Updated level scheme of 172Yb from 171Yb(nth , γ) reaction studied via gamma–gamma coincidence spectrometer

Accepted Manuscript Updated level scheme of 172 Yb from 171 Yb(nth , γ ) reaction studied via gamma–gamma coincidence spectrometer

Nguyen Ngoc Anh, Nguyen Xuan Hai, Pham Dinh Khang, Nguyen Quang Hung, Ho Huu Thang

PII: DOI: Reference:

S0375-9474(17)30103-3 http://dx.doi.org/10.1016/j.nuclphysa.2017.04.032 NUPHA 20884

To appear in:

Nuclear Physics A

Received date: Revised date: Accepted date:

17 February 2017 14 April 2017 25 April 2017

Please cite this article in press as: N.N. Anh et al., Updated level scheme of 172 Yb from 171 Yb(nth , γ ) reaction studied via gamma–gamma coincidence spectrometer, Nucl. Phys. A (2017), http://dx.doi.org/10.1016/j.nuclphysa.2017.04.032

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Updated level scheme of 172 Yb from 171 Yb(nth , γ) reaction studied via gamma-gamma coincidence spectrometer Nguyen Ngoc Anha,c,∗, Nguyen Xuan Haia , Pham Dinh Khangb , Nguyen Quang Hungc , Ho Huu Thanga a Dalat

Nuclear Research Institute, Vietnam Atomic Energy Institute, 1 Nguyen Tu Luc, Dalat City, Vietnam b Hanoi University of Science and Technology, 1 Dai Co Viet, Hanoi city, Vietnam c Institute of Research and Development, Duy Tan University, K7/25 Quang Trung, Danang city, Vietnam

Abstract This paper provides the updated information on the level scheme of cleus studied via

171

172

Yb nu-

Yb(nth , γ) reaction using the gamma-gamma coincidence

spectrometer at Dalat Nuclear Research Institute (Vietnam). The latter is used because of its advantages in achieving the low Compton background as well as in identifying the correlated gamma transitions. We have detected in total the energies and intensities of 128 two-step gamma cascades corresponding to 79 primary transitions. By comparing the measured data with those extracted from the ENSDF library, 61 primary gamma transitions and corresponding energy levels together with 20 secondary gamma transitions are found to be the same as the ENSDF data. Beside that, 18 additional primary gamma transitions and corresponding energy levels plus 108 secondary ones are not found to currently exist in this library and they are therefore considered as the new data. Keywords: Level scheme,

171

Yb(n,γ) reaction, γ − γ coincidence spectrometer.

∗ Corresponding

author Email address: [email protected] (Nguyen Ngoc Anh)

Preprint submitted to Nuclear Physics A

April 28, 2017

1. Introduction The complete information on the nuclear level scheme plays important roles for the study of not only nuclear reaction and statistical model calculations but also adjustment of the nuclear level density parameters. Most of the nuclear 5

level scheme data were compiled in the ENSDF library, which contains 187067 datasets for 3312 atomic nuclei [1]. For each nucleus, the ENSDF library collects data from various experiments and based on these data the library proposes the appropriate values for a certain property of the nucleus. The construction of a complete nuclear level scheme requires the comprehensive spectroscopic data of

10

non-selective reactions such as slow neutron capture reactions [2]. As 172 Yb is a deformed, even-even and rare earth nucleus, its level scheme is absolutely necessary for either confirming or enhancing the predictive powers of the nuclear models for heavy nuclei. Level scheme of 172 Yb has been thoroughly studied in different methods such as beta decay of

15

decay of

172

172

Tm [3], electron capture

Lu [4], neutron inelastic scattering for low-lying states in

[5], (n, n’γ) reactions using fast neutrons from reactors [6], reaction for high-spin states [7],

171

170

172,174

Yb

Er(α,2n)172 Yb

Yb(n,γ) reaction for low-spin states [8,

9]. Additional methods, which are also able to provide the level scheme of 172 20

Yb based on the compound nuclear reactions induced by light ions, in-

clude 173

172

Yb(3 He,3 He’γ)[10],

(d,t) [14],

173

3

172

Yb(α,α’) [11],

3

173

Yb( He,γ), ( He,αγ) [14, 15],

Yb(p,d) [12],

170

171

Yb(d,p) [13],

Yb(t,p)[16], and elastic and

inelastic proton scatterings [17, 18]. Furthermore, lifetimes of a number of levels have been also determined via the

172

Yb(γ, γ’) reactions using the nuclear

resonance fluorescence [19] and Coulomb excitation [20, 21] methods. Through 25

these experiments, the low-lying discrete level scheme of 172 Yb in the low-energy region (E < 2.4 MeV) has been well understood [22]. In this low-energy region, energies of the levels were determined with the accuracy of ten to hundred eV, whereas spins and parities were also identified for a majority of levels. However, information on the excited states and their corresponding primary transitions

30

in the intermediate energy region (2.4 MeV < E < 5 MeV), where the thermal

2

neutron capture reactions (nth ,γ) were mostly employed to extract the data, is sparse and incomplete. In particular, based on the neutron capture reaction with both thermal and 2 keV neutrons, Greenwood et al [8] has detected, by using the Ge(Li) detector, in 35

total 127 primary gamma transitions including their intensities from the prompt gamma spectrum of 172 Yb. At the same time, the prompt gamma yield of 172 Yb has also been determined from the abundance of 171 Yb in natural ytterbium and their relative thermal-neutron capture cross sections. In addition to that, 136 gamma-ray transitions, whose energies are less than 2.5 MeV, have been also

40

reported in this paper. Using the same thermal neutron capture reaction with the use of the pair formation spectrometer, Gellety et al [9] has measured the primary transitions of

172

Yb, whose energies and relative intensities were found

in good agreement with those reported by Greenwood. Although the results obtained from Gellety have improved the level scheme of 45

172

Yb, which had

previously been constructed by Greenwood using the Riz combination principle, the significant differences between the absolute intensities of gamma transitions obtained within those works have not yet been explained. It is obvious that the number of detectable gamma rays in the normal gamma-ray spectrum depends upon the energy resolution of the detectors as well as the number of excited

50

states existing in an interval of excitation energy. Thus, there has been a certain limitation on the results of Refs. [8, 9] due to the restricted energy resolution of the Ge(Li) detectors used in those works as well as the large number of excited states of

172

Yb in the energy region from 3 MeV to 5 MeV, where the discrete

region of the level density interferences with the continuous one. 55

It has been well known that the gamma-gamma coincidence technique [23] is one of the appropriate methods, which is able to improve the ability of detecting the exited states of 172 Yb in the energy region from 3 MeV to Bn - 0.5 MeV (Bn - neutron binding energy). One of the reasons is that within the gamma-gamma coincidence method the cascade events, which are obtained from the decay of the

60

initial compound state to the different final states, are separated into different Two-Step-Cascade (TSC) spectra. The number of gamma-rays contributed to 3

the TSC spectrum in this case is less than that presented in a normal prompt gamma spectrum, leading to the significant reduction of the overlapping gamma rays as well as improving the detecting ability of this method. The other reason 65

is that different from the normal gamma spectra, the TSC spectra obtained using this method, after applying the background subtraction algorithm, have almost no Compton background. Therefore, the detection limit of the coincidence method is much improved. Beside that, the state from which a secondary gamma transition is decayed can be determined in the coincidence method if one of the

70

two gamma rays in the cascade is a known primary transition. In fact, the gamma-gamma coincidence technique was used to measure the level scheme of

172

Yb in Ref. [9], however, it was set to cover the energy range

from 0 to 2 MeV only. This method was later used to measure the TSC intensities of 75

172

Yb from

171

Yb(nth , γγ) reaction in Ref. [24]. The results obtained

were then compared with the statistical-model calculations which base entirely on the experimental level density and gamma strength function extracted from the primary gamma spectra of

173

Yb(3 He, αγ)172 Yb reaction. On the other

hand, in Ref. [24] the spectroscopic data were also presented and compared with the results of Refs. [8, 9] but these data were not shown in detail because 80

of their low statistics, in which only 4000 cascade events corresponding to the decays from the compound state to the ground state are collected. Given the advantages of the gamma-gamma coincidence method, the aim of the present work is to provide the updated information on the level scheme of

85

172

Yb based on the study of the gamma cascades from the thermal neutron

capture reaction

171

Yb(nth , γ). Based on this method, we have achieved the

statistics of the collected data to be about 10 times higher than that reported in Ref. [24]. The spectroscopic data of the coincidence spectrum of

172

Yb will

be reported in detail and comparison with the ENSDF data [22] will be also made.

4

90

2. Experimental Method The experimental measurement was performed on the tangential neutron beam at Dalat Nuclear Research Reactor [25]. The neutron beam from the reactor was filtered by using a single crystal silicon of 48 cm length and collimated by a mixture of paraffin and boron in order to obtain the thermal

95

neutron source, whose size and flux at the irradiation position are 2.5 cm and 1.7 × 105 n.cm−2 .s−1 , respectively. This configuration of the beam is similar as that reported in Ref. [25]. The only difference between the beam configuration employed in the present work and that of Ref. [25] is that we have used the narrower collimator and longer silicon filter, leading to the strong decrease of

100

the neutron flux but the significant increase from 77.5 to 230 (about 3 times) of the thermal-to-fast flux ratio R(Cd/Au). Practically, the current neutron flux is appropriate for the present experiment because the reaction rate causes a dead time of only 2.3% for the Analogue-to-Digital Converters (ADCs) of our spectrometer. Although the use of higher flux would increase the data collection

105

rate, it also increases the dead time of the ADCs, and therefore decreases the quality of the collected data.

HPGE Detector

Lead Carbide Boron Lithium Fluoride Target 2 mm Carbide Boron shields

Thermal neutron beam

HPGE Detector

10 cm thickness Lead Chambers with 2 mm lead in detector window

Figure 1: Experimental setup for measuring the gamma-gamma coincidences.

5

71

SE (0)

3500 3000

0

2500

71

1042.7

1500

1155.9

Ge

2000 1197.3

Events

78.8

1117.4

DE (0)

4000

SE (78.8)

DE (78.8) + SE ( Ge)

4500

1000 500 0 6600

6800

7000

7200

7400

7600

7800

8000

8200

E1+ E2 (keV)

Figure 2: Summation amplitude of coincident pulse (SACP) spectrum for

171 Yb(n

th ,2γ)

reaction. E1 +E2 is sum of energies measured from two detectors. Energies (in keV) of the final levels in the cascades are pointed near the peaks of the full capture energy. The notations SE and DE correspond to the single- and double-escape peaks, respectively.

The target is made in the form of a 0.56 g Yb2 O3 powder, 0.46 g of which is 171

Yb, that is, the enrichment of 171 Yb is 95.5%. Other impurities in this target

contain mostly 110

172

Yb (3.6%) together with

173

Yb (0.4%) and

174

Yb (0.3%),

whereas the percentage of all other elements is less than 0.02%. It is noted here that even the quantity of

172

Yb in the target is comparable with

171

Yb,

its influence on the spectroscopic data is negligible because thermal neutron capture cross section of

172

Yb is only 1.3 barn compared to 58.3 barn of

171

Yb

[26]. This target, after being sealed in a plastic bag, is then irradiated at the 115

center of the thermal neutron beam. The gamma cascades from

171

Yb(n, γ)

reaction are recorded by using a gamma-gamma coincidence spectrometer [27]. The latter consists of two ORTEC coaxial HPGe detectors with efficiency of

6

250

Events

200

150

100

50

0 0

2000

4000

6000

8000

Eγ (keV)

Figure 3: TSC spectrum corresponding to the ground state of

172 Yb.

Eγ (keV) is energy of

the gamma transition.

35% relative to efficiency of the standard NaI(Tl) crystal (3-in.-diameter × 3in.-long). The low -energy threshold of the HPGe detectors was set at 0.52 MeV. 120

The experimental setup is shown in Fig. 1. The distance from the target to each endcap face of detectors is 5 cm. A plate of lead of 2 mm thickness is placed in front of the detector windows in order to filter the back-scattered photons, which lead to the increase in the dead time of the electronics. Lead cone collimators are positioned in front of the detectors to lower the background and bad impact

125

on the energy resolution caused by the gamma rays, which interact with the detector’s crystals in the edge region. In order to protect the detectors from the neutron damage, the space inside the cone collimators are filled with the lithium fluoride. In addition, the boron carbide shields with 2 mm thickness are placed between the neutron beam and the detectors. For each coincident event,

7

1000

800

Events

600

400

200

0 0

2000

4000

6000

8000

Eγ (keV)

Figure 4: TSC spectrum corresponding to the final level with the energy Ef = 78.8 keV of 172 Yb.

130

Eγ (keV) is energy of the gamma transition.

amplitudes of the pulses generated from two detectors are stored and treated off-line. The experiment ran for about 830 hours yielding approximately 67×106 gamma coincidences. The relative efficiency of both detectors from 0.5 MeV to 8 MeV were determined from the single gamma-ray spectra using 35 Cl(n,γ)36 Cl reaction and its known γ intensities [28].

135

The procedure of extracting the cascade events and their intensities was thoroughly presented in Ref. [23], so it is only shortly described in the present paper. The most informative part of the SACP (summation amplitude of coincident pulses) spectrum is shown in Fig. 2. Six distinct peaks, which correspond to the TSC spectra obtained via the transitions from the neutron binding en-

140

ergy to the ground state, and five low-lying states are seen in this Fig. 2. The appearance of other peaks seen in Fig. 2 is known to come from the neutron

8

scattering through the detector material (71 Ge) as well as from the single- and double-escape peaks. The TSC spectra are obtained by gating on 6 peaks, whereas those obtained by gating in the vicinity regions of the corresponding 145

peaks are subtracted from the TSC spectra in order to eliminate the Compton background and to exclude the random coincidences. Figures 3 and 4 show the TSC spectra corresponding respectively to the final levels with energies equal to 0 keV and 78.8 keV and having approximately 31050 and 41160 events each. Each pair of peaks, which is symmetrically positioned with respect to the center

150

of the TSC spectra, represents a cascade, whose relative intensity is proportional to the peak area. The relative intensities are then converted to their absolute values based on the absolute intensity of the 5538.8 keV primary transition and its branching ratio. Through the analysis of the prompt gamma spectrum of the target, which

155

is a mixture of 0.46 g transition of

28

171

Yb and 0.6409 g

27

Al, as well as the 1779 keV gamma

Si produced from the β − decay of

28

Al (formed via the neutron

capture reaction of 27 Al) with the absolute intensity of 100/100 captures [29] in combination with the thermal neutron capture cross section of

171

Yb (58.3±3.8

barn) and 27 Al (0.231±0.003 barn) [26], we have deduced the absolute intensity 160

of the 5538.8 keV primary transition of

172

Yb to be equal to 2.9/100 captures

with the uncertainty of about 9%, which is caused mainly by the uncertainty of thermal neutron cross section of

171

Yb (∼6.5%), relative efficiency curve of

the detectors (∼5%) and the the determination of peak area (∼3%). This value agrees with the value of 2.8/100 captures in Ref. [8] within the experimental 165

uncertainty but far from that given in Ref. [9] (0.95/100 captures). Since the gamma-gamma coincidence technique is not able to indicate which gamma in the cascade comes from the primary transition, it is necessary to introduce two rules for determining the primary transitions. The first rule is that if a transition is found as the primary in the ENSDF library, it is also considered

170

as the primary in this work. The second rule assumes that if the transitions appear in at least two TSC spectra, they are considered as the primary ones. In addition, if the cascades do not have enough information for determining which 9

transition is the primary, they are not being reported in this work. Moreover, in order to ensure the appropriate statistical uncertainty, only the cascades, in 175

which the peak area is larger than 50 counts, are reported in the present work. Based on the above rules, although we have determined in total 479 cascades, only 128 two-step cascades are presented.

3. Results and Discussion All the cascades coming from the compound state to the ground state and 180

five excited states, whose energies Ef are 78.8, 1042.7, 1117.4, 1155.9, and 1197.3 keV, have been identified. The set of gamma cascades and their absolute intensities are given in Table 1. From this table, we have determined in total 79 primary transitions, 61 of which are found to be the same as those currently exist in the ENSDF library. The remain (18 primary transitions) are therefore

185

considered as the new data. Consequently, 18 final levels corresponding to these new primary transitions together with the secondary transitions emitted from these levels are also considered as the new ones. For the secondary transitions, we have detected in total 128 transitions, only 20 of which are the same as those in the ENSDF library and the remain

190

are considered as the new data. Among these new secondary transitions, 87 transitions are those emitted from the levels, which currently exist in the ENSDF library, whereas 21 transitions come from the new levels reported in the present paper. It is also seen from Table 1 that the mean deviation of the gamma energies

195

obtained within the present work to that of the ENSDF data is approximately 0.7 keV. In few cases, the discrepancy between the two data is found to exceed 1.2 keV. The reason can be explained as follows. Within the energy resolution of the detectors, the detected gamma-ray energies obviously depend on the statistics of the peak area. In the gamma-gamma coincidence method, energy of a

200

gamma primary transition or a intermediate level is averagely determined from more than one TSC spectra. In some cases, the statistics are found to be signif-

10

icantly poor in a certain TSC spectrum, leading to a considerable uncertainty in the determination of the average gamma energy. In addition to that the recoil contribution is known to affects also the accuracy of the gamma energies. 205

However, its effect in comparison with the present experimental uncertainty has been found to be insignificant in the energy region from 0 to 10 MeV [31]. The absolute intensities of the cascade transitions, whose uncertainty ranges from 9% to 25% for the most number of cases, are also presented in Table 1. Although uncertainties of the cascade intensities are mostly contributed by the

210

statistical counting and normalization factor, the overlapping of the gamma transitions with nearly equal energies in a given TSC spectrum would lead to the increase of the uncertainty to more than 50% in several cases. Obviously, the gamma intensities are determined within the present work based on a fixed pair of initial and final levels, so their values are apparently different from those

215

obtained by measuring the gamma intensities without considering the initial and final excited levels, for example those obtained from the conventional (n,γ) spectrum. For this reason, it is quite difficult to directly compare the gamma intensities obtained within the present experiment with those obtained from different experimental techniques. Nevertheless, comparing the TSC spectrum

220

obtained within the present work with that obtained within the same reaction and technique as in Ref. [24], a good agreement is clearly seen. Moreover, our normalization factor, which is determined by using the relative thermal neutron capture cross section of Al and 171 Yb together with their corresponding mass in the sample, also coincides with value reported in Ref. [8], which was obtained

225

from the relative neutron capture cross section and the abundance of

171

Yb in

natural ytterbium. The above-mentioned consistencies prove the reliability of the data obtained within the present work. Figure 5 shows the level scheme

172

Yb obtained within the present work. It

is known that the spin of the intermediate levels, which ranges from 1¯h to 2¯ h, 230

is rather low due to the use of the radiative capture of thermal neutrons given on a stable nuclear target [32]. Consequently, within the present work, the spin value of 1¯ h and 2¯h are suggested for all levels, whose spin values can not be 11

found in the ENSDF library [22] as seen clearly in this Fig. 5.

12

0-, 1-

(1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) 2+ (1,2) (1,2) 2+ (1,2) (1,2) (1,2) (1,2) 2+ (1,2) (1,2) 1+,2+ 1+,2 2+ 2+ 1+ 11+ 2+ 2+ 2+ 212+ 0+ 2+ 0+

Jπ

8020.1 (8019.33)

323 3.2 (?) 328 9.2 (?) 341 1.1 (?) 344 7.5 (?) 349 0.7 (?) 357 8.4 (?) 358 8.8 (?) 385 7.3 (38 394 56.5 1.3 ) ( 394 403 1.1) 6.1 (40 410 34.4 2.6 ) (41 411 02.0 2.7 ) (41 414 11.0 4.0 ) (41 421 4 2.9) 9.8 ( 422 425 0.3) 2.4 (42 426 52.8 4.0 ) (42 427 64.6 2.4 ) (42 427 71.7 7.9 ) (42 430 78.4 0.3 ) ( 4 430 300 4.7 .1) (43 432 05.1 1.9 ) (?) 433 8.3 (43 436 38.4 2.0 ) (43 437 62.3 8.7 ) ( 4 438 378 5.1 .9) (43 443 85.0 2.2 ) (44 445 32.4 0.1 ) (44 446 49.3 1.7 ) (44 447 62.0 6.0 ) (44 449 75.9 2.8 ) (?) 451 2.6 ( 4 452 513 3.5 .3) (45 453 24.6 3.1 ) (?) 453 7.4 (?) 455 3.4 (45 459 54.2 6.1 ) ( ? 463 ) 6.1 (46 465 3 7.8) 2.6 ( 465 467 2.6) 3.3 (46 471 72.7 8.4 ) (47 473 19.1 9.3 ) (?) 475 0.7 (?) 475 9.0 ( 4 476 759 3.4 .1) (47 477 64.9 4.6 ) (?) 479 4.4 (?) 480 5.3 (?) 487 4.0 (?) 488 9.8 (48 489 8 8.7) 9.0 (48 492 99.2 1.6 ) (49 494 20.6 5.0 ) (49 498 44.5 2.2 ) (49 499 82.5 8.6 ) (49 501 9 9 6.0 .1) ( 5 507 017 5.2 .8) (50 513 76.2 1.9 ) (51 514 31.9 7.2 ) (51 517 47.0 5.4 ) (51 518 74.9 5.1 ) (51 527 84.6 2.7 ) (52 531 7 1.9) 8.8 ( 5 539 318 2.0 .9) (53 541 91.3 3.0 ) (54 544 11.5 4.9 ) (54 549 43.5 5.3 5) (54 551 95.0 6.7 5) ( 5 553 515 9.6 .28) (55 564 38.8 4.9 ) (56 569 43.6 1.7 3) (56 570 90.8 7.8 9) (57 582 06.9 5.6 ) (58 591 24.8 7.0 4) (59 601 15.8 0.0 6) (60 617 09.1 0.6 5) (61 654 69.8 2.9 4) (65 655 42.4 4.1 4) (65 53.0 3) 686 5.2 (68 690 64.2 2.7 8) (69 01.3 2)

4787.0 4730.7 4610.2 4572.6 4528.2 4440.9 4431.8 4162.8 4078.7 3984.0 3917.5 3908.1 3876.0 3799.8 3767.8 3756.4 3747.7 3742.0 3719.4 3715.2 3698.9 3682.2 3657.7 3641.0 3635.0 3587.5 3569.6 3558.0 3544.0 3526.6 3507.1 3496.5 3486.6 3482.6 3466.3 3423.7 3382.7 3367.1 3346.7 3301.5 3280.7 3269.0 3261.1 3256.3 3245.1 3225.3 3214.4 3145.7 3130.1 3121.0 3098.1 3074.7 3038.0 3021.1 3003.7 2944.5 2888.2 2872.8 2844.8 2834.9 2747.3 2700.9 2627.7 2607.7 2574.9 2524.9 2503.8 2480.3 2374.9 2328.3 2311.6 2194.6 2103.0 2010.0 1849.5 1477.1 1466.2

(?) (?) (?) (?) (?) (?) (?) (4162.8) (4078.2) (3984.9) (3917.3) (3908.3) (3876.4) (3799) (3766.5) (3754.7) (3747.6) (3740.9) (3719.2) (3714.2) (?) (3680.9) (3657) (3640.4) (3635) (3586.9) (3570) (3557.3) (3543.4) (?) (3506) (3494.7) (?) (?) (3465.1) (?) (3381.5) (3366.7) (3346.6) (3300.2) (?) (?) (3260.2) (3254.4) (?) (?) (?) (?) (3130.6) (3120.1) (3098.7) (3074.8) (3036.8) (3020.2) (3001.5) (2943) (2887.3) (2872.2) (2844.3) (2834.6) (2747.3) (2700.3) (2627.9) (2607.3) (2575.6) (2524.1) (2503.9) (2480.037) (2375.37) (2327.58) (2312.9) (2194.331 or 2195.03a ) (2102.944) (2009.8) (1849.173) (1476.784) (1465.875)

1198.472 1155.4 (1154.935) 1117.4 (1117.874) 1042.914 78.7427 0

Energy (keV)

172-Yb

13

Figure 5: Experimental level scheme of

172 Yb

obtained within the gamma cascades from

compound state to six distinct low-lying discrete levels with spin from 0¯ h to 2¯ h. Horizontal lines are excited levels. Vertical arrows are the gamma transitions. On the right, experimental level energies are given together with values from the ENSDF library (in parentheses), if available. On the left, spin and parity from the ENSDF library are given, if available. For the levels, which spin and parity are not found in the ENSDF library, a suggested value is given (in parentheses). At the head of each arrow, which indicates the primary gamma transitions, the experimental energy together with the ENSDF data (in parenthese, if available) of the gamma transition are shown. Continuous lines are data that exist in both present experiment and ENSDF library. Dashed lines are data that only exist in the present experiment. Question marks (?) are unavailable data. a

Two levels 2194.331 and 2195.03 keV are not able to be experimentally separated. Within

the present work, the primary transition 5825.6 keV corresponding to excited level of 2194.4 keV is determined as a coincidence of 5 transitions, namely 996.5, 2116.5, 1039.5, 1076.6, and 2194.4 keV. The first two transitions are recognized in the ENSDF library as the secondary transition emitted from the 2194.331 keV excited level, whereas the last one is detected as that emitted from the 2195.03 keV excited level. The conclusion from which levels the two remain transitions emit is still not reached and therefore further studies are needed to be made.

14

235

In general, the results obtained within the present work are in good agreement with those reported in Ref. [8]. In addition to that, the new primary transitions found in the present experiment have energies in the range from 3200 to 4900 keV, which correspond to the energy region from 3120 to 4820 keV, where the data in the ENSDF library are still sparse. In this intermediate region, it is

240

difficult to determine explicitly the primary gamma rays based on the analysis of the conventional prompt gamma-ray spectrum because the level density and Compton background in this case are very high. However, with the ability of filtering the events with the same final levels as well as the ability of reducing the Compton background based on the gamma-gamma coincident technique, we

245

are able to detect in the present work more gamma rays in this energy region. In addition, with the advantage of identifying the correlated gamma transitions, which were not available in Refs. [8, 9], we are capable of identifying a lot of secondary transitions and levels from which they emit.

15

Table 1: Primary and secondary gamma-ray energies and absolute intensities obtained from the thermal-neutron capture

171 Yb(n

th , γ)

reaction

(the recoil contribution to the gamma energies has not been taken into account). The experimental values are compared with the ENSDF data. Throughout the table, E1 : energy of the primary transition; E2 : energy of the secondary transition; Ei : energy of the intermediate level; Ef : energy of the final level; Iγγ : absolute intensity of the cascade normalized to 106 decays; Present work: the experimental results obtained within the present work; ENSDF: data taken from the ENSDF library; question notations (?): unvailable data (new data in the present work); SP: spin and parity. Notation

a

denotes the result with the uncertainty being the same as that of the primary transition, whereas result given in the parenthesis is the

absolute uncertainty if available. b

This level can not be separated from 2195.03 keV level. See explanation in the caption of the Fig. 5.

Present Work

ENSDF

Ei (keV)

Iγγ

Ef (keV)

SP

Ei (keV)

E1 (keV)

E2 (keV)

6902.7 (0.2)

1039.3

1117.4 (0.2)

310 (43)

78.7427 (0.0006)

2+

1117.874 (0.005)

6901.32 (0.22)

1039.149 (0.010) from 1117.874

6865.2 (0.1)

1076.7

1155.4 (0.6)

703 (82)

78.7427 (0.0006)

2+

1154.935 (0.006)

6864.28 (0.27)

1076.25 (0.01) from 1154.935

6554.1 (0.2)

1387.9

1466.2 (0.3)

441 (59)

78.7427 (0.0006)

2+

1465.875 (0.004)

6553.03 (0.22)

1387.093 (0.004) from 1465.875

6542.9 (0.1)

1398.6

1477.1 (0.5)

965 (105)

78.7427 (0.0006)

2+

1476.784 (0.017)

6542.44 (0.22)

1397.92 (0.05) from 1476.784

6542.9 (0.1)

1477.6

1477.1 (0.5)

348 (48)

0 (0)

0+

1476.784 (0.017)

6542.44 (0.22)

1476.77 (0.07) from 1476.784

6170.6 (0.2)

1771.3

1849.5 (0.2)

798 (96)

78.7427 (0.0006)

2+

1849.173 (0.022)

6169.84 (0.22)

1770.9 (0.4) from 1849.173

6170.6 (0.2)

1849.6

1849.5 (0.2)

530 (76)

0 (0)

0+

1849.173 (0.022)

6169.84 (0.22)

1849.06 (0.03) from 1849.173

6010.0 (0.1)

1931.8

2010.0 (0.2)

1655 (178)

78.7427 (0.0006)

2+

2009.80 (0.03)

6009.15 (0.22)

1931.28 (0.09) from 2009.8

6010.0 (0.1)

2010.3

2010.0 (0.2)

3104 (282)

0 (0)

0+

2009.80 (0.03)

6009.15 (0.22)

2009.92 (0.15) from 2009.8

16

E1 (keV)

Ea2 (keV)

Table 1: (continued)

Present Work E1 (keV)

Ea2 (keV)

5917.0 (0.3)

2025.0

5825.6 (0.2)

996.5

5825.6 (0.2)

2116.5

5825.6 (0.2)

1039.5

5825.6 (0.2)

Ei (keV)

ENSDF

17

Iγγ

Ef (keV)

SP

Ei (keV)

E1 (keV)

E2 (keV)

2103.0 (0.3)

560 (84)

78.7427 (0.0006)

2+

2102.944 (0.03)

5915.86 (0.23)

2024.38 (0.18) from 2102.944

2194.6 (0.9)

810 (120)

1198.472 (0.007)

2-

2194.331 (0.014)

5824.84 (0.23)

995.74 (0.021) from 2194.331

2194.6 (0.9)

834 (116)

78.7427 (0.0006)

2+

2194.331 (0.014)

5824.84 (0.23)

2115.5 (0.3) from 2194.331

2194.6 (0.9)

697 (99)

1154.935 (0.006)

1-

2194.331

b

(0.015)

5824.84 (0.23)

?

1076.6

2194.6 (0.9)

758 (123)

1117.874 (0.005)

2+

2194.331

b

(0.015)

5824.84 (0.23)

?

5825.6 (0.2)

2194.4

2194.6 (0.9)

2027 (201)

0 (0)

0+

2195.03 (0.05)

5824.84 (0.23)

2195.4 (0.3) from 2195.03

5707.8 (0.3)

2234.2

2311.6 (1.1)

464 (80)

78.7427 (0.0006)

2+

2312.9 (0.08)

5706.9 (0.3)

2233.6 (0.3) from 2312.9

5691.7 (0.1)

2250.1

2328.3 (0.2)

2048 (225)

78.7427 (0.0006)

2+

2327.58 (0.07)

5690.89 (0.24)

?

5691.7 (0.1)

2328.4

2328.3 (0.2)

4980 (441)

0 (0)

0+

2327.58 (0.07)

5690.89 (0.24)

2327.3 (0.3) from 2327.58

5644.9 (0.2)

2297.1

2374.9 (0.8)

1167 (161)

78.7427 (0.0006)

2+

2375.37 (0.03)

5643.63 (0.25)

2296.2 (0.4) from 2375.37

5539.6 (0.3)

1282.3

2480.3 (0.6)

811 (151)

1198.472 (0.007)

2-

2480.037 (0.02)

5538.8 (0.3)

1281.89 (0.13) from 2480.037

5539.6 (0.3)

1325.6

2480.3 (0.6)

466 (110)

1154.935 (0.006)

1-

2480.037 (0.02)

5538.8 (0.3)

?

5539.6 (0.3)

1363.0

2480.3 (0.6)

689 (150)

1117.874 (0.005)

2+

2480.037 (0.02)

5538.8 (0.3)

?

5539.6 (0.3)

1437.8

2480.3 (0.6)

482 (105)

1042.914 (0.018)

0+

2480.037 (0.02)

5538.8 (0.3)

?

5539.6 (0.3)

2401.8

2480.3 (0.6)

21685 (1809)

78.7427 (0.0006)

2+

2480.037 (0.02)

5538.8 (0.3)

2401.39 (0.08) from 2480.037

Table 1: (continued)

Present Work

ENSDF

18

E1 (keV)

Ea2 (keV)

Ei (keV)

Iγγ

Ef (keV)

SP

Ei (keV)

E1 (keV)

E2 (keV)

5539.6 (0.3)

2480.2

2480.3 (0.6)

1401 (142)

0 (0)

0+

2480.037 (0.02)

5538.8 (0.3)

?

5516.7 (0.3)

2425.3

2503.8 (0.5)

357 (66)

78.7427 (0.0006)

2+

2503.9 (0.3)

5515.28 (0.23)

?

5495.3 (0.3)

2524.7

2524.9 (0.8)

333 (54)

0 (0)

0+

2524.1 (0.3)

5495.05 (0.25)

?

5444.9 (0.2)

2575.1

2574.9 (0.3)

507 (69)

0 (0)

0+

2575.6 (0.3)

5443.55 (0.25)

?

5413.0 (0.6)

1489.6

2607.7 (0.8)

187 (61)

1117.874 (0.005)

2+

2607.3 (0.2)

5411.5 (0.3)

1489.8 (0.3) from 2607.3

5392.0 (0.4)

1510.4

2627.7 (0.8)

323 (82)

1117.874 (0.005)

2+

2627.9 (0.3)

5391.3 (0.3)

?

5392.0 (0.4)

1585.6

2627.7 (0.8)

323 (87)

1042.914 (0.018)

0+

2627.9 (0.3)

5391.3 (0.3)

?

5318.8 (0.3)

2701.2

2700.9 (0.3)

364 (63)

0 (0)

0+

2700.3 (0.3)

5318.9 (0.3)

?

5272.7 (0.2)

2747.2

2747.3 (0.4)

1283 (145)

0 (0)

0+

2747.3 (0.6)

5271.9 (0.3)

?

5185.1 (0.4)

2756.9

2834.9 (0.3)

381 (67)

78.7427 (0.0006)

2+

2834.6 (0.5)

5184.6 (0.5)

?

5185.1 (0.4)

2834.7

2834.9 (0.3)

269 (59)

0 (0)

0+

2834.6 (0.5)

5184.6 (0.5)

?

5175.4 (0.3)

2844.5

2844.8 (1.4)

467 (80)

0 (0)

0+

2844.3 (0.5)

5174.9 (0.5)

?

5147.2 (0.1)

2794.7

2872.8 (0.2)

2798 (279)

78.7427 (0.0006)

2+

2872.2 (0.5)

5147.0 (0.5)

?

5147.2 (0.1)

2872.9

2872.8 (0.2)

942 (115)

0 (0)

0+

2872.2 (0.5)

5147.0 (0.5)

?

5131.9 (0.3)

2810.0

2888.2 (0.5)

417 (79)

78.7427 (0.0006)

2+

2887.3 (0.8)

5131.9 (0.8)

?

Table 1: (continued)

Present Work

ENSDF

19

E1 (keV)

Ea2 (keV)

Ei (keV)

Iγγ

Ef (keV)

5075.2 (0.3)

2866.8

2944.5 (0.3)

476 (81)

78.7427 (0.0006)

2+

2943.0 (0.6)

5076.2 (0.6)

?

5016.0 (0.3)

3004.0

3003.7 (0.3)

396 (71)

0 (0)

0+

3001.5 (0.9)

5017.8 (0.9)

?

4998.6 (0.3)

2942.1

3021.1 (0.3)

655 (98)

78.7427 (0.0006)

2+

3020.2 (0.6)

4999.1 (0.6)

?

4998.6 (0.3)

3022.7

3021.1 (0.3)

356 (55)

0 (0)

0+

3020.2 (0.6)

4999.1 (0.6)

?

4982.2 (0.5)

1839.8

3038.0 (1.1)

217 (78)

1198.472 (0.007)

2-

3036.8 (0.6)

4982.5 (0.6)

?

4982.2 (0.5)

3038.5

3038.0 (1.1)

428 (65)

0 (0)

0+

3036.8 (0.6)

4982.5 (0.6)

?

4945.0 (0.4)

2997.0

3074.7 (0.3)

500 (92)

78.7427 (0.0006)

2+

3074.8 (0.6)

4944.5 (0.6)

?

4921.6 (0.2)

3098.3

3098.1 (0.7)

546 (71)

0 (0)

0+

3098.7 (0.6)

4920.6 (0.6)

?

4899.0 (0.2)

3042.4

3121.0 (0.5)

1048 (136)

78.7427 (0.0006)

2+

3120.1 (0.6)

4899.2 (0.6)

?

4899.0 (0.2)

3121.5

3121.0 (0.5)

364 (56)

0 (0)

0+

3120.1 (0.6)

4899.2 (0.6)

?

4889.8 (0.2)

3051.5

3130.1 (1.6)

512 (93)

78.7427 (0.0006)

2+

3130.6 (0.6)

4888.7 (0.6)

?

4889.8 (0.2)

3130.9

3130.1 (1.6)

1108 (119)

0 (0)

0+

3130.6 (0.6)

4888.7 (0.6)

?

4874.0 (0.3)

3146.0

3145.7 (0.8)

380 (56)

0 (0)

0+

?

?

?

4805.3 (0.4)

3214.7

3214.4 (0.6)

325 (68)

0 (0)

0+

?

?

?

4794.4 (0.3)

3147.6

3225.3 (0.8)

500 (82)

78.7427 (0.0006)

2+

?

?

?

SP

Ei (keV)

E1 (keV)

E2 (keV)

Table 1: (continued)

Present Work

ENSDF

20

E1 (keV)

Ea2 (keV)

Ei (keV)

Iγγ

Ef (keV)

SP

Ei (keV)

E1 (keV)

E2 (keV)

4774.6 (0.4)

3245.4

3245.1 (0.4)

317 (61)

0 (0)

0+

?

?

?

4763.4 (0.3)

3178.6

3256.3 (0.7)

488 (92)

78.7427 (0.0006)

2+

3254.4 (0.7)

4764.9 (0.7)

?

4759.0 (0.4)

2062.7

3261.1 (0.7)

198 (79)

1198.472 (0.007)

2-

3260.2 (0.5)

4759.1 (0.5)

?

4759.0 (0.4)

2105.9

3261.1 (0.7)

173 (63)

1154.935 (0.006)

1-

3260.2 (0.5)

4759.1 (0.5)

?

4759.0 (0.4)

3183.1

3261.1 (0.7)

417 (79)

78.7427 (0.0006)

2+

3260.2 (0.5)

4759.1 (0.5)

?

4759.0 (0.4)

3261.2

3261.1 (0.7)

2177 (235)

0 (0)

0+

3260.2 (0.5)

4759.1 (0.5)

?

4750.7 (0.3)

3191.3

3269.0 (0.3)

369 (66)

78.7427 (0.0006)

2+

?

?

?

4739.3 (0.4)

3202.3

3280.7 (0.4)

322 (65)

78.7427 (0.0006)

2+

?

?

?

4739.3 (0.4)

3281.1

3280.7 (0.4)

277 (67)

0 (0)

0+

?

?

?

4718.4 (0.4)

3223.3

3301.5 (0.3)

584 (85)

78.7427 (0.0006)

2+

3300.2 (0.6)

4719.1 (0.6)

?

4718.4 (0.4)

3301.9

3301.5 (0.3)

317 (76)

0 (0)

0+

3300.2 (0.6)

4719.1 (0.6)

?

4673.3 (0.2)

3268.5

3346.7 (0.3)

655 (89)

78.7427 (0.0006)

2+

3346.6 (0.5)

4672.7 (0.5)

?

4673.3 (0.2)

3347.0

3346.7 (0.3)

934 (115)

0 (0)

0+

3346.6 (0.5)

4672.7 (0.5)

?

4652.6 (0.5)

3289.4

3367.1 (0.3)

357 (77)

78.7427 (0.0006)

2+

3366.7 (0.7)

4652.6 (0.7)

?

4636.1 (0.7)

2186.7

3382.7 (1.1)

301 (101)

1198.472 (0.007)

2-

3381.5 (0.5)

4637.8 (0.5)

?

Table 1: (continued)

Present Work

ENSDF

21

E1 (keV)

Ea2 (keV)

Ei (keV)

Iγγ

Ef (keV)

SP

Ei (keV)

E1 (keV)

E2 (keV)

4636.1 (0.7)

2227.5

3382.7 (1.1)

340 (96)

1154.935 (0.006)

1-

3381.5 (0.5)

4637.8 (0.5)

?

4636.1 (0.7)

2266.8

3382.7 (1.1)

228 (120)

1117.874 (0.005)

2+

3381.5 (0.5)

4637.8 (0.5)

?

4596.1 (0.3)

3423.8

3423.7 (0.7)

364 (63)

0 (0)

0+

?

?

?

4553.4 (0.4)

3388.6

3466.3 (1.0)

369 (66)

78.7427 (0.0006)

2+

3465.1 (0.6)

4554.2 (0.6)

?

4537.4 (0.2)

3482.6

3482.6 (0.4)

515 (75)

0 (0)

0+

?

?

?

4533.1 (0.6)

3408.9

3486.6 (0.9)

298 (75)

78.7427 (0.0006)

2+

?

?

?

4523.5 (0.7)

2379.7

3496.5 (0.3)

313 (125)

1117.874 (0.005)

2+

3494.7 (0.6)

4524.6 (0.6)

?

4523.5 (0.7)

2453.6

3496.5 (0.3)

459 (239)

1042.914 (0.018)

0+

3494.7 (0.6)

4524.6 (0.6)

?

4523.5 (0.7)

3418.4

3496.5 (0.3)

298 (121)

78.7427 (0.0006)

2+

3494.7 (0.6)

4524.6 (0.6)

?

4523.5 (0.7)

3496.3

3496.5 (0.3)

372 (92)

0 (0)

0+

3494.7 (0.6)

4524.6 (0.6)

?

4512.6 (0.5)

3429.3

3507.1 (0.3)

345 (66)

78.7427 (0.0006)

2+

3506.0 (0.6)

4513.3 (0.6)

?

4492.8 (0.7)

2329.8

3526.6 (0.4)

200 (76)

1198.472 (0.007)

2-

?

?

?

4476.0 (0.2)

3465.8

3544.0 (0.2)

476 (81)

78.7427 (0.0006)

2+

3543.4 (0.6)

4475.9 (0.6)

?

4476.0 (0.2)

3544.2

3544.0 (0.2)

879 (112)

0 (0)

0+

3543.4 (0.6)

4475.9 (0.6)

?

4461.7 (0.4)

2360.7

3558.0 (0.7)

188 (71)

1198.472 (0.007)

2-

3557.3 (0.5)

4462.0 (0.5)

?

Table 1: (continued)

Present Work

ENSDF

22

E1 (keV)

Ea2 (keV)

Ei (keV)

Iγγ

Ef (keV)

SP

Ei (keV)

E1 (keV)

E2 (keV)

4461.7 (0.4)

2403.4

3558.0 (0.7)

372 (139)

1154.935 (0.006)

1-

3557.3 (0.5)

4462.0 (0.5)

?

4461.7 (0.4)

3480.3

3558.0 (0.7)

405 (78)

78.7427 (0.0006)

2+

3557.3 (0.5)

4462.0 (0.5)

?

4461.7 (0.4)

3557.5

3558.0 (0.7)

1069 (128)

0 (0)

0+

3557.3 (0.5)

4462.0 (0.5)

?

4450.1 (0.3)

3491.9

3569.6 (0.8)

643 (108)

78.7427 (0.0006)

2+

3570.0 (0.6)

4449.3 (0.6)

?

4432.2 (0.3)

3587.8

3587.5 (0.6)

744 (112)

0 (0)

0+

3586.9 (0.7)

4432.4 (0.7)

?

4385.1 (0.3)

3634.9

3635.0 (0.6)

523 (83)

0 (0)

0+

3635 (1)

4385.0 (0.7)

3635 (1) from 3635

4378.7 (0.2)

3641.2

3641.0 (0.3)

633 (94)

0 (0)

0+

3640.4 (0.6)

4378.9 (0.6)

?

4362.0 (0.4)

3580.0

3657.7 (0.9)

369 (77)

78.7427 (0.0006)

2+

3657.0 (0.6)

4362.3 (0.6)

?

4338.3 (0.5)

2563.2

3682.2 (0.8)

202 (68)

1117.874 (0.005)

2+

3680.9 (0.6)

4338.4 (0.6)

?

4338.3 (0.5)

2638.0

3682.2 (0.8)

147 (58)

1042.914 (0.018)

0+

3680.9 (0.6)

4338.4 (0.6)

?

4338.3 (0.5)

3605.2

3682.2 (0.8)

310 (76)

78.7427 (0.0006)

2+

3680.9 (0.6)

4338.4 (0.6)

?

4338.3 (0.5)

3682.3

3682.2 (0.8)

618 (87)

0 (0)

0+

3680.9 (0.6)

4338.4 (0.6)

?

4321.9 (0.6)

3620.1

3698.9 (0.7)

298 (75)

78.7427 (0.0006)

2+

?

?

?

4304.7 (0.4)

3637.5

3715.2 (0.3)

369 (111)

78.7427 (0.0006)

2+

3714.2 (0.6)

4305.1 (0.6)

?

4304.7 (0.4)

3715.0

3715.2 (0.3)

594 (99)

0 (0)

0+

3714.2 (0.6)

4305.1 (0.6)

?

Table 1: (continued)

Present Work

ENSDF

23

E1 (keV)

Ea2 (keV)

Ei (keV)

Iγγ

Ef (keV)

SP

4300.3 (0.7)

3641.7

3719.4 (0.3)

333 (99)

78.7427 (0.0006)

2+

3719.2 (0.6)

4300.1 (0.6)

?

4277.9 (0.6)

2543.9

3742.0 (0.4)

150 (106)

1198.472 (0.007)

2-

3740.9 (0.5)

4278.4 (0.5)

?

4277.9 (0.6)

2700.6

3742.0 (0.4)

226 (69)

1042.914 (0.018)

0+

3740.9 (0.5)

4278.4 (0.5)

?

4277.9 (0.6)

3741.7

3742.0 (0.4)

475 (88)

0 (0)

0+

3740.9 (0.5)

4278.4 (0.5)

?

4272.4 (0.2)

3669.6

3747.7 (0.2)

1036 (135)

78.7427 (0.0006)

2+

3747.6 (0.5)

4271.7 (0.5)

?

4272.4 (0.2)

3747.7

3747.7 (0.2)

562 (91)

0 (0)

0+

3747.6 (0.5)

4271.7 (0.5)

?

4264.0 (0.3)

3677.9

3756.4 (0.6)

453 (80)

78.7427 (0.0006)

2+

3754.7 (1.0)

4264.6 (1.0)

?

4252.4 (0.3)

3690.0

3767.8 (1.2)

369 (77)

78.7427 (0.0006)

2+

3766.5 (0.7)

4252.8 (0.7)

?

4252.4 (0.3)

3767.2

3767.8 (1.2)

744 (105)

0 (0)

0+

3766.5 (0.7)

4252.8 (0.7)

?

4219.8 (0.4)

2757.9

3799.8 (0.8)

222 (72)

1042.914 (0.018)

0+

3799.0 (0.6)

4220.3 (0.6)

?

4219.8 (0.4)

3799.8

3799.8 (0.8)

467 (80)

0 (0)

0+

3799.0 (0.6)

4220.3 (0.6)

?

4144.0 (0.3)

3797.3

3876.0 (0.7)

786 (114)

78.7427 (0.0006)

2+

3876.4 (0.6)

4142.9 (0.6)

?

4144.0 (0.3)

3876.7

3876.0 (0.7)

340 (62)

0 (0)

0+

3876.4 (0.6)

4142.9 (0.6)

?

4112.7 (0.3)

3829.3

3908.1 (0.9)

429 (90)

78.7427 (0.0006)

2+

3908.3 (0.7)

4111.0 (0.7)

?

4102.6 (0.2)

3839.3

3917.5 (0.2)

595 (106)

78.7427 (0.0006)

2+

3917.3 (0.6)

4102.0 (0.6)

?

Ei (keV)

E1 (keV)

E2 (keV)

Table 1: (continued)

Present Work

ENSDF

24

E1 (keV)

Ea2 (keV)

Ei (keV)

Iγγ

Ef (keV)

SP

Ei (keV)

E1 (keV)

E2 (keV)

4102.6 (0.2)

3917.6

3917.5 (0.2)

705 (91)

0 (0)

0+

3917.3 (0.6)

4102.0 (0.6)

?

4036.1 (0.5)

3905.9

3984.0 (0.4)

405 (89)

78.7427 (0.0006)

2+

3984.9 (0.7)

4034.4 (0.7)

?

3941.3 (0.2)

4078.7

4078.7 (0.3)

554 (77)

0 (0)

0+

4078.2 (0.7)

3941.1 (0.7)

?

3857.3 (0.4)

4084.6

4162.8 (0.3)

298 (75)

78.7427 (0.0006)

2+

4162.8 (0.6)

3856.5 (0.6)

?

3857.3 (0.4)

4162.9

4162.8 (0.3)

618 (100)

0 (0)

0+

4162.8 (0.6)

3856.5 (0.6)

?

3588.8 (0.6)

3234.1

4431.8 (0.6)

238 (71)

1198.472 (0.007)

2-

?

?

?

3588.8 (0.6)

3275.1

4431.8 (0.6)

176 (71)

1154.935 (0.006)

1-

?

?

?

3578.4 (0.6)

3243.8

4440.9 (0.9)

184 (64)

1198.472 (0.007)

2-

?

?

?

3578.4 (0.6)

3286.3

4440.9 (0.9)

306 (88)

1154.935 (0.006)

1-

?

?

?

3490.7 (0.6)

3486.6

4528.2 (0.8)

221 (73)

1042.914 (0.018)

0+

?

?

?

3447.5 (0.7)

3416.6

4572.6 (0.6)

150 (63)

1154.935 (0.006)

1-

?

?

?

3411.1 (0.7)

3453.0

4610.2 (0.9)

173 (71)

1154.935 (0.006)

1-

?

?

?

3289.2 (0.6)

3533.5

4730.7 (0.3)

174 (65)

1198.472 (0.007)

2-

?

?

?

3233.2 (0.5)

3744.1

4787.0 (0.3)

281 (88)

1042.914 (0.018)

0+

?

?

?

4. Conclusion 250

Present paper studies the level scheme of

172

Yb nucleus from

171

Yb(nth ,γ) re-

action with the use of thermal neutron source from Dalat Nuclear Research Reactor (Vietnam). Using the gamma-gamma coincident technique together with high resolution HPGe detectors, which have advantages in achieving the low Compton background as well as in identifying the correlated gamma tran255

sitions, we are able to detect 18 new levels and 108 secondary transitions in the intermediate energy region from 3-5 MeV, where information on the ENSDF is still spare. This work therefore provides important information on the updated level scheme of

172

Yb nucleus. The forthcoming study is to apply the same

method proposed here to other nuclei, especially those in the rare earth region.

260

Acknowledgement We would like to thank the Ministry of Science and Technology of Vietnam for the financial support through the project coded KC.05/11-15. The authors would like to take this opportunity to thank the operation team of Dalat nuclear research reactor for their kind supports during the experiment.

265

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