Alpha decay of neutron-deficient radon and polonium isotopes

Nuclear Physics Al63 (1971) 277-288;

@ North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

ALPIIA

DECAY OF NEUTRON-DEFICIENT AND POLO~

RADON

ISOTOPES

P. HORNSHBJ and K. WILSKY Institute of Physics, University of Aarhus, Denmark P. G. HANSEN + CERN, Geneva, Switzerland A. LINDAHL and 0. B. NIELSEN The Niefs Bohr Institute, University of Copenhagen, Denmark and The ISOLDE collaboration, CERN, Geneva, Switzerland Received 16 October 1970 Abstract: The use of mass-separated samples of neutron-de~cient radon isotopes, produced at the ISOLDE on-line mass-separator facility at the CERN 600 MeV synchro-cyclotron, has permitted precise half-life determinations and unambiguous mass assignments for a range of light radon isotopes. Alpha/EC-branching ratios are extracted for several radon and polonium isotopes through the use of genetic relationships. The systematics of the a-widths is discussed. E

~DIOA~IVITY 2oo-20aRn [from Th(p, spall)]; 196-202Po from [decay 200-zo6Rn]; measured T+ and Ea ; deduced a-branching ratios. Mass-separated samples.

1.Introduction The decay of alpha-radioactive neutron-de~cient isotopes of radon and polonium has previously been the subject of several investigations ‘-*), the lightest isotope of radon being identified having mass 201. By means of the ISOLDE on-line massseparator facility at the CERN 600 MeV syncho-cyclotron, it has been possible to investigate radon isotopes down to A = 200. The inclusion of the mass-separating stage in the study of short-lived activities produced in the spallation reaction has eliminated the interference from a wide range of other Rn activities (and their daughters) produced simultaneously, thereby establishing a reliable method for element and mass assignment. The present paper describes a study of the alpha decay, mainly energies and halflives. The experiments also provide new information on partial alpha half-lives in the low-mass range. In this connection, the existing information about relative alpha widths for the Rn and the PO isotopes is surveyed. A preliminary account ofthe present work has been given earlier ‘). 7 Visitor 1969-71. Permanent address: Institute of Physics, University of Aarhus. 277

278

P. HORNSHPIJ et al.

2. Experimental methods Short-lived isotopes of radon were produced in the ISOLDE facility lo). A target assembly containing about 300 g of Th(OH).+ was irradiated with a 0.05 PA 600 MeVbeam ofprotons from the CERN synchro-cyclotron. The Rn activity produced in the spallation reaction diffuses continuously from the target into the ion source of the isotope separator. After mass separation, a beam of the Rn isotope under study was selected by a slit system and directed to the collection position where it was stopped in a 1.27 cm aluminized mylar tape ‘I). Following a preset collection time, the sources collected are automatically moved to the counting station, where several detector arrangements can be used to study the radiations. Most of the measurements reported in the present paper were performed with loo-150 mm’, 100 pm thick silicon surface-barrier detectors. For multichannel recording of a-singles spectra, two modes are available: (i) the singles a-spectrum is accumulated from several sources, the collection and counting time being selected independently (ii) after collection, the measurement of (normally) eight consecutive a-ray spectra is initiated, while the separator beam is intercepted until the next collection cycle starts. After the collection period, a variable decay period can be introduced I’). For the branching-ratio measurements, mode (ii) was applied, whereas for the half-life measurements, a single-channel analyzer selected the a-line of interest, and the decay was followed with an automatic scaler-printer system. For the shorter half-lives, the use of method (ii) above eliminated time loss from the printing cycle. The shortest half-life was measured by a method involving a stationary source (see subsect. 3.1., *e’Rn). 3. Experimental results When the half-life of the radon and polonium isotopes studied is known, multispectrum analysis of the singles cc-spectra can be used for extracting the alphabranching ratios, b, = ~(~)~(~(~)+~(EC)). With t, denoting the collection time and t, and t, the beginning and the end of the counting interval (measured from the moment the collection is interrupted) the integrated a-counting rate for the parent activity (Rn) is given by: Nl(&,

y&t $1, h)

(l_e-""')(e-""'_e-""2),

= 1

while for the daughter activities (PO and At), the following expressions are valid:

ALPHA DECAY

(1

x

[

_,-A’&)

2

(e-A’2t1

_e-“““)_(l

219

_e-y

2

(e-Lltl

_e-A”2)].

2

The formulae are based on the observation of alpha lines with an absolute intensity i, in a branch of strength b,. As no definite evidence has been obtained for fine

structure, an intensity factor of unity has been used throughout. The x-branches from Rn, PO and At are denoted b,, , b,, and bi,, respectively. The formation rate at the collector for the first link in the chain is denoted R. Each series of eight spectra allows eight comparisons of line intensities from corresponding radon and polonium (astatine) isotopes. From these, the polonium a-branch is calculated, and in an analogous way, the strength of the radon a-branch is found, provided the a-branching is known for the astatine daughter. The use of eight spectra furnishes a good control that impurity activities do not interfere. In some cases, this method was supplemented by measurements where - after a short exposure - the total number of radon and polonium decays are recorded yielding a more direct measure of the polonium cc-branch. As the recoil energy of the daughter nuclei in a-decay is larger than the implantation energy, there is a loss of daughter activity during collection and counting intervals. In most cases, the correction for this effect was small. TABLE1 Radon isotopes Isotope

T&ec) 1.010.2 7.OhO.4 3.8hO.4 9.910.2 45 *2 28 &2 14 12 170 *4 340 310 555 +10 1461 f8

EJMeV) 6.909 ho.008 6.721 +O.OOS 6.770&0.008 6.63610.003 “) 6.497 kO.005 “) 6.547 10.003 “) 6.416&0.003 “) 6.262kO.006 6.258f0.003 “) 6.135+0.003 “) 6.14410.008

a/total 0.98 b) 0.80 “) 0.90 b) 0.93 b) 0.45 10.08 (1 .OO) 0.70~0.02 0.35f0.02 0.62f0.03 0.23 kO.02 0.52 *0.05

T+a (set)

1.0 xl00 8.8 x 100 4.2 x loo 1.06 x 10’ 1.00 x 102 2.8 x 10’ 1.06 x lo2 4.85 x lo* 5.48 x lo* 2.41 x 10” 2.81 x 103

u-width ‘) 1.13 0.62 0.84 1.03 0.37 0.84 0.71 0.65 0.57 0.42 0.31

“) Energy values taken from the work of Valli et al. 8). As all energies measured in the present work agree within 5 keV with those of ref. *), the latter have been used as reference energies. b, For these isotopes the p-strength function aystematics Is) have been used in the estimate of the partial p half-life. ‘) The a-widths are calculated relative to the a-width for 212Po which in this scale has a value of unity.

The experimental results are summarized in tables 1 and 2; details concerning the individual isotopes are reported below.

P. HORNSHBJ

280

et af.

TABLE 2 Polonium isotopes

Isotope

T&ec)

=ePO 19’Po 1971UP0 ‘a*Po ‘9QPo

5.5*0.5 “) 52 k.4”) 21 33 107 13 324 +20 255 115 675 &15 905 125 2700 190

199mpo =*Po 201Po =*apo “) “) “) d,

a/total

%(MeV) 6.521 &O.OO8 6.279&0.009 6.380&0.009 6.178f0.005 ‘) 5.950&0.008 b, 6.053f0.005 a) 5.860&0.003 ‘) 5.677&0.005 8) 5.57s*o.OO5 b)

0.95 ‘) 0.9 iO.1 0.85 ‘) 0.70 kO.08 0.12 +0.02 0.39 10.04 0.14 10.02 0.016~0.~3 0.022 10.003

T&sec)

a-widths “)

5.8 x10* 5.8 x10’ 3.2 x10’ 1.53 x 10’ 2.70 x lo3 6.54 x lo2 4.82 x lo3 5.66 x 104 1.23 x lo5

0.96 0.86 0.60 0.86 0.44 0.64 0.61 0.36 0.49

Half-lives measured by Siiviola ‘). Alpha energies measured by Treytl and Valli r2). @-strength function systematics r5) used for estimating the partial ,8 half-life. Calculated relative to the E-width for 212Po, which in this scale has the value unity.

R

COUNTS/ SEC

50

(1600

SAMPLES]

COUNTSlSs

T,,,=lO’-02s

103

.103

COUNTS13s ( 260

SAMPLES)

“‘R” 6.721 t&V \

10’ TIME

ISEC

1

196& 6.521 tvitv

2mR”

1 10

202

20LR” -----I

p 1-7 . * * .._.W.”

6909MtV

‘&_____,

1

Rn TIME

.

I&

1

i;/

.

*i I $I

CHANNELS

Fig. 1. The singles a-spectrum recorded at mass-number 200. The 6.909 MeV u-line showed a half-life of l.OhO.2 s (see insert); the daughter 5.5 s rg6Po is known from the work of Siivoia ‘).

CHANNEL: 350

Fig. 2. The a-singles spectrum and decay curves of the two /O’Rn isomers and their lg7Po daughters with half-lives of 26 s and 60 s. The collection time was 5 s and the counting time 10s. AIso identified is the 6.344 MeV a-line from 201At.

281

ALPHA DECAY 3.1. RADON

ISOTOPES

The “‘Rn isotope. “‘Rn decays by an a-g roup of energy 6.909+0.008 MeV, see fig. 1. The half-life was found to be l.OkO.2 s. As this was somewhat too short for detection with our tape-transport system, a special technique was adopted. The ion beam was stopped on a 450 pg/cm2 nickel foil, and after activation of the fast beam shutter, the a-spectrum was recorded as a function of time. The resulting half-life curve is shown in the insert in fig. 1. In the spectrum, the a-group from the decay of the daughter ‘96Po is also identified (E, = 6.521 kO.008 MeV). No evidence was found for a 6.845 MeV a-group tentatively assigned to this isotope “). 202Rn

203mRn

6 636 Me’4

COUNTS/

10s

6.5L7MeV 203Rn 6L97Mevj

1000

I

,103 199mpo

lg8P0 6.176 MeV

6 053MeV

203At 6.081 MeV

! .I

*I-

100

i4 .jri siI .

.

20

t

lo=“-

. 2qo I

10’

LO

60

80

/,o$O

100

’ TIME ’ ’ (SEC) ’ ’

, TIME

, 2:o

1:o (SEC)

..

_ . . 390 I

350

CHANNELS

I

Fig. 3. Singles a-spectra of *02Rn and 1g8Po measured with a collection time of 20 s and counting period 10 s. In the insert the decay curve for the 6.636 MeV line using the multispectrum mode is shown.

250 I.-_-.

.

300

350

Fig. 4. Singles a-spectrum recorded for massnumber 203. The five lines are assigned to the two pairsof Rn and PO isomers and 203At. Collection time and counting interval are both 45 s. In the insert, decay curves for the two Rn isomers are shown.

The 20’Rn isotope. Here two isomers are observed. The 3.8kO.4 s activity (“lmRn) is undoubtedly identical to the 3 + 1 s activity assigned to this isotope by Valli et al. “). The other isomer (denoted ‘OlRn) with a half-life of 7.OkO.4 s and a-ray energy 6.721 kO.008 MeV has not been reported previously. In fig. 2, the a-spectrum and decay curves for the two isomers are presented. The growth and decay of the two

282

P. HORNSH0J

et al.

mass-197 PO isomers indicate that the 6.380+0.008 MeV a-group stems from the decay of the isomer denoted ‘OlrnRn. An a-group of energy 6.344f0.008 M:V is assigned to the “IAt decay, but the strength is not known, and the “‘Rn alpha branch cannot be found. The “‘Rn isotope. The half-life of ‘02Rn (see fig. 3) has been determined as 9.85 + 0.20 s, which is slightly lower than the 13+2 s reported by Valli et al. “). A weak line in the a-spectrum at 6.226kO.015 MeV is assigned to the decay of “‘At on the basis of its half-life 12) of 2.7kO.4 min. Its low intensity does not allow a determination of the “‘Rn a-branch. The ‘03Rn isotope. The existence of two mass-203 isomers of Rn is confirmed and the half-lives Tt(203mRn) = 28 42 s and T+(‘03Rn) = 45_f3 s are in good agreement with previous observations “). Also the half-life of ‘03At(7.3+0.3 min) was found in good agreement with earlier values 13*14). The growth and decay of the PO daughters establish the 45 s Rn activity as the precursor of the 5.947f0.010 MeV PO activity ( lggPo) . As the eight subgroup a-spectra have different relative contents of the two isotopes ’ ggmPo and ‘03At in the complex peak (see fig. 4), the time-dependence of the contents being known, an analysis of these spectra can yield information about the a-branch of ‘03Rn. The a-branch was found to be 0.45+0.10, using a value of 0.14 for the ‘03At cc-branch 13). No indication of a 203mAt is observed; therefore 203mRn has tentatively been assigned a 100 % a-branch. The ’ 04Rn isotope. ’ 04Rn was found to have a half-life of 74 + 2 s in good agreement with previous results “). From the value 13) of 0.045 for the a-branch of ‘04At, the a-branch of ‘04Rn is found to be 0.69+0.03. The a-singles spectrum is shown in fig. 5. The 205Rn isotope. The 170-1-4 s half-life measured for “‘Rn (see fig. 6) is significantly higher than the value 1.8kO.5 min reported by Valli et al. “) who based their analysis upon data from a two-component mixture of “‘Rn and “‘Rn. As the a-ray energies of” ‘Rn and “‘Rn are very similar, the use of mass-separated samples greatly facilitated accurate half-life determinations. The a-branch in the decay of “‘Rn is found to be 0.35f0.03, based on a value 13) of the ‘05At cc-branch of 0.18. The “‘Rn isotope. The “‘Rn half-life was found to be 340-L-10 s, (see fig. 7) slightly lower than, but essentially confirming earlier results ‘, “). The a-branch in the decay of ‘06Rn was determined as 0.62f0.03, in agreement with the result of Stoner and Hyde ‘). Our analysis is based upon a value 13) of 0.0088 for the a-branch . ‘06At. Slight contaminations from “smear” are observed in the a-singles spectrum ;:g. 8). The ‘07Rn isotope. For this isotope, contradictory values are reported for the cl-branch by Stoner and Hyde ‘) (0.04) and Burcham ‘) (0.28). Assuming ‘) that the a-branch of ‘07At is 0.10, the present data yield 0.23 for the ‘07Rn a-branch. To reproduce the value 0.04, the a-branch of ‘07At has to be about IO-‘. The half-life of ‘07Rn was measured to be 9.27kO.25 min (see fig. 7), in reasonable agreement with

283

ALPHA DECAY

earlier values ‘*‘) of 11.O+ 1.O min, respectively lo+ 2 min. The contamination from the adjacent “*Rn with similar cl-ray energy was less than 0.2 %. The “*Rn isotope. The half-life was found to be 1461+ 8 s in agreement with less accurate earlier determinations “). The decay curve for “*Rn is shown in fig. 9. The initial contribution from 2.85 h “‘Rn was less than 0.05 %. Using a value “) of I

I

205R"

COUNTSl2OOs

6.260McV 200 PO 5.660 MeV

! \

I

-103 2O5At 5.696MeV

I

!

t

*.i

Lea 600 I200 1600

10 . .

lo3

TIME (SEC)

’

’ TIME

’ WC

’ I

.. _.

t

..

. .”

2po

250

300

CHANNELS

200 _._.I.._

Fig. 5. ‘04Rn a-singles spectrum recorded with collection time 600 s and counting period 600 s. In the insert is shown the decay curve for *04Rn. The scaler-printer system was used for recording the decay of the 6.416 MeV a-line selected by a single-channel analyzer.

CHANNELS

25i ..-

Fig. 6. Alpha spectrum obtained for A = 205. Both the zOIPo and ‘OsAt daughters are observed with the *OsRn activity. The collection time and counting period are both 200 s. The insert shows the decay of the 6.262 MeV a-line.

0.005 for the a-branch of “*At the cl-branch of 208Rn was determined as 0.52-10.06, significantly higher than previously reported values “) of about 0.2. 3.2. POLONIUM

ISOTOPES

The lg6Po isotope. This isotope was observed in the decay of 200Rn, and the

a-energy was found to be 6.521+0.008 MeV, in good agreement with the assignment by Siivola ‘) of an cc-group of energy 6.526kO.008 MeV to this isotope. The yields were too low to allow a precise half-life determination.

284

P. HORNSH0J

ef al.

The rg7Po isotope. In the decay of the two mass-201 isomers of Rn, two mass-197 isomers of polonium with half-lives 27 2 3 s and 60) 6 s are produced. The a-energies as well as the half-lives are in agreement with the values assigned to these isomers by SiivoIa ‘). The E-branch of lg7Po was found to be about 0.9 whereas the E-branch of ’ g7mP~ could not be determined due to the lack of knowledge of the 201At g-branch, necessary for the resolving of the 20’At-1g7mPo complex line.

6 2! iaM
.COUNT.5

1

?

I 160s

206R” h

Ea= 6.256 MeV

. 202P0 5 581 McV 4

2c6At 5.7OOt.w

I‘I

A

.

I

Fig. 7. Decay curves for 206Rn and zo7Rn. Both experiments were performed with the automatic scaler-printer system, the single-channel analyzer selecting the 6.258 MeV and the 6.135 MeV a-line, respectively.

Fig. 8. Singles a-spectrum for ao6Rn and its daughters *ozPo and 206At. Slight contam~atio~ from “smear” are observed in this spectrum.

The 198Po isotope. lg8Po is observed in the decay of 202Rn. Both the half-life (T, = 107F 3s) and energy (E, = 6,174f0.008 MeV) confirm earlier assignments

to this isotope fir‘). M easurements of the a-branch yielded b, = 0.70+0.08 jn agreement with the lower limit of 0.41 given in ref. “). The “‘PO isotope. The decay of the two mass-203 Rn isomers leads to two PO activities, For the 5.4kO.4 min activity with a-energy 5.950 MeV the a-branch was determined to be 0.121t:O.O2. This result is significantly different from the value 0.026~0.002 reported in ref. “). For the 4.25 mm activity with a-energy 6.055 MeV

ALPHA DECAY

285

(denoted 6t 12) 199mPo), the analysis of the composite peak due to ‘03At and ’ 99mP~ yields b, = 0.39+0.04, a value slightly higher than the value 0.28 kO.02 found by Le Beyec and Lefort ‘). The taoFo ~~u~~~e.‘eOPo observed in the decay of 2”4Rn (see fig. 5) was found to have a half-life of 11.2+0.2 min, in good agreement with previous determinations 4s 5j ‘). M easurements of the a-branching ratio yielded bar= 0.14~0.03, also in agreement with earlier results 4*8)S

Fig. 9. Decay curve for 2oaRn. The initial relative counting rate contribution (3 x lOed) from the 210Rn a-decay corresponds to a mass contamination of less than 1W’+

The zolPo isofope. 20fPo identified in the a-spectrum from the decay of “‘RII (see fig, 6) was found to have an a-branch of 0.016+0.003, in agreement with an earlier value 0.012+0.001 reported in ref. 6). The 201Po isotope. 2*zPo is mainly decaying by electron capture. Its E-branch is observed together with the 206At a-decay in the spectra from the decay of ‘O”Rn. A measurement of the a-branch gave the value 0.022t_O.O03, in agreement with the value of 0.020+0.002 reported earlier “).

4. Conclusion The results obtained in the present study permit an extension of the systemattics of partial half-lives for cc-decay to include the region of low-mass radon and polonium

286

P. HORNSHBJ

et al.

isotopes. In some cases, where the branching ratios are not measured directly, the partial p half-lives have been calculated from p-strength function systematics 15). The results for the even isotopes are shown in fig. 10, where the abscissa JA,/(A, + 4) AL Z$ y(x) contains the dependence on mass, charge, and decay energy ’ “). Over a half-life range of 10 16, the points for the radon and polonium isotopes essentially fall on the same curve; the straight line represents the Bethe I’) one-body

Fig. 10. Partial cc-half-lives of polonium and radon isotopes. The larger symbols indicate results based wholly or in part on ISOLDE data. The abscissa l’A,/(A,+4) AD&Z,+ y(x) is proportional to the logarithm of the barrier penetration parameter I’), x being the ratio of Q, to the height of the Coulomb barrier. The straight line is normalized to reproduce the known half-life of zlzPo with a radius parameter r,, = 1.55 fm.

predictions, using a radius parameter of 1.55 fm and a frequency factor adjusted to reproduce the ‘12Po a-half-life. For the lighter radon and polonium isotopes, a hindrance effect is observed. The hindrance effects are displayed more clearly in fig. 11, which shows the relative alpha widths for radon and polonium isotopes. Here, the alpha width is defined as the ratio of the experimental transition probability to that given by the one-body model used in fig. 10. Again, the normalization is relative to ‘12Po, which then in this scale has a width of unity, Data for the heavier isotopes “) of radon and polonium

287

ALPHA DECAY

have been included to show for both elements the dramatic decrease and subsequent increase in alpha width as the neutron number passes the magic number N = 126. This behaviour has been noted previously for the polonium isotopes; shell-model calculations by Mang 18) and Zeh I’) have accounted qualitatively for this effect. The influence of pair correlations on the widths has been studied by Kao et al. ‘O), who conclude that pair correlations can increase the absolute alpha widths by one RELATIVE Q-WIDTH

EVEN-A

*

ISOTOPES

va

NEUTRON

)

120

130

NUMBER 140

Fig. 11. Relative a-widths 16*17) of polonium and radon isotopes. A radius parameter r0 = 1.55 fm was used with an arbitrary constant chosen to give an u-width of unity for the zlzPo decay. The larger symbols in this graph denote points based wholly or in part on ISOLDE data. The arrows indicate where /?-strength function systematics 15) has been used to infer the partial p-decay half-life. The graph is further based on results given in refs. 6-8~12~2*~24).

order of magnitude without essentially changing the relative behaviour of the alpha widths. Together with the data on a-radioactive Pt and Hg isotopes 22), our data strongly support the idea of the existence of a similar shell effect involving the 82proton closed shell ‘B2”). The hospitality and generous support provided by the CERN Nuclear Chemistry Group, and especially by Dr. A. Kjelberg, is gratefully acknowledged. Our research has been supported financially by the Danish Nuclear Research Committee (Acceleratorudvalget).

288

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References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24)

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