Alpha-decay widths of neutron-deficient francium and astatine isotopes

pxziy

Nuclear Physics A230 (1974) 380-392; Not to be reproduced

ALPHA-DECAY

by photoprint

@

North-Holland

Publishing

or microfilm without written permission

WIDTHS OF NEUTRON-DEFICIENT AND ASTATINE ISOTOPES

Co., Amsterdam from the publisher

FRANCIUM

P. HORNSH0J Institute

of Physics, University Aarhus, Denmark

of Aarhus

and P. G. HANSEN t and B. JONSON tt CERN,

Geneva, Switzerland

Received 10 June 1974 Abstract:

The use of mass-separated samples of neutron-deficient francium isotopes, produced at the ISOLDE on-line mass-separator facility at the CERN 600 MeV synchro-cyclotron, has permitted precise determinations through genetic relationships of cr/(EC+@+) branching ratios for a range of light francium and astatine isotopes. The experiments verify previous mass assignments and provide improved values for the half-lives. The systematics of a-widths and of /?strength functions is surveyed.

E

RADIOACTIVITY 204.205.206.207.208.209.213.220Fr 201.202At; measured ,E,, 1 measured E,, T+; 212Fr, 200.203.204.205,4f; Z,, deduced a-branching ratios, PI: 2’2*214Ra; measured E,, Z,, T+. Mass separated samples. 205~206~207~208~209~z10~z11Fr, 203.204,205.206.207,208.209.210.211Rn 199.*oo.201,202.203,204,205.206po

200.20~.202,203.204.205.206.20.%209.210~~, 198,199,200.201.202.203~204.205.20~.20~~~~

calculated

B-strength functions.

1. Introduction In a paper ‘) parallel with the present one, the systematics of reduced a-widths W, of ground-state to ground-state transitions of even nuclei has been discussed. It was pointed out that the largest W, are found just above the closed shells, and that there is a steady decrease towards the next closed shell. The element lead, which has a closed proton shell, seems to present an exception from this regular bchaviour. In the present work the a-widths of the odd-Z nucleides of francium and astatine are examined. It is well known ‘) that the fastest a-transitions in odd and odd-mass nuclei have reduced rates that are close to those found for the neighbouring even nuclei. These so-called favoured a-transitions are assumed to involve only the paired nuclear system so that the configuration of the unpaired nucleon remains the same for the initial and final states. This information is clearly a valuable clue in nuclear-structure studies. It is also possible in certain cases ‘) to understand the hindrance of the unfavoured transitions. + On leave from the Institute of Physics, University of Aarhus, Denmark. ++ On leave from the Department of Physics, Chalmers University of Technology, Sweden. 380

Gothenburg,

Fr, At a-WIDTHS

381

New developments in target-ion-source techniques 3, “) made neutron-deficient isotopes of francium (2 = 87) and its a-decay daughter astatine (2 = 85) available at the on-line isotope separator ISOLDE at the CERN synchro-cyclotron. Because of the very clean conditions obtained with on-line electromagnetic mass separation, it was possible not only to study the a-branching ratios but also to verify previous ’ - ‘) mass assignments and to make greatly improved measurements of half-lives and energy spectra. The new information is presented in sect. 3, while sect. 4 deals with the systematics of CI-and b-decay transition probabilities.

2. Experimental methods Short-lived isotopes of francium and radium were produced in the ISOLDE facility r “). A molten metal target assembly “) containing an alloy of La and Th (30 % by weight) at about 1400 “C was irradiated with an external 50 nA beam of 600 MeV protons from the CERN synchro-cyclotron. The activities produced in the spallation reactions with the two metals diffuse continuously from the target to the tantalum

1

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Fig. 1. (a) The singles a-spectrum recorded at A = 204. The left and right side correspond to counting intervals of 16 and 6 set, respectively. For both parts the collection time was 3 sec. The inset shows a decay curve for the 7.027 MeV *04Fr a-group. (b) The singles a-spectrum and decay curve for 205Fr. The right- and left-hand part of the figures were obtained with 12 set and 32 set counting intervals, respectively. In both cases the collection time was 4 sec.

382

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et al.

-,6.785Mc 206F, 206Rll

202At

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6 221 McV 1wO

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Fig. 2. (a) The singles a-spectrum recorded for mass number 206. The left-hand side of the figure was obtained with 180 set waiting time and 180 set counting time, the right part with 180 set counting time. Both cases were run with a collection time of 20 sec. Both the zo6Rn and 202At daughters were observed with the ‘06Fr activity. The inset shows a decay curve for the 6.785 MeV a-group of 206Fr. (b) The singles a-spectrum for *“Fr and ‘O’At and *O’Rn daughters measured with a collection time of 20 set and counting intervals of 180 set and 60 set for the left and right part, respectively. The decay curve in the inset was measured for the 6.761 MeV cc-group of 207Fr.

tip (1000 “C) of the surface ionization source 3*11) where only atoms of the alkali and (in a much smaller yield) earth-alkali metals are ionized and subsequently separated. A general description of this system and of its main performance characteristic was given in ref. “). The measuring equipment and techniques were essentially identical to those applied in earlier investigations 12, 13) of mercury and radon isotopes and their daughters. The mass-separated beam was stopped in an aluminized tape, and after a pre-set collection time the source was moved to a silicon surface barrier detector. A series of a-spectra were then recorded consecutively before the next source was brought in. Examples of the cl-spectra are given in figs. l-6. The cl-resolution was typically about 20 keV. The energy calibration was made with a mass-separated “*Th source and a 23gPu, 241Am, and 244Cm combination source. In several cases the spectra under study contained well-known a-lines which served as substandards for internal cali-

383

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r

a.

COUNTS/

20 s

6 646 MeV

b.

209F,

206Rn lco0

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150

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Fig. 3. (a) The singles a-spectrum recorded for A = 208. The decay curve for the 6.636 MeV a-group of *OsFr is shown in the inset. The spectrum was recorded with collection and measuring intervals both of 20 sec. (b) The singles a-spectrum obtained at mass number 209. The a-groups of *OgFr and its daughters *OsRn and 205At are observed.

bration. A slight loss in resolution due to the penetration of the 50 keV Fr ions and their daughters into the collector tape was noticed. The growth and decay of daughter lines in the spectra were used for the calculation of a/(~ + /?’ + EC) branching ratios as described in detail in ref. 13). A small correction for recoil losses was included. 3. Experimental

results

The following two subsections contain a brief description of the results at each mass number, while a selected part of the data, normally the cases where timing has been chosen to optimize the conditions for the Fr isotopes, is shown in figs. l-6. The measured branching ratios are given in tables 1 and 2, which also list the energies and half-lives. Our data, based on mass-separated samples, essentially confirm the assignment made previously ’ - 9), but because of the clean conditions in the present experiments the new half-life values are more precise, and only these values are given here.

384

P. HORNSHQJ

et al.

2ogFr

C

T,,s= 5o.oto.3s

A \lOOOO

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-\'\

i i

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100

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I

\

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I

Fig. 4. Decay curves for “‘Fr and *“‘Fr. The ‘09Fr decay was observed for the 6.646 MeV a-line whereas the z20Fr decay was recorded via the 7.803 MeV a-line of 216At. Owing to its short half-life (0.3 msec) this isotope is in equilibrium with 220Fr. The counting intervals were 20 set and 10 set for “‘Fr and “‘Fr, respectively. In both cases a 1 set printing period is present between the counting intervals.

3.1. FRANCIUM

ISOTOPES

The ‘04Fr isotope. Francium-204 cl-spectra are characterized by two strong agroups with energies 7.027 & 0.005 MeV and 6.967 + 0.005 MeV. The two lines exhibit half-lives of 2.lkO.2 set and 2.lkO.3 set, for the high- and low-energy group, respectively. The possible existence of two isomers, as suggested in ref. 6), is not supported by the present data which favour an interpretation in terms of fine structure in the cl-decay of the ‘04Fr ground state. In this case the relative intensities are determined to be 70 o/0and 30 o/ofor the high- and low-energy group, respectively. In the a-spectrum shown in fig. la, lines from the heavier Fr isotopes are noticed, in addition to the 6.415 MeV and 6.466 MeV lines from the “‘At daughter. These contaminations - most clearly seen with the isotope with the lowest yield - are due

385

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Fig. 5. (a) Singles a-spectrum recorded at A = 212. In addition to the decay of 212Fr which shows several fine-structure lines, the 6.902 MeV a-line characteristic of 212Ra was observed. The inset shows a decay curve for *r2Ra. The collection and measuring intervals were 20 set and 15 set, respectively. (b) The a-singles spectrum at A = 213. With 213Fr is observed the shortlived (25 msec) 213Rn daughter at Ea = 8.090 MeV. The inset shows a decay curve for 213Fr. The second component is due to 153 set 213Ra separated with a yield of about 5 % of the Fr yield.

to “wings” on the ion beams and to a “smear” of neutral background. It should be remembered, however, that the yield of, for example, “‘Fr is about three orders of magnitude higher than that of ‘04Fr, and that the yields of 21oS‘ilFr are still higher. In the inset is shown a decay curve for the 7.027 MeV a-line of ‘04Fr. The “‘Fr isotope. The half-life of ‘05Fr was found to be 3.7kO.l set, and the aenergy is 6.912+0.005 MeV. The a-spectrum shown in fig. lb also contains weak contaminations in addition to the strong 6.347+0.005 MeV characteristic of the “IAt daughter. The 6.262 MeV line from the decay of “‘Rn is not observed; this allows an upper limit to be set on the p-decay branch of ” ‘Fr of 3 %. The 206Fr isotope. In this case a strong a-group is observed at 6.785+0.005 MeV, and the half-life for this line is found to be 16.OkO.l sec. In the spectrum shown in fig. 2a one observes the 6.133 + 0.005 MeV and 6.227f0.005 MeV lines from the decay of ’ 02At as well as the 6.258 + 0.005 MeV line from ’ 06Rn. From the intensity of the latter, one can deduce the a-branch in the decay of 206Fr. On the basis of a

386

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Fig. 6. (a) The tr-spectrum recorded at mass number 220. It shows the fine structure in both the 220Fr and the *l6At decay. The collection time was 1 set and the recording time 25 sec. (b) This figure combines a-spectra at A = 213 and 214. In addition to the 213Fr and 213Rn x-lines, the 7.138 MeV K-line of 2L4Ra is observed. A decay curve for &l“Ra is shown in the inset.

value 13) of 0.351.0.02 for b,( 206Rn), the present experiment yields 0.85 kO.03 for the “‘Fr cc-branch. The 207Fr isotope. Spectra recorded at A = 207 are characterized by a strong agroup at 6.761 f0.005 MeV and a composite peak with energies 6085~0.005 MeV and 6.130 f 0.008 MeV (see fig. 2b). These two lines are assigned to 203At and ’ “Rn, respectively. The half-life of 2O’Fr is determined as 14.8 4 0.1 sec. From the knowledge of the line-shape one can determine the intensity of the “‘Rn a-group, and with a value r3) of 0.23 kO.02 for b,( zo7Rn) one obtains an a-branch in the decay of 2O‘Fr of 0.93 _+0.03. The “‘Fr isotope. The ct-spectrum obtained for mass number 208 is shown in fig. 3a. The tine at 6.636t0.005 MeV is assigned to ‘08Fr. The present experiments yield a value of 58.OkO.3 set for the ‘O*Fr half-life in agreement with ref. 6, but significantly longer than the 37.5 t 2.0 set reported in ref. ‘). With the “*Fr activity, both ‘04At and 2oSRn daughter activities are observed (see fig. 5), and from the value 13) of 0.52-&0.05 for the a-branch of “‘Rn, the a-branch of 20sFr is found to be 0.74kO.03.

387

Fr, At a-WIDTHS TABLE 1 Francium isotopes Isotope

204Fr zosFr zosFr 207Fr 2osFr 209Fr 212Fr

z13Fr 220Fr

“) b, ‘) “)

a/total

=+ (set)

(ME?)

2.1+ 0.2 2.lf 0.3 3.7* 0.1 16.0& 0.1 14.8& 0.1 58.0f 0.3 50.0& 0.3 1160 &30”)

34.6* 27.4*

0.3 0.3

7.027+0.005 6.967=bO.O05 6.912&0.005 6.785&0.005 6.761 ztO.005 6.636*0.005 6.646*0.005 6.406~0.007 6.384f0.007 6.338kO.005 6.262+0.005 6.17510.007 6.129*0.007 6.779*0.005 6.686kO.005 6.641 &to.005 6.582AO.005 6.531+0.005 6.484kO.007 6.400&0.007”)

(0.70 iO.15) (0.30 f0.06) 0.97 0.85 &IO.02 0.93 10.03 0.74 kO.03 0.89 ztO.03 0.23 &0.02 0.25 zhO.02 0.12 hO.01 0.37 50.02 0.018&0.004 0.012+0.004 0.991*0.001 0.61 +0.05 0.17 io.02 0.11 10.02 0.07 kO.01 0.02 fO.O1 0.02 kO.01

=t= (set)

a-widthd)

3.OO(O)f) 7.00(O) 3.80(O) 1.88(l) 1.59(l) 7.84(l) 5.62(l) 5.01(3) 4.61(3) 9.60(3) 3.12(3) 6.40(4) 9.60(4) 3.49(l) 4.49(l) 1.61(2) 2.49(2) 3.92(2)

0.30 0.21 0.59 0.34 0.47 0.28 0.34 0.03 0.04 0.03 0.20 0.02 0.03 0.14 0.17 0.07 0.08 0.08 0.04

1.37(3)

Ref. Is). Complex line. The number in parentheses denotes the power of ten. Calculated relative to the cc-width for 212Po, which in this scale has the value unity. TABLE 2 Astatine isotopes

Isotope

zooAt zolAt 2ozAt 203At ao4At 205At “) b, ‘) d,

a/total

=* (set) 42f2a) 42&2”) 88+5 183&4 179f5 444+1gb) 558&12b) 1572f30”)

6.466hO.008 6.415&0.008 6.347+0.005 6.227&0.005 6.133&0.005 6.08510.005 5.948+0.005 5.901*0.005

0.32 &to.06 0.21 +0.04 0.71 *0.07 0.09 AO.01 0.06 50.01 0.31 f0.03 0.042+0.003 0.10 AO.02

=t= (set) 1.31(2)=) 2.00(2) 1.25(2) 2.02(2) 3.03(2) 1.43(3) 1.34(4) 1.51(4)

a-widthd)

0.16 0.16 0.49 0.09 0.15 0.43 0.21 0.28

Ref. *). Ref. 7). The number in parentheses denotes the power of ten. Calculated relative to the a-width for “*PO, which in this scale has the value unity.

The 209Fr isotope. The spectrum at mass 209 (fig. 3b) shows a strong a-group at 6.646 kO.005 MeV with a half-life of 5O.OkO.3 set (fig. 4). The mass-separated samples have greatly facilitated accurate half-life and energy determinations for the

P. HORNSH0J

388

et al.

two very similar cases “*Fr and ’ “Fr. The a-branch in the decay of 209Fr was determined as 0.89+0.03; the analysis is based upon the value 14) of 0.17 for the M-branch in “‘Rn. The ‘12Fr isotope. For this isotope no attempts were made to remeasure the wellknown half-life “); only energies and relative intensities were determined, as advantages might be obtained from the very fresh mass-separated samples which were free from 212Rn, which has an m-line of similar energy. The intensities are listed in table 1 and a spectrum is shown in fig. 5a. The results are in good agreement with values given in ref. 15). The 213Fr isotope. The half-life of 2l3 Fr was found from the decay of the 6.779~~: 0.005 MeV a-line to be 34.6kO.3 sec. As the half-life of the 2’3Rn daughter is very short (25 msec), equilibrium is obtained in less than 1 set and thus the 8.090f0.008 MeV u-line intensity is a direct measure of the P-decay branch of 213Fr. Any loss of 213Rn activity is very unlikely as the recoil energy of the 2’3Rn atoms is very low compared to the 50 keV implantation energy of the ” 3Fr ions. From three different runs the value 0.009+0.001 was found for the /?-branch of 213Fr - a result about 50 % higher than previous determinations ‘* “). This effect might be due to an unnoticed escape of Rn activity in cases where it is not (as here) trapped by implantation. The a-spectrum and a decay curve for 213Fr is shown in fig. 5b. The 220Fr isotope. The half-life was determined to be 27.4kO.3 set (see fig. 4) and the relative intensities of the various a-groups are listed in table 1. The singles a-spectrum of 220Fr is shown in fig. 6a. 3.2. ASTATINE

ISOTOPES

MeV and 6.466kO.008 MeV are observed and assigned to 42 set “‘At. From their intensity, the “‘At a-branch is deduced to be 0.53f0.08, the relative intensities being 60 % and 40 y0 for the high- and low-energy components, respectively. Any trace of an a-branch from 4.3 set loomAt is masked by the 6.54 MeV complex line from weak contaminations of 21OS21‘Fr produced in high yields. It has not been possible to ascertain whether the 6.415 MeV line contains a contribution from 204Rn (T+ = 74 set) as the poor statistics wash out the time dependence of the intensity. Tentatively the a-branch is assumed to be 100 % for ‘04Fr. The 201At isotope. The line observed at 6.347+_0.005 MeV in the decay of “‘Fr decays with a half-life of 88+5 set, and these results are in agreement with earlier determinations 7, “). From the intensity of the 201At a-line, the a-branch of “‘At is found to be 0.71 kO.07. The 202At isotope. In the spectra recorded for 206Fr, two a-lines at 6.133+0.005 MeV and 6.227 f 0.005 MeV are assigned to 202At on the basis of the genetic relationship and previous data 7S*). Ambiguities exist whether the two groups represent fine structure in the a-decay of 202At ground state ‘) or two isomers “) of “‘At. In the present experiments, the two groups were found to have half-lives of 179+5 The “‘At

isotope. In the decay of 204Fr, two a-groups at 6.415+0.008

Fr, At a-WIDTHS

389

see and 183 Ifr4 set for the low- and high-energy groups, respectively. Our results thus favour an interpretation in terms of fine structure in the a-decay of ‘OzAt. If this is assumed, the total a-branch is found to be 0.15 + 0.03 and in agreement with the results of Latimer et al. ‘). The relative intensities are 54 % and 46 % for the high- and low-energy groups, respectively. The 203At isotope. The line at 6.085&0.005 MeV observed in the decay of “‘Fr is connected with the decay of 7.4 min 203At. The present experiments yield an abranch of 0.3 1 f 0.03 for ’ 03At, a value twice as large as the result 0.14 + 0.01 reported in ref. ‘). The 204At isotope. In the A = 208 a-spectrum the line at 5.948 +0.005 MeV is assigned to 204At and the line at 6.145f0.005 MeV to “*Rn. The assignments are confirmed by the growth and decay of the intensity of the lines. The a-branch of 204At is found to be 0.042 + 0.003 in good agreement with an earlier, indirect, result ‘). The ‘05A t isotope. In the decay of 2ogFr the strongest daughter a-line is the 5.901+ 0.005 MeV line from “‘At. From the intensity of the “‘At line one deduces an a-branch of O.lO+ 0.02, which is significantly weaker than an earlier result ‘) of 0.18 kO.02. Possibly this discrepancy could be due to a too low value of b,(’ 05Po) used in ref. ‘) for the calculation of b,( 205At); however, this point needs further experimental evidence. 3.3. RADIUM ISOTOPES During the present experiments it was noted that the surface ionization source was effective also for the element radium. In most cases the timing was chosen to study the decay of Fr isotopes and did not allow a simultaneous study of the Ra isotopes. For the lower mass numbers investigated, the half-lives of the Ra isotopes are very short, and the technique at hand did not allow a study of these isotopes. In two cases the half-life and a-energy of Ra isotopes have been determined: ‘12Ra has E, = 6.902+0.005 MeV and T, = 13.0f0.2 set, while 214Ra has E, = 7.138+ 0.005 MeV and T+ = 2.410.1 sec. In fig. 6b are displayed on the same graph the a-spectra recorded at A = 213 and A = 214. 4. Conclusion

The present experiments have confirmed the mass assignments ‘, “) for a range of a-groups from light francium isotopes, and have provided improved half-lives and energy spectra. We consider below the a-branching ratios, for which little experimental data were available previously. [Extrapolations based upon P-decay systematics “) turn out to have been reasonable estimates of p-branches in these cases.] From the results obtained in the present study, one may extend the systematics of partial half-lives for a-decay to include several more low-mass Fr and At isotopes. The results including fine structure are listed in column 5 of tables 1 and 2. Fig. 7 shows the reduced a-widths of the strongest transitions. The unit for the a-widths,

390

P. HORNSH0J

et al. RELATIVE

a-

WIDTH

d -50

EVEN

-A

ODD-A 1SOTOPES

ISOTOPES

-10

-05

-01

I 110 LOO01

120 k

130 L

NEUTRONS 140 I

NEUTRONS 110

1

120

,

130

1

Fig. 7. Relative cc-widths (as defined in ref. ‘)) of astatine and francium isotopes. The larger symbols in this graph denote points based wholly or in part on ISOLDE data. The graph is further based on results given in refs. 5-9*‘5).

here defined as the ratio of the ex~rimental transition probability to that given by a simple one-body model, was discussed in detail in ref. “). The strong decrease at N = 126 and smoother increase with decreasing neutron number observed 13) for both PO and Rn isotopes is again observed for the Fr and At isotopes. It may also by noted that the a-widths for the Fr isotopes is in general higher than the At u-widths in keeping with the general trend ‘) that the cc-widths increase with increasing distance from the Z = 82 closed shell. Further, the a-widths for even-A isotopes with Z = 87 and 85 are roughly a factor of 2 lower than the widths for the odd-A isotopes, which have close to the values found for even isotopes. The interpretation of this is probably that the odd-A nuclei have favoured a-decays to a single final state, while this strength in odd nuclei is spread between several levels: fine structure is actually observed in a number of cases. From the knowledge of the a-branches one can deduce the partial half-lives for /Idecay. This may be used to calculate - in a model ’ “) where the p-strength function is assumed constant above a cut-off energy - the average strength function. The trend of this parameter is presented in fig. 8 where data from ref. 16) are displayed together with results for the francium isotopes 205-212 and astatine isotopes 200-208. The present data support the occurrence of a pronounced minimum just below the

Fr, At a-WIDTHS

391

-

+

sa

Bi

ORI

A

At

l

Rn Fr

n

-

11

Fr (Z-87)

At l2=85)

-Po(2=8LI -1

Rn(Z-66)

+ Bi fZ=83)

NEUTRONS

\/ -l( 1-9

Fig.

8.

I

118

I

+

I

I

12L

I

1

I

Average B-strength functions calculated as outlined in ref. 16), The Q-values were taken from ref. I’) and the experimental data from this work and ref. Is).

N = 126 closed neutron shell, and also the increase with atomic number beyond Z = 82 is brought out more clearly through the inclusion of the present results. The apparent even-odd staggering in fig. 8 cannot be taken to be real because of the rather arbitrary nature of the pairing cut-off correction introduced in ref. 16). The authors are indebted to the technical staff of the ISOLDE facility for excellent running conditions and to H. L. Ravn, S. Sundell and L. Westgaard for decisive developments and improvements in the target techniques. We wish to acknowledge financial support from the Danish Nuclear Research Committee (Acceleratorudvalget) and from the Swedish Atomic Research Council.

392

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et al.

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