Yrast traps and high-spin states in 210Rn

Nuclear Physics A390 (1982) 335-364 © North-Holland Publishing Company

YRAST TRAPS AND HIGH-SPIN STATES IN st ° Rn Depwrtment qf Physics,

Unicersitr

A. R. POLETTI qf Auckland, Pricute Bup, Auckland. Netr Zeulund

G. D. DRACOULIS and C. FAHLANDER Deportment of Nuclear Pht~sics, Research School ol~Pht~sicul Sciences, Australian Notional Unirersitt', P. O. Bo .~ 4, Cunherru, ACT2600, Australia and I . MORRISON School of Physics, Unirersitr of Melbourne, Parkrille, Victoria 3052, Australia Received 3 November 1981 Abstract : Yeast and near-yeast states have been investigated in Z`°Rn to high spin (J > 30) and high energy (E, > l0 MeV) . Three different(HI, xn) reactions were used to populate the states of interest and several different y-ray spectroscopic techniques were utilized. Three high-spin yeast traps were discovered . Two de-excite by strong E3 transitions while the third decays mainly via an extremely inhibited E2 transition . The E3 decays are interpreted as allowed single-particle transitions between proton or neutron states above the =°sPb shell clostue while the inhibited E2 transition is interpreted as indicating a substantial change in structure as the decay proceeds down the yeast line . The interpretation has been given in terms of shell-model calculations.

E

NUCLEAR REACTIONS x°sTl(t°B, Sn) E x 70 MeV ; "°Pt(` 60, 4n) E x 90 MeV; "BPt ("O, Sn),E z 96 MeV; measured yy-, ny~oin, ny(t), Eï, L,, y(0), ICC. ~`°Rn deduced levels J, n, T, configurations, yeast traps. Ge, Ge(Li), NE213, Si(Li) detectors, enriched targets. Shell-model calculations .

1. Introduction Yeast traps are at present a subject of lively interest, both theoretical and experimental, in heavy ion nuclear physics. An yeast trap in (HI, xn) reactions collects the decay intensity of higher levels and, by its lifetime, delays the further decay to lower states. Yeast traps of two types can be distinguished . An yeast level can become a trap if its most probable decay must be via either a low energy or high multipolarity transition (or both). The transition however can be a strong one of its kind. The second type of trap occurs for nuclear structure reasons. Although decay can take 335

33 6

A . R . Poletti et ai. / ~'°Rn

place via a transition of low multipolarity and reasonably high energy, it is inhibited because of a substantial structural difference between the two states involved . In the region of the periodic table near the Z éz1'b closed shell, yrast traps of both types abound . In order to understand their properties and their significance it is however necessary to undertake a systematic study of the yrast spectroscopy of the nucleus concerned and of related nuclei . This work describes inves~igations of the yrast states of Z ééltn up to an excitation energy of 10 MeV and an angular momentum of more than 3011 . As a consequence, the yrast spectroscopy of three Rn isotopes is now established to spins of 3011 or over : z i zRn [Horn et ul. ' )], z "Rn [Poletti et ul . z) and Dracoulis et ul . z' °Rn 3)] and 4. [Poletti et ul . s) and the present work] . We have used three different reactions and several different spectroscopic techniques in the course of our investigations . The results considerably extend the understanding obtained from a previous investigation [ref. ")] of Z 1 °Rn and much of that work has been confirmed ; however, some of the conclusions reached in that earlier study have been corrected . We have discovered several strong E3 transitions between high-spin states in z ' °Rn : The isomeric levels involved are yrast traps in the first sense. These E3 transitions are related to those previously discovered in other Rn isotopes' Z) and in the At isotopes : z°9 At [Bergstrom et ul. e)], zioAt [Rahkonen et ul .')] and z"At [Bergstrom et ul . $) and Maier et ul . 9 )], and their systematic occurrence and significance in this region of the periodic table is discussed. An yrast trap of the second type, involving a substantial structural change, has also been identified . This illustrates the way in which core excited states intrude into the yrast line once the highest spin yrast level obtainable from the valence particles is reached.

2. Experimental procedure To elucidate the high-spin structure of 2 '°Rn three different (HI, xn) reactions were used, each initiated by beams provided by the ANU 14UD Pelletron accelerator . The reactions were so5~(ioB~

Sn)Z1°Rn, ro) 198~(160~ 4n)sio Rn, (c)'9aPt("~, Sn)Z1°Rn, (a)

Eb = 68 to 75 MeV, Eb = 85 to 97 MeV, Es

= 94 to 98 MeV,

where Eb is the bombarding energy of the corresponding incident particle . Reaction (a) optimises the population of states of intermediate excitation (population of states up to J = 26 is expected, while states up to J = 25 were observed). The second reaction investigated in conjunction with an experiment on Z° 9 Rn enabled us to observe the same states, but the relative excitation of the highest states (with

A. R. Poletti et al. / s' °Rn

33 7

respect to the 644 keV 2+ -. 0+ transition) was increased by about 50 %. The maximum input angular momentum of reaction (c)~ is about 32iß, thus the population of the high-spin states is expected to be enhanced . The highest-spin state we actually observed is either J = 31 or J = 32. For each reaction a number of y-ray spectroscopic measurements were undertaken. Depending on the requirements of the experiment either large-volume Ge(Li) detectors or intrinsic Ge LEPS detectors were used. The former achieved optimum detection efficiency and energy resolution for high energy y-rays (E < 1400 keV) while the latter were used to optimise energy resolution, efficiency and timing for y-rays ofenergy between 50 and 250 keV. In addition a Compton-suppressed Ge(Li) detector was used in two separate experiments using the "O+' 98 Pt reaction . In each case and for each detector the detection efficiency was measured by placing sources at the target position . The sources used were `S Z Eu, `ZSSb and `33Ba. In addition to excitation function measurements, for all three reactions over the range of energies specified above, we undertook the following experiments : 2.1 . 7~- ;-t AND n-ï-t MEASUREMENTS (a) An experiment using the Z°STl(' °B, Sn)Z ` °Rn reaction together with event mode recording of coincident events was carried out at a bombarding energy of 69 MeV using a rolled self-supporting 13 mg/cm2 enriched Z°STl foil ( 2 °STl : 96 .4 %, ~o3-Il : 3 .6 %. Two y-ray detectors were placed at backward angles of about ± 135° and the beam was stopped in a distant, shielded beam dump . A neutron detector (NE213 liquid scintillator), with pulse shape discrimination, detected neutrons at a forward angle in coincidence with one of the y-ray detectors. The time range used in both the y-y and n-y concidence experiments was ±4 ~s, because of the known existenceofisomers in s `°Rn with mean lives of the orderof 1 ~s. A similar experiment was also tamed out in which one of the Ge(Li) detectors was replaced by a high resolution intrinsic Ge LEPS detector . Coincidences between y-rays detected in this detector and neutrons were also recorded as described above. (b) For ` 98 Pt(` 6 0, 4n)2 ' ° Rn : similar experiments to those in (a) were carried out using this reaction at bombarding energies of 90 and 91 MeV respectively with a 4 mg/cm Z enriched Pt foil target (' 98 Pt : 95 .8 %, ` 96 Pt : 2.5 ~, ` 95 Pt : l .2 %. In the first experiment with two Ge(Li) detectors placed at about ± 135° thé beam was stopped immediately behind the target (which was normal to the beam) in a 0.13 mm thick Pb foil, and the neutron detector was placed at 0° . For the second experiment one of the large-volume Ge(Li) detectors was replaced by an intrinsic Ge LEPS detector and the detectors were placed at ± 90° in a close geometry . The beam was again stopped in the distant beam dump and the neutron detector was placed at a convenient forward angle. (c) For ' 98 Pt("O, Sn)Z `°Rn : In order to elucidate puzzling features of the decay scheme at intermediate excitations and to investigate higher lying yrast states, a

3~8

A . R . Poletti et al. / Z~°Rn

further coincidence experiment was undertaken using this reaction at a bombarding energy of 95 MeV. The target (placed normal to the beam) was the same as for the ~e~+198 reaction while the beam was again stopped in a Pb foil immediately behind it. In this experiment two large-volume Ge(Li) detectors at approximately 45° and 135° to the beam were used, each in coincidence with a Compton-suppressed Ge(Li) detector at about -45°. Coincidences between this detector and a neutron detector positioned at about 135° (and out of the detection plane defined by the other detectors) were also measured . A time range of ±2 p,s was used. Use of the Compton suppressor led to a significant improvement in spectrum quality, the effective (in-beam) suppression factor being about 3 :1 in the low energy region of the spectrum . This is comparable to that obtained in reactions leading to deformed nuclei, but without the equivalent peak rejection observed in those cases' °) . The rejection of Compton events also reduces the data storage requirements, and in this experiment about 1 .3 x 108 Compton-suppressed coincidence events were recorded . 2 .2 . ;-RAY ANGULAR DISTRIBUTIONS

Several measurements were undertaken . In each case normalization was to a fixed Ge(Li) detector . For the ' ° B+ Z°sTl reaction two measurements were made . The first used a large-volume Ge(Li) deteçtor while the second used a planar Ge LEPS detector . In both cases the angular range was 90° to 150° . The targets, respectively 13 mg/cm2 and 3 .5 mg/cmZ thick were placed at 45° with respect to the beam axis while the beam was stopped in the remote beam dump. Bombarding energies were 70 and 68 MeV respectively . For the ' 60+'98Pt reaction an angular distribution measurement at a bombarding energy of 89 MeV was carried out. The target was 4 mg/cmZ thick and the beam was stopped in a 0.13 mm thick Pb foil immediately behind the target so that the Ge(Li) detector could move over the full angular range of 0° to 90°. Information on the highest lying transitions was obtained from an angular distribution measurement using the "O+' 98Pt reaction at 95 MeV and the same experimental disposition as for the'60+' 98 Pt reaction above. In this case the target was 3 .6 mg/cmZ thick. 23 . CONVERSION COEFFICIENTS

As well as the indirect determination of total conversion coefficients by intensity balances, especially from delayed intensities, electron conversion coefficients were measured directly for both the'°B+ Z°sTl reaction at 69 MeV and the "O+' 98 Pt reaction at 95 MeV bombarding energies . In both cases the in-beam~electron spectrum was detected in a cooled Si(Li) detector at 125° to the beam direction, in conjunction with a "mini-orange" magnetic filter "). The y-ray spectrum at 55° to the beam axis was recorded simultaneously . For the '°B+ZOSTI reaction the target used was

A. R. Poletti et at. / Z ~°Rn

339

3.5 mg/cmZ z°STl evaporated on to a thin carbon foil placed with its plane at about 30° to the beam direction. Forthe "O + 198Pt reaction the target was a self-supporting rolled 4.1 mg/cm2 198Pt foil at about 60° to the beam direction. In both cases target thickness and angles were chosen as a compromise between electron resolution, target spot size and loss of recoiling nuclei . The electron and ~-ray efficiencies were measured (simultaneously) using ` SZEu, 2°'Bi, " 3Sn, "'Cs and 65Zn sources. 2 .4. PULSED BEAM LIFETIME MEASUREMENT

Information on the longer-lived isomers in 2 ' ° Rn was obtained in a pulsed beam experiment using the 1°B+ Z°STl reaction . The time relationship to the beam pulse and the energy of y-rays observed in a large volume Ge(Li) detector were recorded. The pulse width was 0.6 ps and the pulse separation was 8 ks. Mean lives (and limits on mean lives) were obtained for several of the isomers . 3. Results 3.1 . LEVEL SCHEME

The level scheme for the z ' ° Rn as determined in the present work is summarized in fig. 1 . Table 1 lists the observed Y-rays together with their placement, and the relative intensities as determined from the l 'O+ 198Pt reaction at 95 MeV. The low-spin region has already been studied by Poletti et a/. 4) but a number of new features in this part of the scheme have been established in the present work, part of which was reported earlier S). In addition, the scheme above the isomer at 3812+d keV has been deduced from the present experiments (d is the energy of the unobserved 8+ -+ 6+ transition). For reference, table 2 summarizes the measured mean lives and lifetime limits . 3.1.1 . Levels beloK~ the 8 + isomer at 1665+d keV. The y-rays at 120, 203, 644, 818 and 901 keVcorresponding to de-excitation of the levels below the 8+ isomer are seen in the upper part of fig. 2 as negative-going peaks. They are delayed with respect to the gating y-rays by the lifetime of the 8 + isomer . (The 325, 546 and 712 keV lines also appear as negative peaks because of the 104 ns mean life of the level at 3248 + d keV.) As shown in fig. 1 these y-rays correspond to de-excitation of the 6+ state at 1665 keV via two 4+ states to the 2+ state. Table 3 lists the measured branchings, the significance of which has been discussed in a separate paper S). An analysis of n-y time spectra taken with a LEPS detector gave the mean life of this 6+ state as T m = 11 ±2 ns. The mean life df the 8+ isomer was established from appropriate projected time spectra obtained from the y- y coincidence experiments. One of these spectra is shown at the top of fig. 3. A mean life of 910±50 ns was obtained . This is somewhat shorter than the results quoted by Poletti et al. °) or Maier et al. ' Z) ; however both of these results were obtained by analysis of y-ray time spectra synchronized with respect to a pulsed

340

A.

R. Poletti et al. / z'°Rn

beam . With two higher lying isomers with mean lives near 1 .5 yes, extraction of a reliable mean life for the 8+ isomer is difficult from such measurements . No Y-ray which could be ascribed to the direct de-excitation of the 8+ state was observed. The very large total conversion coefficients (aT) for low-energy E2 transitions (for instance aT = 2400 for E = 31 keV while for E = 50 keV, aT = 225) makes their observation difficult. The measured efficiency for low-energy transitions (which would allow the

TABLE I

Energies, intensities and placement of ~-rays in Z'°Rn Initial state

Final stae

Energy (keV)

Intensity

11 I .2(1) 119.6(1) 127 .4(5) 159.6(1) 185.4( I ) 191 .6(1 x' 203.1(1) 294.3(2) 303.5(2x' 322.3(2) 325 .3(1) 354.8(2) 367.4(3) 378.9(2)`' 390.2(2x' 415.9(2x' 426.2" 433.0( I ) 467.2(3)d' 482.2(3) 492.3(2x' 515.4(3)

- 19 47 < 10 127 -66 l03 546 - 10 123 42 662 -90 - l4 ~ 30 96 - 29 - 34 350 -17 17 S9 61

2317+d 1665 5025+d 6036 + d 2563 + d 5876+ d 1665 3404+d 5684+d 10086+ d 3248 + d 5381 +d 2033 +db'

2266+d 1545 4899+d 5876 + d 2377 + d 5684+ d 1462 3110+d 5381 +d 9764 + d 2923 + d 5025+d 1665 + de'

5384+d 7310 + d 6895 + d 6469 + d

4994+d 6895 + d 6469+ d 6036 + d

5381 +d 5876+d 9764+d

4899 + d 5384+d 9248+d

545.6(1) 547.5(2) 564.2(1) 601 .1(1) 602.0(5) 616.2(3x' 643.8(1) 671 (lx' 693.4(3) 712 .2(I ) 817.6(I)

760 - 30 606 223 87 32 1000 ~IS " 65 670 83!

2923 +d 3110+d 3812+ d 2266 + d 6469 + d 3864+ d 644 3919+d 9248+d 2377 + d 1462

2377 +d 2563 + d 3248 +d 1665 +d 5867 +d 3248 + d 0 3248+d 8555+d 1665 + d 644

341

A . R . Poletti et ol. / z'°Rn TABLE

I (wntinued)

Energy (keV)

Intensity

Initial state

Final state

841 .5(5) 841 .8(2) 872.8(2) 897.6(2) 882.5(2) 901 .3(1) 1035 .5(Sr' 1086 .6(5) 1106 .1(SY' i 181 .2(2) 1245 .0(7) 1358 .9(7)

~- 6 225 133 -15 80 218 ~ 15 38 15 373 85

3404+d 7310+d 5867+d 2563 + d 5876+d 1545 4899+d 4899+d 5025 + d 4994+d 8555 +d

2563+d 6469+d 4994+d 1665 + d 4994+d 644 3864+d 3812+d 3919+ d 3812+d 7310+d

') Placement ambiguous, see fig. 1 . b) Possible placement. °) Above 3812+d keV level, also possibly 3783+d ~ 3404+d . °) Above 3248+d keV level and beneath 5381 +d . `) Above 3812+d, below 6036+d .

TABLE 2 Summary of lifetimes (and limits) of states in ~'°Rn

Mean life (ns)')

Level (keV)

y-rays

1665 1665 +d

120, 203, etc. 120, 203, etc.

2563+d

185

3248+d

325, 546, etc.

104 f 5

3812+d

564, 325, etc.

1590 ± 90

4994+d 6469+d 7310+d

1181 433, 159,etc. 842, etc.

present 11 f 2 910 t 50 92 ± 5

16 .5 f 1 .0 1500t'l00 49 ± 3

other 1070 t 50 °) 1082 f 58 °) 98 f 6 ") 84± 6 `) 147t 26 b) 143t 12 °) 1440 t 120 b) 1441 f 180 °)

') For all other levels not listed in the table, sm < 8 ns, except for the levels at 2266+d keV (< 30 ns), 3864+d (< 12 ns), 5876+d (< 10 ns), 6036+d (< 10 ns) and 6895+d (< SO ns). ") Ref. 4). `) Ref. ' 2) .

342

A . R. Poletti et al. 10086+p 9764+p

/

Z'°Rn (31,32)

322(D)

(30,31)

51~(D)

9248 +p

(29,30) 693

8555 +p

28+ 1245(E 3)

7310 +p

25 F 49m

w~ns~~ . 416

+ 22 F ~ 1 .50~.s 20+ 19 + 21 +

20 + <--- 17 ns 118

3)

IÔ 6

1035

1106

3919+ 864+p ~_

r;

i i _~_ 3812 +p ~--17 ~ 1 .59i.s 616 ) 671 564 .~ ;_ 3404+p104 ns - 12~-12+ I1

E-- 92 ns

-IIIIMI)-~~ 0+ 897 9

601(MI/E?a

g+ F- 910ns Ilns

644 (E2) 0+

0 210 Rn

Tm

Fig. 1. Level scheme of Z`°Rn determined in this work. The width of the vertical arrows represents the y-rayintensity observed in the "O+' 9sPt reaction at 95 MeV. The positions of the levels marked with an asterisk is uncertain since the order of the feeding and de~zciting transitions may be reversed .

343

A. R. Poietti et al. / Z '°Rn 3000 Rn X~r-a. Ys

(407) 390 379 433 3221 5 1 , 192 304~

120

564

210Rn 325+546+564+712 (early' -'lofé )

539

I

~9~ 6 ~l _ I . 416'J 546 426

6616 r693

842, , t

r I ~883

712

1086 10351

1181

1245

901

325 818 644

-3000

203

_N C O U

325

203+644+818 (early'-'I ate')

5000

x8

(8 I B)

546 564

II~I r.

160 r185 304~ 192 294:~

433 355 ~ 5391~ Î3~ 468 3681 x390

~8 873 18897

712

I I81 1086l f1106 1035 I I

1245

II

601 ~ 616 ~(f44) 1672

500

1000

1500

channels Fig. 2. Plots obtained by subtracting the spectra observed to be late with respect to the gating transitions from thoseobserved to be early. For this figure and fig. 8 also, "early" corresponds to a time rangefrom 33 to 817 ns before time zero (i .e . the time for a true coincidence event) while "late" corresponds to the same range after time zero. Since there are no y-rays delayed with respect to the 203, 644 and 818 keV y-rays, there are no negative-going peaks in the lower spectrum and all positive-going peaks correspond to transitions in Z' °Rn above the 8 + isomer. Thesmall peaks at 644 and 818 keV in the lower spectrum occurbecause of slow rise time effects from some of the pulses in the 203 keV gate. The upper spectrum illustrates clearly the time relationship of all y-rays observed in a ' °Rn . Because the 3248+d keV level has a mean life of 106 ns, peaks wrresponding to each of the gating y-rays also appear (as expected) in the spectrum . We ascribe the small peak at 407 keV to incomplete subtraction of the very prominent ' 9sPt Coulomb excitation line.

observation of unconverted y-ray transitions down to about 25 keV) places a limit of d < 50 keV on the energy of this unobserved E2 transition . 3.1.2. Levels below the 17 - isomer at 3812+d keV. The y-rays de-exciting this isomer are the prominent peaks at 325, 54b, 564 and 712 keV (and the less prominent

A . R. Poletti et al. / Z`°Rn

344 103

r-r

start 325,546 stop 203, 644, Blö 10 2 C

10

,/ rm = 910ns

C

10 2 c i3 u

start 564 stop 325.546 ~-104 ns

10 A

yin start 818 stop 644

10

0

400

800 1200 time (ns)

1600

2000

Fig. 3. Time spectra obtained from the analysis of the y-y coincidence data for the "O+ " B Pt reactiort.

The gating y-rays are indicated .

peaks at 111 and 601 keV), in the lower spectrum of fig. 2. We have directly observed the 111 keV y-rays de-exciting the level at 2377+d keV . Because of the high conversion coefficient (aT = 10.1) for this M1 transition, although it comprises (23± 1)% of the decays of the level, the 111 keV y-ray comprises just (2.1 ±0.3)% of the y-rays which de-excite it. Although both the 325 and 546 keV n-y time spectra show prompt components, we attribute the prompt component in the 325 keV line to a contaminant

A . R . Poletti et al. / Z ` °Rn

345

TABLE 3

Observed ) .-ray branchings in ='°Rn E, (keV)

E~ (keV)

EÏ (keV)

Branching (°,)

1665

1462 1545 1665+d 2266+d 2377+d 1665+d 2563+d 3I10+d 4899+d 5025+d 4994+d 5486 + d {5384+d

203 120 712 ll I ISS 897 (842) 294 482 355 468 883 390 492

"

93 .4 f 0 .5 6.6 ± 0.5 97 .9± 0.3 2 .l f 0.3 81 .0± 4 .0 19 .0± 4.0 61 .0±10.0 39 .Ot10.0 II .Ot 8 .0 59.0± 5 .0 30.0 t 8 .0 45 .Of 7.0 31 .0 t 4 .0

5684+d 5571 +d

192 304

"

24 .0 f 8 .0

5867+d 6036+d 6895+d 6885+d

602 433 416 426

"

21 .0 f 4 .0 79 .Ot 4 .0 13 .Ot 3 .0

2377+d 2563+d 3404+d 5381+d 5876+d

6469+d

') Relative ordering of y-rays is not established.

transition ofthe same energy in s°9Rn [ref. ' a)]. Intensity considerations, taking into account total electron conversion, place the 325 keV y-ray above the 546 keV y-ray . The 325 keV transition then directly depopulates the isomeric level at 3248+d keV, and the 12+ state which it feeds is placed at 2923+d keV. This disagrees with the placement in two recent papers 4" t Z) which transpose the 325 and 546 keV transitions. Analysis of the y-y time spectrum, the middle plot shown in fig. 3, gives the mean life ofthe 14+ isomer at 3248+d keV as 104±5 ns. This result dil~ers significantly from the previous, less accurate, measurements 4 . tz) . The level at 3812+d keV which feeds the 14 + isomer is also long lived. Its mean life was measured as 1590±90 ns in reasonable agreement with the previous measurement by Poletti et ul. ") and we adopt a weighted average of 1540±70 ns. 3.1.3. Levels below the isomer ut 6469+d keV. The decay from this isomer is complex. The prominent, positive going peaks in the upper spectrum of fig. 2 correspond to transitions which follow this isomer, and precede that at 3248+d keV. The major transition path involves a 433, 160, 833 and 1181 keV cascade. This cascade is illustrated in fig. 4 which shows spectra in prompt coincidence with gates set on three of the above y-rays . Intensity, excitation function and coincidence measure-

346

A . R . Poletti et a/ . / 2'°Rn

500 channel

1000

1500

Fig. 4. Prompt coincidence spectra from the Compton-suppressed detector illustrating the decays of the levels including and immediately below the isomer at 6469+d keV . The prompt time gate had a width of 64 ns. The pcak at 245 keV in the lowest spectrum is due to the i 181-245 keV coincidence in Z '°Po following the q-decay of z' ° Rn. In all three spectra small contaminant peaks are shown with their energies bracketted .

ments all place the 1181 keV y-ray at the bottom of the sequence with the 883 keV transition feeding into the corresponding level. Analysis of n-y time spectra places the 433 keV y-ray at the top of the sequence :there was no significant prompt peak in the 433 keV time spectrum, whereas the 160 keV y-ray (see fig. 5) and the 883 keV y-ray showed such a feature. Analysis ofthe time spectra gave a mean life for the level at 4994+d keV of 16.5±1 .0 ns. This is illustrated by the n-y time spectrum of the 1181 keV y-ray obtained using the t 'O+ t98 Pt reaction, shown in fig. 6. The mean

A . R. Poletti et ai. / z' ° Rn

time(ns)

34 7

time(ns)

Fig. 5. Neutron-gamma time spectra observed for gating y-rays of 433 and 159 keV. The lowest left-hand spectrum demonstrates also the feeding of the 22* state by the higher lying isomer . Note the difFerent time ranges.

life of 1500± 100 ns for the isomer at 6469+d keV is apparent in the n-y time spectra of the 433 and 160 keV y-rays shown in fig. 5 (especially those taken with the longer time range) . As can be seen by reference to fig. 4 the 1181 keV gate established a further branch (via a 492-390 keV cascade) of the level at 5876+d keV while the .602 and 873 keV y-rays are also in cascade and establish a .second branch out of the 6469+d keV level. The 433 gate showed that there were several other decay paths (see the uppermost spectrum of fig. 4). The main one is via a sequence of y-rays (with energies of 192, 304, 355, 128, 1035 and 616 keV) ending at the 3248+d keV level . There are in addition a number of other side branches involving these y-rays . The complex situation encountered in this weak side chain is illustrated in fig. 7 which displays a y-ray spectrum in coincidence with a prompt composite gate set on

A . R. Poletti et al. / 2 ' ° Rn

348

21 103

neutron-y (170 . 5n) 1181

Rn

185

10 2

~m, 92 ns

'

,r16 .5ns

I~ 10

~'

......il

!y

100

Ilt

a.

200 300

ùi n

4Ô0

~i

r

100

200

842

300 400

1245

10 2 ~,49ns

r

10

0

100

~~~~~~~~V~ ~Î ~

200 300 400

time (ns)

~

i

Ô

~~t,~~i~~i ~~~ 100

200 300

400

time(ns)

Fig. 6. Neutron-gamma time spectra observed for selected gating y-rays. The spectrum illustrated for the 185 keV gate shows the lifetime of the 11 - state at 2563+d keV . The high background to the right of the peak for the 1181 keV case is caused by the two long-lived isomers above the 20 * state . The small prompt peak in the 842 keV spectrum is ascribed to the 842 keV transition which directly feeds the l l - state at 2563+d keV . The small peak to the left of the prompt peak in the 1245 keV spectrum is an artefact of the detector and timing electronics .

the 192, 304, 355, and 61 .6 keV y-rays. Our conclusions are summarised in the decay scheme (fig. 1) . In order to understand the many features observed in an examination of the coincidence information we have had to hypothesize (in addition to the firm placements as in fig. 1) decay branches via unestablished y-rays to both the 3404+ d and 2563+d keV levels . These are shown dashed in the level scheme . 3.1.4. Levels above the isomer at 6469+d keV. As illustrated in the lower part of fig. 8, y-rays of 322, 416, 426, 516, 693, 842 and 1245 keV lie above the isomer at 6469+d keV. The 842 keV y-ray together with a 41626 keV cascade de-excites an isomeric state at 7310+d keV. The mean life of this level was obtained from analysis of n-y time spectra (see fig. 6.) as 49 ± 3 ns. That the other four y-rays are in simple cascade, with the 1245 keV transition at the bottom, was established by the coincidence data . Analysis of n-y time spectra gave an upper limit of < 8 ns on the mean life

A . R . Poletti et al. / z'oRn

349

210Rn

1000 N C O U

500

channel

1000

1500

Fig . 7 . A composite prompt coincidence spectrum obtained in coincidence with gates set as indicated . Small peaks known to be contaminants are indicated either by brackets or by the symbol C. The 521 keV line is in' 96 Pt . All the labelled peaks have been placed in z ' ° Rn although we have not been able to demonstrate the exact connection to the 2563+d keV level which de-excites by the I85 keV y-ray (see text) .

of any of the corresponding levels as illustrated by the n-y time spectrum for the 1245 keV y-ray shown in fig. 6. .i'.1 .5. Decays into and out of the 11 - state at 2563+d kev. As can be seen by reference to table 3 we have established a further decay mode for this level ; in addition to the established decay to the 10 + state, there is also a (19±4) ~ branch to the 8+ state. The 897 keV y-ray which corresponds to this transition is clearly seen in the lower spectrum of fig. 2. In addition we established the existence of two further levels which decay to the 2563+d keV level . These appear to be populated (at least partly) by decays from the level at 5025+d keV. 3 .2 . SPIN AND PARITY ASSIGNMENTS

Spin and parity assignments have been made on the basis of angular distribution, lifetime, lifetime limit and conversion coefficient measurements . Information on total conversion coefficients was obtained from y-ray intensity considerations while partial conversion coefficients (generally aK) were deduced from direct observation of electron lines using the mini-orange spectrometer . Fig. 9 presents electron and y-ray spectra observed in these measurements. Table 4 summarizes the results obtained from angular distribution measurements using two of the reactions studied. For each reaction the observed intensity is given, normalized so that the intensity ofthe 644 keV transition is 1000. In addition each distribution was fitted to a sum of Legendre polynomials : W(6) = IY [1 +aZ PZ(cos B)+a 4P4(cos 9)].

A . R. Poietti et ai. / a'°Rn

350

210 Rn

300 322

416

Rn X-rays

516

x426

322+ 516+693+ 1245 (Prompt) 693

r(546) r(564) , I (664)I

842 1245

~Il~"~?"~~~I ~~~~T,~l~~~i~,~I~~R~ .~~,1~`~ 300 322

426 416~ Î

ôU

842 516

693 901

185

601

-300 203

1245

I

325

-600

I

~ 712 644 L 564

BIB

1181 + 433 (~early~-~laté )

546 500 channel

1000

1500

Fig. 8. See caption for fig. 2 for "early-late" definition. The upper (prompt) coincidence spectrum illustrates the relationship between the ;-rays characteristic of the decays of the highest lying levels observed . Of the peaks labelled by bracketted numbers, the 546 and 564 keV peaks occur because of some overlap in the 322 keV gate from the 325 keV peak. The peak at 664 could possibly be in Z' ° Rn.

The normalised coefficients are given as a percentage . It will be observed that the angular distribution coel%cients are strongly attenuated, especially for the t 'O+ t 98 Pt reaction . This is due to the fact that most of the observed ^~-rays follow at least onelong-lived K > 1 ~s) isomer . Furthermore thallium has a hexagonal structure so that the ions coming to rest at a lattice site will be subject to a quadrupole electric field and hence suf%r a relaxation of their alignment. On the other hand, although platinum has a cubical structure, the relaxation was observed to be even more severe in this case . For the transitions which were already well established (those below the 14+ isomer), reference to the paper by Yamazaki t4), or by der Mateosian and Sunyar t 5 ), showed that typical second-order attenuation coefficients a2 were of the order of 0.29 f 0 .05 [a Z = (aZ)exptl(u2),~x~ for the ' ° B+ 2° STl reaction . For the t'O+ t 98 Pt reaction, a2 ~ 0 .10±0.03 . For the transitions above the long-lived isomers the attenuation is significantly less . In spite of the attenuation the observed

A . R . Poletti et al. / Z ' °Rn

351

21oRn aaz 546

644

( 863

(a)

y -r°ys

901

564 601

xs

(s6

7I2 (797}~ (7901~ (7az

1161

i

i ~~ ~1

^(91~ (1013)

~I 1371 12as

na4~

°u

0, Ia 16 14 12 10

500

,195(L ~200(L 203(L 3 6

564(M) 667 712

325 433 ~376a)

r 546 ~5sa I

9

1000

x4

I' 644,

601

6

(b) electrons

~799 ral9

~~

ra42 r873 Ira83 901

1161

L

4 2 500

channel

1245

1000

Fig. 9 . Singles y-ray spectrum observed in a Compton-suppressed Ge(Li) detector, and the electron spectrum observed with the mini-orange spectrometer from the "O+' 98 Pt reaction . ~-rays in ~' °Rn and 2° 9 Rn are labelled while some of the known contaminants and activity lines are given in parentheses. The large electron peak at 376 keV is due to an M2 transition in Z°9 Rn .

angular distribution could still be usefully interpreted in terms of the (HI, .rn) fusion evaporation mechanism . Further aid in interpreting the observed angular distributions came from the results of the conversion coefficient measurements which we summarize in table 5. These were compared with the theoretical calculations of Rosel et al. 'e) in order to assign multipolarities and therefore parity changes. The multipole assignments in their turn rendered the interpretation of the observed angular distributions much less ambiguous. 3.2 .1 . Spins and parities of levels below the 17 - isomer ut 3812+d keY. We have already commented briefly on the branching of the 6+ level at 1665 keV. The results presented here provided the basis for the spin-parity assignments made in that work 5). The conversion coeffcients summarized in table 5 also provide an independent verification [cf. ref. 5)] that the 203, 644 and 818 keV transitions are all E2 in nature . That the spin and parity of the 2266+d keV level is 9+ is firmly based on the observed conversion and angular distribution coefficients . The combined information from the' °B+ Z°STl angular distribution analysis and the measured conversion coefficient gives the E2/M1 mixing ratio of the 601 keV transition as 8 = -0 .20±

A . R . Poletti et al . / z'°Rn

352

TABLE 4

Angular distributions and intensities of ~~-rays in z'°Rn

E ;(keV)

~oB+zosTl, 70

intensity

100 A z /A o

"O+'gept, 95 MeV

MeV 100 A,/A °

Ill 12o

25 s9

OS(09)

- Il(l7)

1q(06)

160

20

185 192

120 25

- 28(09) Ol(02)

-os(o9> - 09(14)

203

544 12

294 304 322 325 355 390 433 492 516 546 548 564 601 Go2 616 644 712 818 842 873 883 901 1086 1181 1245

44

n .o . 519 20 31 84 14

n .o . 553

109 374 258 31 1000 584 797 58 55 < 104

-08(07) 01(Ol)

02(Ol)

-54(13) -16(07)

OS(21) 09(10)

ll(03)

- 07(OS)

04(OS)

l4(08) 08(l6)

- 13(l0) ll(04) - 27(IS) 14(02) -4b(04) l0(02) - 23(02) -12(11) ll(02) 14(02) 12(02) 42(02) -26(09)

197

- 19(04) I1(02)

153

43(03)

n .o .

-03(03) 16(l3)

-09(07) - 10(22) 00(OS) -02(OS) -08(03) Ol(03) 23(13) - OS(03) -06(03) - 04(03) Ol(03)

-06(15) ll(06) -04{03) 04(OS)

intensity

100 Az/Ao

~19'~ a7

06(12)

100 A 4 /Ao

-ll(1z>

127 ^- 66')

- 11(OS)

02(OS)

103

-16(04)

546 " 10') 123

01(Ol)

07(04) 01(01)

- l2(03)

42

-18( l 1)

-02(03) -05( 11)

- 13(OS) OS(03) - 04(08) - 32(06)

-01(03) 01(OS) 08(06)

662') - 90') 96 350 59 61 760') ~ 30

OS(01)

606 223 87 32

- l4(03) - 38( 10)

1000 670

-13(13) 03(Ol)

831 225 133 80 218 38 373 85

OS(01) 04(Ol) 18(03) -31(04) - 13(11) 07(02) 00(09) 19(01) 36(OS)

06(OS)

Ol (Ol ) 04(03) 07(10) 20(16) 00(01) -02(01) - Ol(Ol) 02(03) 06(04) - 14(13)

-08(03) -23(09) -02(01) - 04(OS)

') Contaminated in singles, intensity estimated from ;-~~ coincidence spectra.

0.05. All information on the 712, 546 and 325 keV transitions is consistent with their being stretched and E2. The corresponding levels are therefore J" = 10+, 12+ and 14+ respectively . The total conversion coefficient of the 111 keV transition is consistent only with it being M1, while the limit on the conversion coefficient for the 185 keV transition is consistent only with it being E1 . The multipolarity ofthe 897 keV transition to the 8+ state will therefore be M2 or E3 for spin assignmentsof 10 - or 11 respectively . Although the 897 K-conversion line is not resolved from the 901 K-

A . R . Poietti et al. / z ' ° Rn

353

TABLE S

Summary of conversion coefficient measurements in z '°Rn EÏ (keV)

Type')

Exp .

III 120 160 185 192 `) 203 304 °) 325 390 433 516 546 `) 547') 564 601 °)~

T T T T T T T K K K K K K K

616 644

K

K

< 0 .007 (1 .4f0 .2)x 10 -z

El E2

712 818 842 873

(I .1t0 .1)x10" z (8 .4±0 .8) x 10 -3 (1 .7t0 .2) x 10" z (2 .4f0 .3)x 10 -z ~ 8 .0 x 10 -3 (2 .9±0 .4) x 10 -z

E2 E2 E3 M1

883

K K K K L K

897

K

< 0 .03

901 1181 1245

K K K

< 14 x 10" 3 (8 .4±0 .9) x 10 -3 (8 .9 t 1 .3) x 10 -3

') ") `) °) `) ~), ~)

K

14+4 4 .4+0.4 5 .2+2 .0 < 0.07 1 .7t0.6 O .St0 .2 0 .9±0.4 (6 .8±0.7) x 10 -z 0 .28±0 .04 (3 .6t0.7)x 10 - z O .I I t0.03 (1 .6±0.4) x 10 -z (8 .3 f 1 .6) x 10 -z (5 .1 f0.5)x 10 - z (6 .Sf0.7)x 10 -z

Multipolarity Ml E2 MI E2 M1 E2

MI E2

(Ml)

E2 (Ml) E2 MI E3 MI/E2

Theory x

b)

10 .1 3 .9 3 .6 0 .10 2 .2 0 .50 0 .60 6 .0 x 10 -z 0 .25 3 .2 x 10 -z 0 .12 2 .0 x 10 -z 10 .1 x 10" z 4 .6x 10 -z

~ M1 :7 .Sx 10" z E2 : 1 .6x 10" z 0 .006 1 .4x 10" z

Ml M2

1 .2x10" z 9 .2 x 10" 3 2 .0 x 10" z 3 .Ox 10" z 6 .8 x 10" 3 2 .9 x 10 -z 0 .064

_ E3 E2 E3 E3 Ml

{ 0 .017 7 .6 x 10 -3 9 .8 x 10 -3 8 .8 x 10 -3 11 .6x 10 -3

T : total ICC deduced from intensity balances ; K, L : direct measurements of ar , a~ . Ref. ' 6). Assuming 304 keV transition is M1 . Assuming 192 keV transition is M1 . Assuming 547 keV transition is MI . Assuming 546 keV transition is E2 . Consistent with 601 being mixed M I /E2, 602 could be E t or M 1 /E2 .

and 818 L-conversion lines, the limit on its conversion coefficient of a K < 0 .03 rules out M2 as a possible multipolarity . The level at 2563+d keV is therefore 11 - . The negative a Z coefficient for the 548 keV transition, together with its measured K-conversion coefficient, leads to a 12 - assignment for the 3110+d keV state. A higher state at 3404+d keV decays to both the lower lying odd-parity levels . The

35 4

A. R.

Potetti et at. / z'°Rn

conversion coefficient of the 564 keV transition determines its multipolarity as E3. Although strongly attenuated the angular distributions are consistent with this being a stretched transition . The level at 3812+d keV is therefore 17- . This conclusion (and J" = 14+ for the level at 3248+d keV) is in agreement with an earlier measurement 1 z) . 3.2.2. Spins . and parities of levels below the isomer ut 6469+d keV. For these levels a consistent set of spin parity assignments which involves decays through non-y~rust levels is suggested. The large uZ coefficient together with the measured conversion coefficient of the 1181 keV transition leads to a J" = 20 + assignment for the level at 4994+d keV. This level is populated via three decay paths from the isomer at 6469 + d keV. For one of the paths we measured the conversion coefficients (oc, t, or aK) for all three transitions. These transitions, at 883, 160 and 433 keV are respectively M1, M 1 and E2, thus the parity of the 6469 + d keV level is even. The spin of this level is not as rigorously established ; however the 873 keV transition is M1 and has an angular distribution which is consistent with it beinga stretched transition, while the 602 keV transition has an angular distribution which suggests a substantially mixed E2/M 1 transition . The spin of the state at 6469 + d keV is therefore no higher than 22. Taking into account the above consideration, the relative intensity of the 433 keV y-ray as a function of beam-target combination (and bombarding energy) and the branching of the 5381 +d, 5876+d and 6469+d keV levels, it seems most likely that J" = 22 + for this isomer at 6469 + d keV. The 433 and 160 keV transitions can then be understood as stretched E2 and Ml transitions respectively with the 5876+d keV level being 19+ . The angular distributions of the 192 and 304 keV y-rays imply that they are both largely dipole . In that case their relative intensities in coincidence with the 433 keV y-ray implies that both must be M1 . 3 .2.3. Spins and parities or levels above the 6469+d keV isomer. The 842 keV transition (in parallel with the 416-426 keV cascade) feeds directly into the 6469+d keV level . The K-conversion coefficient and angular distribution coefficient values are only consistent with the 842 keV transition being stretched electric octupole. The 1245 keV transition is also a stretched E3 . The levels at 7310+d and 8555+d keV are therefore ./" = 25 - and 28+ respectively . The 693 keV transition could be dipole or quadrupole while the 516 and 322 keV transitions are both dipole . The uppermost levels at 9248+d, 9764+d and 10086+d keV must therefore be of spin (29, 30), (30, 31) and (31, 32) respectively . 4. Discussion 4.1 . E3 TRANSTTIONS IN ~' ° Rn

The influence of octupole vibration-particle coupling on the strengths of E3 transitions between low-lying levels in the region of the z°BPb closed shell has been recognized for some time l'~ le). The octupole vibration-particle coupling hamil-

A . R. Poietti et ai. / Z ' ° Rn

35 5

tonian mixes states of the form ~j®3 -~ with states of the form U) where j, j' label single-particle states and 13 - ~ is the vibrational octupole state. For instance, strong mixing occurs between states involving the proton 2fi and li_,~ orbits, and the neutron 2gß and lj,~ orbits. Consequently, strong E3 transitions can be expected between these statés. Weaker mixing and a weaker E3 transition occurs between the configurations which involve a spin flip such as the proton 1 h~ and li ~ orbits and between the neutron li,~ and lj,~ orbits. The difference in strength occurs because the vibrationparticle coupling hamiltonian HPV has a radial dependence (R oî~Ylc'r) but does not depend on the spin operator. In particular ' 9) CJ

_ 3 ® 3 IHPVIhi ^' ~~ 0 z

~ t Zl

<11

Ro ~r Ij'),

consequently the mixing is sensitive mainly to the angular momentum coupling involved and Hamamoto ' 9) shows that the matrix element for the non-spin-flip case is on average four times greater than for the spin-flip case. In z ' ° Rn with its four valence protons it is obvious therefore that the 1hß , 2f7 and li,~ orbits will be important in any description of E3.transition strengths. For the neutrons it is not until an excitation across the shell gap occurs that neutron E3 transitions contribute. Consequently it is again the orbits above the shell closure (lj~, li,~ and 2gg) which are important when E3 transition strengths are being discussed. Recently also, the octupole vibration-particle coupling has been shown to have important consequences for the strengths of E3 transitions between high-spin states in several nuclei in this region (for instance, a° 9At : Bergstrom etul . 6), zioAt : Rahkonen et ul.'), Z "At : Bergstrom et ul. e) and Maier et ul. 9). In addition Poletti et ul . 2) and Dracoulis et ul. s) have pointed out the effect of octupole vibration coupling on transitions between high-spin states in 2 "Rn. Indeed in ref. s) the properties of the high-spin ~- yrast trap at 8856+d' keV are interpreted in terms of a strong E3 transition between the configurations

The strength of the 30+ -" 27 - E3 transition') in 2 ' ZRn can also be understood in terms of the related configurations . InCh~i~7zo+vCfg l js~]io~, 30+i,

I~Ch~i~lso " vCfg lg~l~ -, 27 ~. For the 30+ state the assignment suggested above, which was one of two possible configurations originally suggested by Horn et al. '), is in reasonable agreement with the measured g-factor whereas, for the 27 - state, the measured g-factor (g = + 0.63 ± 0.03) differs from the calculated one (g = + 0.81). There is however, some

35 6

A . R. Poletti et nl.

/

2'°Rn

TABLE 6

Strengths of isomeric transitions in Z' °Rn Transition (keV7 185 602 d 120 203 325 433 564 842 897 1181 1245

.1, -, !, 11 22 * 8* 6* 6* 14* 22* 17 25 ~ 11 20* 28 *

~ 10* ~ 21 * -+6* -+ 4* -+ 4 * - . 12* -+ 20* ~ 14* -. 22 * -. 8 * - . 17-. 25 -

Multipolarity E1 (M 1) E2 E2 E2 E2 E2 E3 E3 E3 E3 E3

Strength') (s.p .u .) 3 .6 x 10 - ' 1 .7 x 10' e ~- 0 .16b) 1 .5 1 .5 0 .026 3 .5 x 10 -° 21 .0 38 .0 2 .9 12 .1 > 17 .0

') Weisskopf single-particle units . ") Strength is rather insensitive to the value of d within quite wide limits .

uncertainty in this theoretical value since it neglects configwation mixing and core polarization both of which are likely to contribute significantly. In ref. 2 ), attention was also drawn to the systematic occurrence of enhanced E3 transitions in both the At and Rn isotopes . Ow present results on E3 transitions (summarized in table 6) extend these systematics to transitions between highly excited states in z t°Rn and enable us to make some tentative configwation assignments . In brief, for they are discussed in more detail below, the strong E3 transitions in Z' °Rn can be ascribed to the same neutron transition Ü~ -. g~) involved in the high-spin transitions in z' 1 Rn and i' ZRn, or the proton iT, -. f~ transition and the weaker i~ -" h~ transition . Furthermore, for three of these transitions the energy is sufficiently low that an yrast trap is formed : at 7310+d keV (J" = 25 - ), 4994+d keV (J" = 20+) and at 3812+d

4 .2 . SHELL-MODEL CALCULATIONS

Shell-model calculations were carried out for s t °Rn using the Glasgow shellmodel code Z° ) . These were similar to those carried out for Z "Rn, as described previously Z). The fow valence protons were restricted to the 1 h~, 2fß and li,~orbitals, and the neutron holes to the 3pß, 3pß and 2fß orbitals. Although basis size limitations precluded the inclusion of the i,,; neutron orbit, especially for the low-spin positiveparity levels, its effects were simulated by fitting the parameters of the interaction and the neutron effective charge to the schemes for the Pb isotopes, as in the previous calculation for Z t' Rn [ref. 2 )] . However, this technique is inadequate for those

A. R.

Poletti et al. / z' °Rn

35Z

levels dominated by 2-neutron negative-parity configurations (such as i ;,' ®j) and for high-spin positive-parity configurations (which involve iß,2 components). The results of the calculation are summarized in fig. 10 and the wave functions of the yrast (and near yrast) levels are presented in tables 7 and 8. Although an examination of the tables shows that the wave functions are generally rather complex, in most cases it is possible to identify levels with those obtained from a naive weakcoupling calculation in which proton excitations, as represented by levels in 2' ZRn, ~"

3s+n

I

5684+~

n

+ +

5812

5309

19 +

Î8+

5O58

18+

(16") 1~ 17 -

4020 3753

16+ ~(h92 f7/2II4 Uij-2)2 I - } 1r(h~'132)J

53B4+A

902ß

_

3404 "n 3248+0 3110+G 2923+A

(13-) 14+ 12 12+

2563+G 2377+A

II10 "

2266+D

9"

6

I

"~ 4+

1462

4+

644

12- ~g(h

) U(j-2) 2 923 i 13211

3138

3069

I I - 7r(hy~3 i13lL) 14+7r(hg/~f7/2)

2567

12+ 7T (hg~2 )

2211 2180

I 9+~~(~ 4)8 U(1-2) 2

L789

1545

7r(hg2 2 i13/22 )

~r(h~3f7/2)14v (j-2)4 2 + 2 19- 7f1 h92i13/2) 7r(hgy23 i13/2)17 U (1Z)2

~n. - _____ 4994 (19-) 4738

aooa + 4899+~

19+n 3~4+p 3812+G

19+~ 7r(hyr22i13/22)20U(I-2)2

5718

4+~(h9/2 )

L54 1507

4+

( mia~d)

906

2+

293

0" -R (hg~4)

v (1 -2 )

2+

0

0"

EXPERIMENT 210Rn

THEORY

Fig. 10 . Comparison between experimentally observed energy levels in s'°Rn and the shell-model predictions (see text for details).The bracketted spin assignments are consistent with the properties of the levels concerned but are tentative. The assigned configurations are the main ones involved . More complete wave functions are given in tables 7 and 8 . The shell-model energies have bcen shifted so that the calculated 20 + yrast state lies at 4994 keV .

35 8

A.

R . Poletti et al . / z`°Rn

TABLE 7 Wave functions') of even-parity states in z ` ° Rn

E(keV) ") 0 2, 2z 4, 4z 6 8 9, 9z 10, lO z 12, 12 z 13 14, 14z 16, 16 z 17 18, 18 z 19, 19 z 20, 20 z 21 22, 22z 23 24

293 906 1456 1507 1789 1543 1573 2180 2524 2211 2539 2567 3043 3202 3069 3322 4020 4349 4560 5058 5169 5309 5735 4994 5861 5718 6164 6407 6637 7214

Protons l4h z i z 13h z i z IOh z i z 9h z i z 9h z i z 9h z i z 8hz i z 6h z i z 84h4 88h° 91 h~ 81h ° 72h ; 93h ° 93h`

69h° 71h° 77h° 77h° 80h° 79h° 78h° 79h° 84h °

16fh 3 28ih' 96th 3 98fh 3 97th' 98fh~

all 100 ° ;, hziz

Neutrons 16fzhz 16fzhz 13fzhz 13fzhz lOfzhz 12fzhz 12fzhz Ilt~hz 7fzhz

26fz 13fz 53p,f 23fISp,f 26fz lOp; 23fz 34p,f 27flOp~ 26fz lOp ; 12fz 57p,f 15f44p,f 15fz 44p,f 21fz 24p,f 27fz 9p ; 16fz Slp,f 19fz 52p,f 24fz Ilp,f lOfz 61p,f 9fz 66p,f 26p3f 74f 8fz 90p,f 87fz 12p3 f 24fz 12p,f 23fz 16p,f Ilfz 60p,f 22fz 18p,f 16fz 44p,f 13fz SSp, f 65p,f 15p,p, 76p,f 16f 81p,f 16fz 96f

59p; 17p,p, 49p ; 59p ; l4p~f 58pî 59p ; l8p,p,, 14p,P 3 14p,p, 40p S8p; ISp,p } l5p,p~ SOp; 18p,p 3 l7p,p~

SOp ; 45p ; l9p,p~ 41p ; Ilp,p~ I8p, pa

12p,p,

12pi

18pz 18p;

Ilp

llp;

') The numbers are the probability (%) with which a given configuration is occupied. Only dominant probabilities are given . For protons, h = hv,z , i . i,a,z, f - f,, z . For neutrons, f . fs,z, p, - Pirz . Pa --_ Pa-z~ ") Energy normalized to place 20 ; state at 4994 keV .

are coupled to neutron hole excitations represented by the levels in Z° 6 Pb. These configurations are given in fig. 10. It is also of interest to note that the energies calculated merely by adding the appropriate excitation energies in the two nuclei generally correspond closely with the experimental excitation energies or those given by the shell-model calculation. In the following section we will use the information summarized in fig. - 10 and tables 7 and 8 to interpret the observed properties of the levels below the 22 + yrast trap at 6469+d keV. E2 and E3 transition strengths were also calculated using proton and neutron effective charges of 1 .5 and 1 .0 respectively, and a harmonic oscillator parameter

A . R. Poletti et al . / ~' ° Rn

35 9.

TABLE 8

Wave functions') of odd-parity states in 2 ' ° Rn !

E(keV) ~) 11 3 1

10, lO z 11 12 13 14 IS 16 17 18, I8 2 19, 19 2 20, 20 2 21 22, 222 23 24, 24 2 25

3209 3492 3l38 3517 3619 3749 3753 3815 3714 4259 4393 4738 4998 5251 5465 5774 6525 6803 7933 8414 8805 8980

Neutrons

Protons

100 100 100 100 100 100 100 100 100 100 100 100 100

fh 2 i

hi 3 22f2 22f2 24fZ 20f2 23f2

100 100 100 100 100

l00 100 100 100

19p,f 19p,f 13p,f 26p,f 16p,f

8p,p3 41p ; 9p~ ISp,p 3 45p

24fZ llp,f 24fZ llp,f 23f2 14p,f 21f20p,f 12f2 60p,f lOf2 66p,f 13f2 58p,f 11f2 87p,f lOf2 IOp 3 f 91 f2 9p 3 f 97f2 97p3 f IOf2 66p,f 15f2 82p,f 49f2 46p 3 f 95f2

SOp Slp 47p~ 39p l8p,p 3 16p,p3 18p,p3 14p,p 3

41Pi 48p; 34p;

64p,f

ISp,p 3

') The numbers are the probability (% with which a given configuration is occupied . Only dominant probabilities are given. For protons h = he ;2, i = i,3n, f = f 2 " For neutruns f =_ ß,2, p, __ p, ;2, P3 --_ P3n~ n) Energy normalized to place lowest 20 + state at 4994 keV .

b = 2.2 fm. Since the coupling with the 3 - octupole phonon was not included, the E3 transitions are necessarily underestimated . 4 .3 . THE STRUCCURE OF THE LOW-SPIN STATES (1 < 22)

It is useful when interpreting the wave functions of tables 7 and 8 to keep in mind the wave functions z') of the four states in s ° 6 Pb which contribute most to the neutron-hole excitations in z' °Rn . If they are expressed in a similar way to that used in tables 7 and 8 they are : ~0+, 0 keV ~ : ~68 % p~, 16 % f~, 13 % p~, . . .~, ~2 +, 803 keV) : ~52 % P, f~, 27 % p } p~, 8 % f~, . . .~, ~3 +, 1341 keV : ~ 100 % p~ f}~,

~4+, 1681 keV : ~32 % f~, 53 % f~ p~, 9 %

pf

f~ . . .~ .

360

A . R. Poletti et al.

/

Z'°Rn

It can then be seen for instance that the 2 + state at 644 keV can be associated with the 2+ neutron hole excitation of 2°6Pb . Above this the two 4+ levels arise from a complete mixing of the two forms of excitation as previously discussed 5) . The properties of the 6+ and 8 + states are consistent with their arising primarily from the proton h~ configuration. The strength of the 8 + -" 6 + E2 transition is 0.16 s.p .u. compared to the calculated value of 0 .4 s.p .u. The experimental strength is comparable to the strengths of similar transitions in Z °B Rn [0 .15 s.p .u., ref, az)] and z ts Rn [0 .12 s.p .u ., ref. ' )], a factor of 100 greater than that for the 8 + ~ 6+ transition in 2 'aRa [1 .3 x 10 s s.p .u., ref. z 3 )] and a factor of ten less than that for the 8+ -" 6+ transition in z '°Po [1 .07 s.p.u ., ref. Za)] . The decrease in strength in going from the is) . The further ~(hg) configuration tô the rz(h~) Rn configuration is understood decrease in strength in going to the rz(h~) configuration is not. In the weak coupling picture the most significant component of the 9+ state at 2266+d keV should be that arising from the ~(h~)e+v(p#'f~')s " configuration, since in zero order it should be at 2474 keV . The ><(h~)9+ state has not been observed in Z'ZRn but is calculated to lie higher in energy, at 2686 keV . The shell-model wave functions confirm this picture. The lowest 10 + excitation is expected to be mainly Z'~Rn(8 + )® zoe pb(2 + ) whose weak coupling energy is 2474 keV, i.e. 158 keV below the excitation energy of the Z'ZRn(10+) level at 2632 keV. The wave function for the lowest 10 + state listed in table 7 also confirms this conclusion . It is worth remarking at this stage that the proton neutron-hole interaction is generally only a6) except for configurations in which both the neutron hole and weakly repulsive proton are in high spin orbits - and then only if J = j +jp is either J, = j +jp or J~ ; - Vn -Jp l. As a consequence the weak coupling 10+ state is not expected to be far removed from the predicted (zero order) calculation of its energy . The two observed branches from the 11 - state are consistent with it being mostly n(h~i T,)I 1 - . As expected .for such an assignment, the E1 branch is extremely inhibüed while the E3 strength (2 .9 s.p .u . - see table 6) is characteristic of a n(h~ -. i,~) spinflip transition . For comparison the n(i,~ -" ht ) E3 transition in z °9 Bi has a strength of 3.6 s.p .u . [ref. Z')] . The 12+ and 14+ states are mostly ~(h~) and ~(h~ f~) respectively . The E2 transition connecting them is very weak, B(E2), a * ~ , Z * = 1 .95 e2 " fm a (0 .026 s.p.u .) . Its strength, which is comparable to that of the corresponding transition in s' i Rn, can be understood in terms of an inhibited spin-flip transition . Because both the 14+ and 12+ states are the maximum spin states of each configuration, the transition strength is readily calculated in terms ofthe single-particle ~f~-"nh~ E2 transition in 2°9Bi[B(E2) = 30±3 e2 " fma, or 0.4 s.p .u ., ref, zs)] . It is given by B(E2)ia " -" ,~ " = 0.097B(E2)( zo9Bi) = 2 .9 e2 ~ fm°, which is in accord with the measured B(E2) value of 1 .95 e2 ~ fma. The shell-model calculation correctly predicts the strength as 2 eZ ~ fm°. The strong E3 transition from the 17 - state at 3812+d keV is consistent with

A . R . Poletti et ai. / ~' ° Rn

36 1

the main configuration of the state being a(h~ i,~. The strength of this transition (21 s.p.u.) is comparable to several other non-spin flip E3 transitions in this region 2 ) and to the strength of the E3 transition (29 s.p.u.) from the octupole state in Z°B Pb. This, together with the g-factor measured for this state by Maier et u!. ' 2 ) confirms the configuration assignments (see fig. 10 and tables 7 and 8) to the 14 + and 17 states . The E3 transition from the 20+ state at 4994+d keV has a strength of 12 s.p.u. Examination of table 7 confirms that the main configuration of this state is indeed ~(h~ i;l,), hence the 20 + -" 17 - E3 transition would be expected to be comparable to the n(i,3iz-" ht) E3 transition in 2 °9 Hi which has a strength of 3 .7 s.p.u. Although stronger than expected this is in reasonable accord with the structure of these states as presented in tables 7 and 8. The decay scheme above this yrast 20+ state and below the 22+ yrast trap is complex (see fig. 1). There are several non-yrast transitions with several parallel decay paths and multiple branches from a number of states . An interpretation of this can be given in terms of the shell-model calculations together with the hypothesis that the level at 6469+d keV is the lowest lying (particle-hole) core excited state, as dîscussed in the next section. The shell model predicts several states of spin between 18 and 22 ofeither parity in this region . From fig. 1 it can be seen that the majority ofthe decays from these states are caught by the 20+ state at 4994+d keV. Furthermore the configurations (see tables 7 and 9) are complex, hence no one particular decay branch would be expected to dominate . At the yrast spin 20 state at 4994+d keV all the intensity is channelled into the main yrast decay sequence. 4 .4. THE YRAST TRAP AT 6469+d keV AND THE STRUCTURE OF CORE EXCITED STATES

The 433 keV transitionde-exciting the level at 6469+d keV is very inhibited, with an E2 strength of only 3.5 x 10 -a s.p.u. This inhibited decay could be partly explained by the shell-model calculations which predict 22+ states at 6164 and 6407 keV, the higher ofwhich is very close to the observed 22+ isomer . Further, the calculated B(E2) for the 22 2 -"202 transition is essentially zero, due to a cancellation effect in the wave functions. However, although the strength of the predicted 222 -+20i transition from the same state is also weak, about 3 x 10 s s.p.u., it corresponds to a partial lifetime of 0.06 ns, and hence would not result in the observed isomer . Instead, the inhibited decay, and a consideration of the positions of core excited states in 2 "Rn and 2 ' Z Rn, suggests that the 22+ state arises from a core excitation and that the states feeding it have a related structure. This suggestion would also explain the decay through non-yrast states below the isomer, and the fact that the expected yrast shell-model states, such as the 22 i state, are not strongly populated. The suggested core excited configurations are given in table 9. The configurations chosen are related to each other, in order to explain the strong E3 transitions connecting the states . The extra attraction required to bring these states low enough in energy is similar to that observed for other core excited isomers in this region 1 " Z ),

A . R . Poler[i et al. / z'°Rn

36 2

Tnat.e 9 Suggested configurations of higher spin states in z° 'Rn State

Energy (keV)

Suggested configuration

Required attraction (keV)

(31 . 32) (30, 31) (29,30) 28 + 25"

10086+d 9764+d 9248+d 8555+d 7310+d

22 }

6469+d

,>,<,~(hé/zii3a)zo " v~,(~üs/zlls/z)11 -, lz," \11 9/2 1 1312)20 " '~i l7ngen)lo ", 11 " n( 11 9/2 1Î3 ;z)zo~v(fs~%lls/z)e " .la " ><(hé~zlia ;z)zo " v(Pi/ills/z)an(hé~zli~~z)zo " v(Pi~?ge ;z)sor a(h9/zilj/z)li -v(Pi/ills/z)e " rz(h9/z 1 1a/z)1~ -v(Pl /âBe/z)s -

1388 300 1166 1289 1124 1352 783

and arises presumably through the MONA mechanism z9), or through core polarisation. The core excitation of~the 22+ level is chosen to be v(p~ tg~)5 - . This is possible because the ground state of z° 6 Pb isvery mixed. For instance, using the wave functions given in subsect. 4 .3, it can be seen that the p} level is occupied with a 32 % probability. Excitation of the p} particle across the shell gap and into the gt orbit is the least expensive of all such excitations. This has been demonstrated by the calculations of Matsuyanagi et ul . s°) and has been applied in descriptions of core excited states in Zt i Rn [ref. ' ) and a t t Rn [ref. z)]. In this simple picture the decay of the 22+ state to the upper 20+ state is therefore completely forbidden, and the decay must proceed through small components in each wave function . It is an excellent example of a level which is an yrast trap for nuclear structurE reasons. The E3 strengths of the 1245 and 842 keV transitions are respectively > 17 and 38 s.p.u. (See table 6 .) With the suggested configurations of table 9 this E3 cascade (28+->25-~22+) is readily understood . Significant mixing of the two components listed for the 25 - state allows the 28+--+25 - decay via the strong, non-spin-flip, E3 transition vj,~--.vg t and the weaker spin-flip ~ti~-~nh~ transition . The strong E3 decay of the 25 - state is explained in a similar way. Tentative configurations are also suggested for the three highest states observed (which unfortunately do not have firm spin assignments) in table 9 . Two of these configurations involve non-stretched combinations includingthe i} neutron hole . This is not unreasonable since the proton neutron-hole interaction is only strongly repulsive for stretched configurations . However, if deformation effects or core polarisation are the source of the extra attraction then the configurations shown would be unlikely to fall low in energy since alignedhigh-spin particles favour oblate deformation, whereas aligned high-spin holes favour prolate deformation so) . 5. Summary and conclusions

The yrast and near-yrast level scheme of Z t °Rn has been established up to spins of

A . R. Poletti et ai . l ai° Rn

363

about 30~. Most of the states up to spin 22 can be understood in terms of shell-model configurations . The 1 .5 ~s 22+ state, and several of the states which feed it are suggested to be core excited configurations . The long lifetime of this 22+ state is attributed to the forbidden decay to the lower lying states which do not involve core excitations. Eight isomers have been observed and of these, the transition from the 1 .5 ~s 22+ state is the only one which involves a major change in configuration. It occurs at an excitation energy where the core excited states intrude into the yrast sequence. A recurring feature of the decay of yrast states in this region of the periodic table is the observation of enhanced E3 transitions which are a valuable signature of the presence of particular configurations . Four strong E3 transitions have been observed in 2 ' ° Rn. We would like to thank the academic and technical staff of the ANU 14UD accelerator facility for their support in this work. In particular we are grateful to Mr. A. P. Byrne who gave able assistance with the data analysis towards the end of our experimental work and to Mr. A. Muggleton for target preparation. A. R. Poletti gratefully acknowledges the support of the University of Auckland Research Committee and the Nuelear Physics Department, ANU. References 1) D . Horn, O. Hâusser, T . Faeste_rmann, A . B . McDonald, T . K . Alexander, J . R. Beene and C . J. Herrlander, Phys . Rev . Lett . 39 (1977) 389 ; D. Horn, O. Hâusser, T . Faestermann, A . B . McDonald, T . K . Alexander, J . R. Beene and C. J . Herrlander, Proc . Int . Conf. nuclear structure, Tokyo, 1977, J . Phys. Soc. Japan 44 (1978) suppl . p. 605 2) A . R. Poletti, G . D . Dracoulis, C . Fahlander and I . Morrison, Nucl. Phys . A359 (1981) 180 3) G . D . Dracoulis, C . Fahlander and A . R . Poletti, Phys . Rev . C24 (1981) 2386 4) A . R . Poletti, T. P . Sjoreen, D . B . Fossan, U. Garg, A . Neskakis and E. K . Warburton, Phys . Rev . C20 (1979) 1768 5) A . R . Poletti, G . D. Dracoulis and C . Fahlander, Phys . Rev . Lett . 45 (1980) 1475 6) I . Bergstrôm, C . J. Herrlander, Th . Lindblad, V. Rahkonen, K .-G. Rensfelt and K . Westerberg, Z . Phys. A273 (1975) 291 7) V . Rahkonen, I . Bergstr5m, J . Blomgvist, O . Knuuttila, K .-G . Rensfelt, J . Sztarkier and K . Westerberg, Z . Phys . A284 (1978) 357 8) I . Bergstrôm, B . Fant, C . J . Herrlander, K . Wikstrbm and J . Blomgvist, Phys . Scripta 1 (1970) 243 9) K . H . Maier, J . R . Leigh,F . Pulhofer and R . M . Diamond, Phys . Lett . 35B (1971) 401 l0) G . D . Dracoulis, Nucl . Instr . 187 (1981) 413 11) M . Ishü, Nucl . Instr . 127 (1975) 53 l2) K . H . Maier, D. J . Decman, H . Grawe, H . Haas and W.-D . Zeitz, preprint H .M .L-P6/8 0 13) A . R . Poletti, G. D . Dracoulis, C . Fahlander and A. P. Byrne, unpublished 14) T . Yamazaki, Nucl . Data A13 (1967) 1 15) E . der Mateosian and A . W. Sunyar, Atomic Data and Nucl . Data Tables 13 (1974) 391, 407 16) F . RBsel, H . M . Fries, K. Alder and H . C. Pauli, Atomic Data and Nucl . Data Tables 21 (1978) 291 17) B . R . Mottelson, Proc. Int . Conf. nuclear structure, suppl . J . Phys . Soc. Japan 24 (1968) 87 18) A. Bohr and B . R . Mottelson, Nuclear structure, vol . II (Benjamin, NY, 1975) 19) I . Hamamoto, in Elementary modes of excitation in nuclei, ed . A . Bohr and R . A . Broglia (NorthHolland,Amsterdam, 1977) 20) It . R . Whitehead, A. Watt, B. J. Cole and I . Morrison, Advances in nuclear physiscs, vol . 9, ed . M . Baranger and E . Vogt (Plenum, NY, 1977) p . 123

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21) W . W . True, Phys. Rev . 168 (1968) 1388 22) W . J . Triggs, G . D . Dracoulis, C . Fahlander and A . R . Poletti, unpublished 23) D . Horn, O . Hâusser, B . Haas, T . K . Alexander, T . Faestermann, H . R . Andrews and D. Ward, Nucl. Phys. A317 (1979) 520 24) O . Häiusser, T . K . Alexander, J . R . Beene, E . D . Earle, A . B . McDonald, F . C . Khanna and J . S . Towner, Nucl. Phys . AZ73 (1976) 253 25) A . de Shalit and I . Talmi, Nuclear shell theory (Academic Press, NY, 1963) eq . (28 .40), p . 315 26) J. P. Schiffer and W . W . True, Rev . Mod . Phys. 48 (1976) l9l 27) P . Ring, R . Bauer and J . Speth, Nucl . Phys . A206 (1973) 97 28) O . Hâusser, F . C . Khanna and D. Ward, Nucl . Phys. A194 (1972) 113 29) A . Faessler, M . Ploszajczak and K . R . S . Devi, Phys . Rev. Lett . 36 (1976) 1028 30) K. Matsuyanagi, T . Dosing and K . Neerg~rd, Nucl . Phys . A307 (1978) 253