Prompt and delayed spectroscopy of 199 At

U. Jakobsson,1,* J. Uusitalo,1 S. Juutinen,1 M. Leino,1 P. Nieminen,1 K. Andgren,2 B. Cederwall,2 P. T. Greenlees,1 B. Hadinia,2,† P. Jones,1 R. Julin,1 S. Ketelhut,1 A. Khaplanov,2 M. Nyman,1,‡ P. Peura,1 P. Rahkila,1 P. Ruotsalainen,1 M. Sandzelius,1 J. Sarén,1 C. Scholey,1 and J. Sorri1 1Department of Physics, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland 2Department of Physics, Royal Institute of Technology, SE-10691 Stockholm, Sweden (Received 2 July 2010; published 6 October 2010)


I. INTRODUCTION
The neutron-deficient nuclei in the light lead region exhibit a variety of different shapes.These coexisting configurations involve an excitation of one or more pairs of protons across the Z = 82 shell gap, interacting with an increasing number of valence neutrons as the 82 N 126 shell becomes depleted [1,2].The low-lying yrast states in even-mass polonium nuclei, for instance, show a transition from spherical structures toward oblate-deformed and onward to prolate-deformed collective bands when approaching the N = 104 mid-shell (see, for example, [3,4] and references therein).Similar behavior could be expected in the odd-mass astatine nuclei, as they have only one proton more than their polonium isotones.
The low-lying states of the neutron-deficient astatine nuclei down to A = 201 have been found to be dominated by spherical structures [5][6][7][8][9][10].However, a recent in-beam γ -ray spectroscopic study, supported with theoretical total Routhian surface (TRS) calculations, showed evidence of shape coexistence in 197 At [11].The 9/2 − ground state is predicted to be nearly spherical, with β 2 ≈ 0.1, in agreement with the observed vibrational level pattern.In contrast, the single-proton 13/2 + state, known throughout the odd-mass astatine isotopes down to A = 193 (excluding 195 At), was observed at an excitation energy of 311 keV and predicted to have an oblate deformation with β 2 ≈ 0.2 in 197 At.This conclusion is supported by the observation of the beginning of a strongly coupled band built on this state.
Earlier results for 199 At are somewhat varying.Me ˛czyński et al. [12] presented a strongly coupled rotational band on top of the 9/2 − ground state.Styczeń et al. [13], however, placed it on top of the 13/2 + isomer, which was suggested to have an excitation energy of 466 keV and a half-life of the order of 1 µs.The isomeric decay itself was not detected.In a later study, Lach et al. [14] observed a delayed 573-keV γ -ray transition showing a half-life of 580(130) ns.They assigned it as the single-step de-excitation of the 13/2 + isomer to the 9/2 − ground state.
The present work aims to resolve the discrepancies among the previous results for 199 At and to investigate more thoroughly the shape competition in 199 At.In addition, an updated level scheme for 197 At is presented, based on an independent analysis of the data used in Ref. [11].

II. EXPERIMENTAL DETAILS
The measurements were conducted in the Accelerator Laboratory at the Department of Physics of the University of Jyväskylä (JYFL), Finland.The nucleus 199 At was produced in two different fusion-evaporation reactions, 150 Sm( 52 Cr,p2n) 199 At and 120 Sn( 82 Kr,p2n) 199 At, where the main goal was an in-beam study of 199 Rn [15].In the first reaction, the beam of 52 Cr, provided by the K-130 cyclotron, was accelerated to an energy of 231 MeV with an average beam current of 17 particle-nA (pnA) (80 h irradiation time).The target consisted of two foils of 150 Sm with a total thickness of 500 µg/cm 2 .The targets were evaporated on a 10 µg/cm 2 thick carbon backing foil and an additional carbon reset foil, with a thickness of 42 µg/cm 2 , was used behind the stacked target.The target material included contamination by small amounts of more abundant heavier samarium isotopes.This resulted in a high population of 202 Rn, which has a half-life and α decay energy similar to those of 199 At.For the second reaction a beam of 82 Kr, with an energy of 355 MeV and an average intensity of 10 pnA (60 h irradiation time), was used to bombard a self-supporting 120 Sn target.The target had a thickness of 500 µg/cm 2 , and a 42 µg/cm 2 thick carbon reset foil was used.In these two reactions, 199 At was populated with cross sections of approximately 10 and 15 µb, respectively.
The JUROGAM Ge-detector array was used to detect prompt γ rays at the target position.The array consisted of 43 Compton-suppressed high-purity germanium (HPGe) detectors of EUROGAM Phase1 [16] and GASP type [17].
The recoiling fusion-evaporation products were separated from beam particles and other unwanted reaction products by the gas-filled recoil separator RITU [18] and transported to the GREAT spectrometer [19].When arriving in GREAT, the recoils passed through a multiwire proportional counter (MWPC) and were finally implanted into a 300-µmthick double-sided silicon strip detector (DSSD), which has 4800 pixels in total.A planar and a clover germanium detector were used to detect delayed γ rays close to the DSSD, and a silicon PIN detector array, situated upstream from the DSSD, was used for measuring conversion electrons.All data channels were recorded synchronously using the triggerless total data readout (TDR) [20] data acquisition system, which gives each event an absolute time stamp with a time resolution of 10 ns.
An additional third measurement, using only RITU and GREAT, was conducted to improve the quality of the previously obtained focal-plane data.An 40 Ar beam, with an energy of 200 MeV, was used to bombard a self-supporting 165 Ho target with a thickness of 340 µg/cm 2 .A carbon reset foil with a thickness of 70 µg/cm 2 was used behind the target.The nucleus 199 At was produced in the 6n fusion-evaporation channel with a cross section of approximately 0.7 mb.Alongside 199 At, the polonium isotopes 198 Po, 199 Po, and 200 Po were relatively strongly populated through pxn fusion-evaporation channels.A beam current of 6 pnA was chosen to obtain clean γ -ray spectra, reducing random correlations caused by the contaminating polonium isotopes.A higher beam current of 40 pnA was used to collect γ -γ coincidences.The total irradiation time was 30 h.Two clover HPGe detectors [21] were added to the focal-plane setup alongside the GREAT clover detector for an improved high-energy γ -ray detection efficiency.
The nucleus 197 At was produced in a separate experiment.The 118 Sn( 82 Kr,p2n) 197 At reaction was used with a beam energy of 362 MeV and a target thickness of 500 µg/cm 2 .A description of the experimental details for the 197 At measurement is presented in the work by Andgren et al. [11].

A. 199 At
The data were analyzed using the recoil-decay tagging (RDT) technique [22] and processed using the GRAIN [23] and RADWARE [24,25] software packages.The recoiling fusionevaporation products were selected by their time of flight between the MWPC and the DSSD and their energy loss in the MWPC.Furthermore, the different isotopes were identified by linking the recoils with their subsequent α decays in the DSSD, using spatial and temporal correlations.The α-decay branch of approximately 90% [26] and half-life of 6.92 s [27] for the 9/2 − ground state allowed for an effective identification of the 199 At recoils, using a maximum correlation time of 21 s between the detection of a recoil and its subsequent α decay.The α-particle energy spectrum obtained from the 82 Kr + 120 Sn reaction is shown in Fig. 1.Prompt and delayed γ rays belonging to 199 At were identified based on their time correlation with the α-tagged recoil observed in the DSSD.For the prompt γ rays detected in the JUROGAM detector array, this time gate was set by taking into account the flight time of approximately 600 ns of the recoils through RITU. 1.The α-particle energy spectrum obtained using the 82 Kr + 120 Sn reaction.A maximum correlation time of 21 s was set between the implantation of the recoil and its subsequent α decay.
The measured prompt data from the 150 Sm( 52 Cr,p2n) 199 At and 120 Sn( 82 Kr,p2n) 199 At reactions were summed to gain statistics for a γ -γ coincidence analysis.A spectrum of prompt γ rays tagged with the α decay of 199 At is shown in Fig. 2(a).Figure 2(b) presents the energy spectrum of the coincident γ rays obtained by setting an energy gate on the 600-keV γ -ray peak, which is the strongest γ -ray peak in the spectrum of Fig. 2(a).The gated spectrum confirms the doublet nature of the 600-keV peak, as reported by Styczeń et al. [13].In the present work the 600-keV transitions were placed lowest in a cascade on top of the ground state, according to the high intensity of the doublet peak, at variance with the previous work.By examining γ -γ coincidences, the 559-keV transition was identified to precede this doublet.The 92-keV γ -ray transition reported in Ref. [13] was not observed.The intensity division between the transitions in the 600-keV doublet was made by comparing the intensity  of the 600-keV peak in the singles spectrum and the gated spectrum [Fig.was not in coincidence with any strong transition assigned to 199 At.It is assumed to depopulate the expected 11/2 − state to the ground state.These levels are indicated as the newly assigned Band 1 in the level scheme shown in Fig. 4. The properties of the γ -ray transitions assigned to 199 At are listed in Table I.
The delayed γ -ray energy spectra in Fig. 3 are collected from α-tagged events detected in the clover detectors, with a 5-µs correlation time gate.It is evident that several γ -ray transitions (236, 373, 434, 609, and 670 keV) in the singles spectrum of Fig. 2 [Fig.3(a)], indicating the existence of a new high-lying isomer in 199 At.These transitions belong to the previously reported level structure [13], placed to feed the expected 13/2 + isomer.This study focused on prompt γ -ray spectroscopy, and no high-lying isomer was reported.
Lach et al. [14] placed the 13/2 + isomer at an excitation energy of 573 keV.At the same time, they noted that the level energies suggested by Styczeń et al. [13] have to be revised.In the present work, the 573-keV γ ray is the most intense delayed transition assigned to 199 At [see Fig. 3(a)].Additionally, the 573-keV transition is the only transition that shows two components in the recoil-γ time-difference spectrum.These observations support the assignment made by Lach et al. [14].A 466-keV transition, suggested by Styczeń et al. [13], was not observed.Their suggestion that the half-life of the isomer was on the order of a microsecond and that the excitation energy was 466 keV was based on systematics.The properties of the isomeric states in 199 At, as well as results from the conversion electron measurements, will be discussed in the following.
The present results, utilizing γ -γ coincidence studies and energy-sum arguments, largely support the previously reported level structure of the i 13/2 band based on the 13/2 + state [13] (Band 2 in Fig. 4).However, γ -γ coincidences support the placement of a previously unknown 631-keV transition to precede the 670-keV transition, establishing a new level at 1874 keV.The relative intensity of the previously reported 700-keV transition is distinctly weaker in Fig. 2(a) than in Fig. 3(a), indicating that it lies closer to the higher lying isomer and is not part of Band 2. Furthermore, a new level at an energy of 2544 keV was tentatively established.
The delayed 133-and 163-keV transitions visible in Fig. 3(a) do not belong to Band 2 and are thus candidates for transitions in the de-excitation path of the indicated higher lying isomer.The energy spectrum of γ rays gated by the 163-keV transition, presented in Fig. 3(b), shows mostly the same transitions as those in Fig. 3(a).Thus, it is likely that the majority of the decay intensity of the isomer proceeds via the 163-keV transition.Although the 163-keV transition is clearly visible in Fig. 3(a) it is not seen in Fig. 2(a), suggesting its placement directly below the isomeric state.In contrast, the 133-keV transition is weakly visible in the α-tagged spectrum of prompt γ rays [Fig.2(a)], excluding it from being the transitions that depopulates the isomer itself.The suggested de-excitation path of the isomer is shown in the level scheme of Fig. 4, indicated as Band 3.
Figure 5 displays a delayed γ -ray spectrum from the planar detector up to 5 µs after the detection of the 199 At recoils.In addition to the 133-and 163-keV transitions, lines at energies of 54, 70, 187, 202, 236, 258, and 327 keV are also observed.The very weak 187-keV transition is in coincidence with the 163-keV transition [see Fig. energy sum arguments, is tentatively placed to proceed in parallel with the 54-and 133-keV transitions.The 631-and 258-keV transitions show weak coincidences with the delayed 163-keV transition [Fig.3(b)], implying a connection from the high-lying isomer to the 21/2 + level at 1874 keV.Weak 70-and 202-keV transitions were tentatively identified (see Fig. 5).They are possibly connecting transitions from states in Band 3 to the 21/2 + state in Band 2, allowing the 631-and 258-keV transitions to be detected at the focal plane.Owing to a lack of statistics, the ordering of the transitions in Band 3 remains tentative.
The total internal conversion coefficient of the intense 163-keV transition was estimated by comparing its γ -ray intensity to the total intensity of the 573-keV transition.The resulting value close to 1 indicates an E2 multipolarity for the 163-keV transition.By assuming the 133-and 54-keV transitions are in a cascade with the 163-keV transition, with minimal side feeding, an M1 multipolarity for both transitions was evaluated.These multipolarities suggest a spin-parity of 29/2 + for the isomeric state at 2293 keV.By fitting the time difference between the implantation of the 199 At recoil in the DSSD and the detection of the 163-keV transition in the planar detector, a half-life of 0.80(5) µs was determined for the 29/2 + isomer (see Fig. 5).
Previously, a half-life of 580(130) ns [14] had been assigned for the 13/2 + state.To obtain the half-life of the 13/2 + isomer, time differences of transitions in the recoil-gated data below and above the 13/2 + state were measured.An energy versus time difference matrix, gated on the 163-keV transition detected in the planar detector, was constructed.The energies of the subsequent transitions, detected in the clover detectors or PIN detector array, were plotted against the time difference with the 163-keV transition.An example of the procedure, using the 474(2)-keV K-conversion electron peak of the 573-keV transition, is shown in Fig. 6.The decay curve was fitted from a point after the drop of the prompt time component.Here the remaining prompt component in the time spectrum originates mainly from the background underneath the peak designated as being due to K conversion.The background component originating from random correlations between the detection of the recoil and the decay event was neglected in this study.A half-life of 70 (20) ns was obtained for the 13/2 + state, corresponding to a single-particle transition strength of 0.16(5) W.u. for an M2 transition.This result is comparable with corresponding values of an M2 transition in neighboring odd-mass astatine nuclei, 0.086(2) W.u. for 197 At [11] and 0.182(22) W.u. for 201 At [29].
Additionally, a comparison between the 573-keV γ and K-conversion electron peak intensities [in Fig. 3(a) and the inset of Fig. 6] gives a K-conversion coefficient of 0.19(4) for the 573-keV transition, which supports an M2 multipolarity as well.The 434-keV M1 transition has been used for normalization.

B. 197 At
An independent analysis of the 197 At data of Andgren et al. [11] has been performed, allowing for additional γ -ray transitions to be assigned to the level scheme.The prompt γ -ray spectrum is shown in Fig. 7.The strongly coupled rotational band on top of the 13/2 + isomer is extended by two new states with level energies of 1193 and 1796 keV.The latter of these states is denoted as tentative owing to a lack of γ -γ coincidence data.Some γ -ray transitions evident in Fig. 7 are not associated with the 13/2 + isomer, although they clearly belong to 197 At.The 505-keV transition is not in coincidence with any of the strong transitions in Band 2 nor with the 536-keV γ -ray transition.It is therefore assigned to depopulate the 11/2 − state.The placements of the new 412-, 446-, 477-, and 491-keV γ -ray transitions are based on energy sums and intensity arguments.The extended level scheme for 197 At is shown in Fig. 8.

IV. DISCUSSION
Figure 9 presents energy systematics of negative-parity levels in odd-mass astatine and corresponding yrast levels in polonium isotones.The polonium isotopes with 114 N 124 have been interpreted to have a vibrational character.Below N = 114 the drop in the level energies, particularly visible for the 6 + and 4 + states, has been interpreted to be caused by a 2p-2h excitation across the proton shell gap, enhancing the effect of the oblate component in the wave functions [30].A similar drop, though more gentle for the lower lying states, can be seen for the astatine nuclei in 199 At.Because of the resemblance of the energy levels between the corresponding isotones, the states in the astatine nuclei can be interpreted as the odd h 9/2 proton weakly coupled to the polonium core.
The growing deviation of corresponding states between the isotones around N = 112 in Fig. 9 may indicate a strengthening of the coupling between the odd proton and the core.An 11/2 − state is assigned tentatively for both 197 At and 199 At.The energy of this state in 199 At is higher than that of the 13/2 − state, as can be expected based on the systematics of heavier astatine nuclei (see [5] and references therein).In 197 At our candidate for the 11/2 − state is below the 13/2 − state.This observation further supports the suggestion of a strengthening of the coupling between the mixed configuration of the h 9/2 /f 7/2 proton and the polonium core.A similar behavior has been observed earlier in the odd-mass bismuth isotopes (see [33] and references therein).The suggested configuration for each state is shown in Table II.
The excitation energy of the 13/2 + state decreases, with decreasing neutron number, and becomes yrast in 199 At.A similar decrease for the 13/2 + level is seen for example in the odd-mass bismuth isotopes, when approaching the neutron mid-shell.This is interpreted as an effect brought on by a coupling of the i 13/2 proton to an increasing number of valence neutron holes.This interaction is less repulsive for a proton in the i 13/2 shell than in the h 9/2 shell, thus bringing the 13/2 + state down in energy [34].
The 13/2 + isomer in 197 At is interpreted as being oblate deformed [11].The level spacing in the i 13/2 band decreases when moving from 199 At to 197 At.It can also be noted that the signature splitting is larger for 199 At in comparison with the corresponding band in 197 At. Figure 10  compared to the 193 Bi nucleus.A theoretical estimate was calculated using the Dönau-Frauendorf model [35,36].The B(M1)/B(E2) values for 199 At are lower than one would expect for a collectively behaving nucleus.This may be caused by a stronger contribution of the spherical components in the wave functions in 199 At than in 197 At, resulting in a weaker deformation for the states in 199 At.The values for 197 At and 199 At are, nonetheless, of comparable magnitude with those of the i 13/2 band in 193 Bi.
Plots of experimental kinematic moments of inertia I (1) for the i 13/2 band in 197 At and 199 At show similar linear behavior FIG. 9. Level energies of negative-parity states in odd-mass astatine nuclei compared with the yrast states in their even-mass polonium isotones. 199At has a neutron number of 114.For the polonium data, denoted by closed symbols, see the studies [3,31,32] and references therein.The data for the heavier astatine nuclei are taken from the studies [5,[7][8][9][10].
as bands in neighboring oblate deformed nuclei (see Fig. 11).This, together with the observed strongly coupled character of the band, supports the interpretation of oblate structure for the i 13/2 band in 197 At and in 199 At.
The 29/2 + isomer and the subsequent states in Band 3 of 199 At (see Fig. 4) need a more detailed examination.In the polonium isotone 198 Po, an 11 − π (h 9/2 i 13/2 ) isomer has been identified at an excitation energy of 2565 keV [38].The following states have also been identified: 9 ), and 7 − ν(p −1 3/2 i −1 13/2 ).The 11 − isomer populates the 9 − state through an E2 transition.The 29/2 + isomer in 199 At is assumed to originate from the odd proton in the i 13/2 shell coupled to the maximally aligned h 9/2 proton pair.However, the subsequent 25/2 + , 23/2 + , and 21/2 + states can be interpreted as theJ max−1 states of the odd h 9/2 proton coupled to the 9 − , 8 − , and 7 − neutron states of the polonium core just discussed.This major configuration difference between the states results in the 29/2 + state becoming isomeric.Figure 12 shows a comparison of the level energies of the yrast 29/2 + and 25/2 + states with the 11 − and 9 − states of the proposed polonium isotone core.Although the states are not well known throughout the astatine nuclei, a similarity between the curves is clearly visible.
they are expected to stay rather constant with decreasing neutron number.This results in the 29/2 + isomer favoring a feeding to the positive-parity states rather than to the 23/2 − state.In 205 At, the 29/2 + isomer feeds the 23/2 − state through an E3 transition [10].However, there is also an E2 branch between the 29/2 + and 25/2 + states in this nucleus.Whereas the single-particle transition strength for this 163-keV E2 transition is 0.044 W.u. in 199 At, the transition is even more retarded in 205 At with a strength of 0.00018 W.u. The 23/2 − state is also fed by an E1 transition from the 25/2 + isomer in the heavier astatine nuclei.The level energy of the 23/2 − state, however, rises steadily with decreasing neutron number, making the feeding to it from both the 25/2 + and the 29/2 + isomer increasingly unfavored.FIG.11.Experimental kinematic moments of inertia of the rotational band feeding the 13/2 + isomer 197 At and in 199 At in the studied astatine nuclei compared with neighboring oblate deformed nuclei [3,31,33,37].High-spin data, which exhibit a back bend, have been omitted for 193 Bi, for purposes of low-spin comparison.  19At has a neutron number of 114.The data point indicated with an arrow is floating in the level scheme and has been lifted 30 keV in the figure for visual purposes.For the polonium data see the studies [3,31,32] and references therein.
An alternative scheme for the depopulation of the 29/2 + isomer is that it decays through the 54-keV M1 transition to a 27/2 + state, followed by the 133-keV M1 and 163-keV E2 transitions.This would indicate a strong retardation of 1 × 10 5 compared to the Weisskopf estimate for the 54-keV M1 transition.Similarly hindered M1 transitions have been found in the decay of the 10 − π (f 7/2 i 13/2 ) isomer in the 204,206,208 Rn isotopes [39].In this case, the subsequent 27/2 + and 25/2 + states in 199 At would be interpreted as the J max and J max−1 states from the h 9/2 proton coupled to the 9 − polonium core.A similar pair of states has been reported by Mabala et al.  in 197 Bi [40], which is moreover an isotone to 199 At, and by Lönnroth et al. [34] in the α-decay daughter 195 Bi.In this scheme, the transition between the 25/2 + state and the 21/2 + state is believed to mimic a core transition between the 9 − and the 7 − states in 198 Po.The 25/2 + state is reported to be isomeric in the heavier astatine isotopes, with a half-life of some tens of nanoseconds.However, its possible isomeric nature could not be resolved in this study.

V. CONCLUSIONS
The present study clarifies some ambiguities related to the previous results for 199 At.We present a strongly coupled rotational band on top of the 13/2 + isomer and a series of transitions to the 9/2 − ground state.The transitions observed feeding the ground state point to a vibrational structure in 199 At.The 11/2 − state is above the 13/2 − state in energy in 199 At, but it drops below the 13/2 − state in 197 At, as being part of the nonfavored signature partner in the ground-state band.This indicates a strengthening of the coupling between the odd h 9/2 /f 7/2 proton and the rotating polonium core.The strongly coupled character of the i 13/2 band is a sign of collective behavior in both 197 At and 199 At.The coupling is, however, weaker in 199 At, showing a sign of the nucleus being of transitional nature between spherical structures and the oblate deformed 197 At.A 29/2 + isomer has been identified to feed the i 13/2 band through states originated from the odd proton coupled to a neutron pair.The configuration for this isomer is suggested to be π (h 2  9/2 i 13/2 ).
FIG.1.The α-particle energy spectrum obtained using the 82 Kr + 120 Sn reaction.A maximum correlation time of 21 s was set between the implantation of the recoil and its subsequent α decay.

FIG. 2 .
FIG. 2. (a)Spectrum of singles prompt γ rays tagged with the α decay of 199 At, collected from the 120 Sn( 82 Kr,p2n) 199 At reaction.(b) Spectrum of γ rays in coincidence with the 600-keV doublet, obtained from the 150 Sm( 52 Cr,p2n) 199 At and 120 Sn( 82 Kr,p2n)199 At reactions.The dashed lines are inserted to guide the eye for some relevant coincidence-anticoincidence relations.

FIG. 3 .
FIG. 3. Delayed γ rays tagged with the 199 At α decay, detected in the clover detectors.(a) Spectrum of delayed singles γ rays obtained with lower beam intensity.(b) Spectrum of γ rays in coincidence with the 163-keV transition detected in the planar detector.The 484-keV peak, denoted by a diamond, belongs to 200 Po.The data are obtained from the 165 Ho( 40 Ar,6n) 199 At reaction; a maximum correlation time of 5 µs was set between the recoil and its isomeric decay.
FIG.5.A spectrum of delayed γ -ray transitions detected in the planar detector within 5 µs after the detection of the 199 At recoil in the DSSSD, obtained with lower beam intensity.The inset presents the time distribution between the implantation of a 199 At recoil and the observation of the 163-keV γ ray.The filled circles denote the astatine x-ray peaks and the diamonds denote peaks belonging to the polonium isotopes.A spectrum acquired within the time interval of 5-10 µs has been subtracted.The data are obtained from the 165 Ho( 40 Ar,6n) 199 At reaction.

20 FIG. 6 .
FIG.6.Time-difference spectrum between the 163-keV γ -ray transition observed in the planar detector and the K-conversion peak of the 573-keV transition observed in the PIN-detector array.The solid line presents the decay curve fitted from 60 ns on.The dashed line, with the intensity compressed by a factor of 20, shows the total projection of the time spectrum for comparison.The inset presents the delayed conversion electron spectrum tagged with the 199 At α decay.The K-, L-, and M-conversion peaks of the 573-keV transition are marked in the spectrum together with the K-conversion peak of the 434-keV transition.The data are obtained from the 165 Ho( 40 Ar,6n) 199 At reaction.

FIG. 7 .
FIG. 7.A prompt γ -ray spectrum tagged with the α decay of 197 At, including escaped α particles.Peaks containing events from the contaminating194 Po are denoted with diamonds.

FIG. 12 .
FIG.12.Level energies of the 29/2 + , 25/2 + , and 23/2 − states in the odd-mass astatine nuclei, denoted by open symbols, compared with the 11 − and 9 − states in their polonium isotones, denoted by closed symbols.199At has a neutron number of 114.The data point indicated with an arrow is floating in the level scheme and has been lifted 30 keV in the figure for visual purposes.For the polonium data see the studies[3,31,32] and references therein.

TABLE I .
The γ -ray energies and relative intensities for transitions assigned to 199 At.
depicts the B(M1)/B(E2) values for the i 13/2 band of 197 At and 199 At

TABLE II .
The proposed configurations for some of the states in 199 At.