.[Helene]
3
2
41%
3
пЂ«
2
пЂ«
G.S.
2
3
59%
пЂ«
3
2
пЂ«
88%
G.S. 12%
Partial level scheme ref. [5] of 229Th with Eg in keV.
Fig. 1. The two rotational bands are displaced relative to each other for clarity, and
labeled by the Nilsson asymptotic quantum numbers. The gamma rays of the [631]
band are distinguished by green coloring.
XRS spectra in 29 and 42 keV energy regions.
ref.[5]
Fig. 2.
Data illustrated for the 29 keV doublet (пЃ„E29) is the sum of 11 data sets and 25 pixels (a), and
data illustrated for the 42 keV doublet (пЃ„ E42) represents the sum of 11 data sets only for pixel 0 (b).
Black lines represent the data; red lines represent the least-square fitting results.
Full-energy peaks are labeled by Jp of the corresponding 229Th transition.
In ref.[5] the 29185.6 eV, 29391.1 eV, 42434.9 eV and 42633.3 eV transitions
were taken with the resolution of 26 - 30 eV. The energy of the first isomeric state
E(229mTh) was determined from relationship
E(229mTh) = пЃ„E29 пЂ­ пЃ„E42 = 205.48 п‚± 0.50 eV пЂ­ 198.44 п‚± 0.22 eV = 7.0 п‚± 0.5 eV,
where пЃ„E29 = (29.39 пЂ­ 29.19) keV = 205.48 п‚± 0.50 eV, which means the error of the
29.39 keV transition is 0.50 eV or larger (intensity of the 29.19 keV transition
пЂЇintensity of the 29.39 keV transition п‚» 40). However judging from fig. 2a of ref.[5]
the intensity of the 29.39 keV transition within 26 eV FWHM is about 300 counts. So
the minimal achievable error is 26пЂЇ3000.5 = 1.5 eV. Even if one takes the intensity of
the 29.39 keV transition п‚» 0.5 per day per pixel, its total intensity (not within FWHM)
is 2 п‚ґ 11 days п‚ґ 25 pixels = 550. Within FWHM the intensity is about 400, so the
resulting error is 26пЂЇ4000.5 =1.3 eV. So the accuracy of the energy error of the 29.39
keV transition and hence of пЃ„E29 is overestimated, which possibly result in poor fit
(3пЃі) of the 29.39 keV transition in the level scheme of Helmer ref.[1]. The value of
7.0 п‚± 0.5 eV was corrected by branching b = 1пЂЇ13 from the 29.19 keV level to the
g.s., which gives 7.6 п‚± 0.5 eV. However branching 25% from [2] results in the value
of 9.3 eV.
Energy calibration in the range 0 -33 keV was done using a 4th order polynomial
with zero-energy offset.
Conclusions
Therefore in reality the energy of the 3.5 eV state lies in the range
0 п‚®15 eV and it is not clear whether this state does exist. One
may propose two ways to determine its energy:
1. The direct measurement of the energy of the transition from
the 3.5 eV state. However, the attempts to find such a
transition failed Richardson [10], S.B.Utter et al [11]. The task is
very difficult because the possible range of the transition
energies is very wide,
2. The direct measurement of the energies of the strongest 42
and/or 97 keV transitions to calculate the 3.5 eV state energy.
A double crystal spectrometer similar to that from Gavrikov [12]
with the resolution of the order of 1 eV is very suitable for this
aim. For the source of 100 cm2 area containing 20 g of 233U
one can expect to register 3 g/h for the 97 keV transition and
14 g/h for the 42 keV transition (assuming 100 % effectiveness
of registration and a solid angle of 10-9).
References
1.
2.
3.
R.G. Helmer and C.W. Reich, Phys. Rev. C49 (4), 1845 (1994).
V. Barci et al., Phys. Rev. C68, 034329 (2003).
Z.O. Guimaraes-Filho, O. Helene and P.R. Pascholati, Contributions to
Conference on Nuclear Physics, Santa Fe, 2005, p.257;
4. Z.O. Guimaraes-Filho and O. Helene, Phys. Rev. C71, 044303 (2005).
5. B.R. Beck, J.A. Becker, Beiersdorfer et al., Phys. Rev. Lett. 98, 142501
(2007)
6. J. Tuli. http://www.nndc.bnl.gov./nndc/ensdfpgm/
7. E. Ruchowska et al., Phys. Rev. C73, 044326 (2006)
8. R.B. Firestone, S.Y.F. Chu, and C.M. Baglin, Table of Isotopes CD-ROM, 8th
ed. (Wiley- Interscience, New York, 1999)
9. K. Gulda et al., W. Kurcewicz, A.J. Aas et al., Nucl. Phys. A703, 45, (2002).
10. D.S.Richardson et al., Phys. Rev. Lett. V.80, 3206 (1998).
11. S.B.Utter et al., Phys. Rev. Lett. V.82, 505, (1999)
12. Yu.A. Gavrikov et al., Preprint NP-24-2367, Gatchina, 2000, p.42.
A portion of the low-energy level scheme of 229Th showing the g rays that are used in this
determination of the energy пЃ„ of the first excited level.
The energy, пЃ„, of the first excited level is
given by each of the four combinations
of g–ray energies:
пЃ„= 97.1 - 25.3 - 71.8
пЃ„= 97.1 - 67.9 - 29.1
пЃ„= 148.1 -118.9 +117.1-146.3
пЃ„= 148.1 -76.4 + 74.6 -146.3
An excited state of
Table VII
aThe
bThe
229Th
at 3.5 eV
The energy, пЃ„, of the first excited level
energies for Eg (25) and Eg (29) are from Table V.
energies for Eg (25) and Eg (29) are derived in the text from the computed 217-187 and
164-135 differences for the 29 line and from the measured 29-25 (Table III) difference for 25 line.
Comparison of (d, t) strengths for rotational band members in
229Th and 231Th.
Table 1.
aThis
upper limit for the strength is obtained assuming the full cross section of the
unresolved doublet is for this level.
bUpper
limit only. Peak is obscured by a larger one for a nearby level.
Cross section (Ојb / sr)
Angular distribution for (d, t) cross section of some 229Th levels.
Fig. 2
Angle (deg)
The solid curves are DWBA calculations for the l values appropriate for the states indicated, adjusted
in the vertical direction to give the best visual fit to the data points.
Документ
Категория
Презентации
Просмотров
8
Размер файла
1 147 Кб
Теги
1/--страниц
Пожаловаться на содержимое документа