- Email: [email protected]
The decay of 227Ra
THE DECAY OF ‘“‘&la
Received 4 March 1971
(Revised 10 May 1971) Abstract: The radioactive decay of 227Ra has been investigated with Ge(Li) and Si(Li) detectors Fifty-six new y-rays have been detected. The endpoints of four groups of &branches have been determined with a Si(Li) detector. The Q-value for the decay of 227Ra to 227Ac has been established as 1335&20 key. The results of yy coincidence experiments have been used in constructing a level scheme for 227A~. In this construction data known from the a-decay of’ 23’Pa have also been used. Some levels known from these studies have been reinterpreted and replaced. Seven new levels at 304.70, 428.4, 639.4, 698.4, 789-8, 863.5 and 874.7 keV only showing up in the @-decay of az7Ra have been added to the tevel sheme. The half-life of 2z7Ra was remeasured as 42.210.5 min. E
The &disintegration of (40 min) “%a has been studied before by Butler and Adams “). They reported a D-branch with an endpoint of about 1250 keV, three y-rays at 1300, 500 and 27.5 keV. Gamma rays have been reported by Stephens “)_ The level scheme of the daughter nucleus 227Ac is very weil studied by several authors from the a-decay of 231Pa. The early studies have been carried out mainly by means of magnetic ~~~~r~~~~e~s,both for electron- and aipha-radiation spectroscopy, and by NaI detectors in y-spectroscopy studies 2-4). In a more recent publication, Lange and Hagee “) describe y-spectroscopy studies with the aid ofGe(Li) and Si(Li) detectors. They also describe an q coincidence experiment. Chin-Fan Leang “> performed e~per~me~ts on the a-spectrum, especially on the very weak tra~sitio~s, as well as high~r~solutio~ y-ray and q coincidence spectroscopy studies. Very recently, de Pinho et al. ‘) published results from experiments with Ge(Li) detectors with very high energy resolution. From the last three mentioned experiments it is certain that in 227A~ at least two rotational bands, constructed on the Nilsson orbit& -j” 15301 and $-[532], can be identified. A third rotational band belonging to the -$* [65i ] orbital is quoted in refs. ‘* ‘1. These authors suppose a very strong Cariolis interaction for this band with possible 1)+[MO] or +* [642] bands. The present work is intended to correlate these data with those to be obtained from a study of the radiations following @-decay of 227Ra. 337
338
W. LOURENS et al.
2. Source preparation The 227Ra activity was obtained by the exposure of 226Ra to neutrons in the 2 MW Swimming pool reactor of the “Interuniversitair Reactor Instituut” (IRI) in Delft. The reported ‘) rather high cross section for neutron capture (20 b) permitted us to use samples that contained as little as about 30 pg of 226Ra nitrate. The irradiation time of the sample varied from 30 to 45 min in a neutron flux of IOX neutrons * cmd2 * s-l. With these conditions, sources of about 30 @i were obtained. The commercially available 226Ra contains stable Ba isotopes which must be removed since the 13’Ba isotope induced by (n, y) reactions cause a troublesome background during the measurements. The carrier-free 226Ra was produced in the following way: radium chloride was brought onto a Dowex-50 EDTA column. Ba and Ra are then both fixed. The fractions are then washed out by means of a solution of 0.01 N EDTA + NH,NO, (5.8 < pH < 6.0). The barium leaves the column first. After this the radium is removed by adding more NH4N03 to the eluting agent. The Ra fraction is brought onto a smaller Dowex-50 column in order to remove the EDTA from the solution by means of O.lN HNO, as eluting agent. The purified radium is then obtained by eluting with 8N HNO,. The solution is then evaporated to dryness in a quartz container. During the separation, the daughter products of 226Ra were also removed. But some of them grow into the sample in about 3 hafter the chemical separation to an extent that cannot be tolerated in the measurements. Fortunately the first daughter of 226Ra 9 222Rn, is a gaseous product which can be easily removed from an aqueous solution by bubbling nitrogen through it at a slow rate. In about three hours the shortlived daughters ” 4Pb (RaB) and 214Bi (RaC) will decay to a negligible amount, and only the primary cl-radiation and 186 keV y-radiation of 226Ra is left in the sample. During the chemical separation the original radium chloride is also converted into nitrate because this radical does not contribute to the background after irradiation. For the y-ray measurements, aqueous solutions of radium-nitrate were sealed into a polyethylene tube and irradiated in the reactor by the use of a transfer system. After the irradiation the samples were put into another polyethylene tube, as the activated argon in the air left in the irradiated tube caused a very troublesome background. A further reduction of the background was achieved by the use of a quartz container in which the radium is kept and in which purification by means of the “bubbling” method takes place. Normal glass contains too much sodium and manganese which dissolve into the liquid and contribute to the background after irradiation. Beta-sources were made by drying a drop of the purified radium solution deposited on a mylar tray. This sample was then exposed to the neutron flux for 40 min. The backing material caused only some very short-lived background. The radium solution cannot be used many times because the traces of Na, Mn and Cl become too strong, and another chemical separation has to be performed.
339
227Ra DECAY
In the measurements of the y-spectra and the yy coincidence spectra, two Ge(Li) detectors were used with efficiencies‘of 2.5 % and 4 % (compared with 7.5 cm x 7.5 cm NaI) and 2.8 keV resolution for the 1.33 MeV line of “*Co. The low-energy y-rays were recorded with a Ge(Li) X-ray detector (resolution 640 eV at 122 keV). A Si(Li) X-ray detector was used in a coincidence experiment. The relative detector efficiencies were calibrated by means of the known relative intensities of y-lines in the isotopes f82Ta; s7Co-f 241Am*, 75Se and 226Ra.
t
I
I
8
200
LW
600
600
CHANNEL
I
1000 NUMBER
Fig. 1. Low-energy spectrum taken with Ge(Li) X-ray detector.
The &spectrum was recorded with a cooled (77” K) Si(Li) detector with a depletion depth of 3 mm. The coincidence measurements were performed with a commercially available ~‘~ro~ross-uver detection” type of coincidence arrangement (2r = 100 ns). The singles and three coincidence spectra ~102~hannels each) were recorded sim~taneuosly by the use of a home-made routing system for our 4096channel Laben pulse-height analyzer. The coincidence gates were selected by three modular-type single-channel analyzers. The analysis of /?- and y-spectra was carried out by means of FORTRAN IV programs, running on our PDP-9 and the IBM 360/65 of the university. 4. Experimental results 4.1. GAMMA
SPECTRUM
The gamma spectrum of the low-energy region below 125 keV, taken with theX-ray detector is shown in Sg. 1. It is difficult to distin~ish any ~-transitions between the close-lying L X-rays.
W. LOURENS et al.
340
The origin of the bump on the front slope of the 27.3 keV transition is mainly caused by the detector response. This was made sure by measuring y-lines of 2J’Am and lo9Cd under the same circumstances. We therefore believe that the detection of the 24.6 keV transition is problematic. The line at 22 keV shown in fig. 1 is a result of background radiation as is the 58.3 keV line. In some spectra, traces of peaks at about 37 keV and 57 keV were observed. Probably, they do not belong to the decay of 227Ra. The shape of the front slope of the K,, line of Bi, caused by background of the 226Ra daughter shows some evidence for the existence of a 74.2 keV line. The peaks due to the decay of 227Ra are listed in
COUNTS
I 1600
8
/
mu0
2000
r
--
2200
2;oo CHANNEL
NUMBER
COUNTS
CHANNEL
NUMBER
Fig. 2a, b. Gamma spectrum below 400 keV taken with Ge(Li) X-ray detector.
227Ra DECAY
342
W. LOURENS et al.
TAB&El Gamma lines in z31Pa(er) (de pinho) and az7Ra(B-) (present work) (y-ray energies above 200 keV of de Pinho corrected for systematic diEerence) Transition
4630 4627 211-187 351-330 IlO85 270 300
IlO85-
74 46
1430 460 438-387 llO30 8430 330-273 8427 187-127 llO46 199-127 271- 199 740 187-110 127- 30 211-110 187- 85 199- 74 271-127 501-355 27?74 514-30s
Ev(231Fa) per 10% 14.1 &O.l 16.5 +0.1 18.9 23.6 24.5 fO.1 25.54f0.06 27.35kO.02 29.95 kO.02 31.00~0.05 31.54kO.05 35.82j20.03 38.20&0.02 39.57&0.04 39.97&0.02 42.48 IO.05 43.05 f0.05 44.16&0.05 46.3710.02 50.98 ho.05 52.74f0.02 54.61 f0.02 56.76f0.04
I, per lO=+p
bOt
30 84 27 74 46
387-121 2730 387-110 30527
ZY per IO38
W W
s 1 w 10 1000 9.9 1.0 0.7 1.7 16.0 0.15 1.3 0.6 0.7 6.5 22.4 0.15 9.1 :;
ww9
(95P)
(~~)*) 2180
!%3
24.9 +0.1
174 0.36
27.3710.02 29.95&O&4
5 1130
14 43 (4) (228) 13.6
< 0.2
46.38fO.M
(54.6)
199
+1
:: 514 0.7 0.2 0.4 2.7 7.3 9.5 3.4 B2 0.5 1.3 0.6
680 1.0 490 (2) (4)
1.0
242.2S;tO.lO 243.05&0.10 24X45&0.10 246.05f0.20 255.85&0.07 258.47&0.10
0.9 3.7 0.8 c 0.1 10.9 0.25
< 0.1 13.8 0.43
260.22;tO.OS 273.17&0.09 277.09&0.09
18.6 6.2 7.2
34 9.4 7.4
6.3
< 0.1
< 1.6
W
<3
(146.9) AO.5 198.9 &to.2 209.6 kO.2 218.19f0.10 219.9 kO.15 226.6 AO.1 228.00&0.10 232.20&O-l 2.42.10~0.2 243.15&0.10 245.9 &O.l 255.76&0.10 258.30&0.10 259.7 kO.2
0.3 1.1 2.1 2.1 0.3 4.2 3.0 0.3 5.4
“)
277.39&0.10
0.3 2.0 20 0.3 9.6
273.15f0.10 a}
< 14
< 1.5 3.6
74.2 kO.2
$)a) 133 39 % 22 2.2 3.6
870 79
< 2.7 1.9
0.24 < 0.2
57.1 k0.1 57.19;to*o3 60.5010.03 63.67&0.03 70,50j$.05 71.9 fO.1 72.5 SO.1 74.18&0.04 77.3610.03 96.88 10.03 100.92,tO.O4 102.6 124.6 fO.l 144.5 kO.1
I 1(11
w
30585 27346 501-273 537-305 273330273330305-
E, (227Ra)
29
5 < 32
7(if E2)
0.5 3.6 zG.5 0.3 11.7 8.4 9 0.3 3Y 14.6 43
343
227Ra DECAY TABLE 1 (continued) Transition 330
-
46
330 330
-
0 27
387 - 74 354 - 27 330 0 387 - 46 426 84 426 74 355 0 387 - 30 469 -110 438 - 74 426 469 387 435 438 426 426 428 43s 438 438 428 43s 438 -
46 84 0 46 46 30 27 30 27 30 27 0 0 0
514 501 563 514 537 501 537 563 563 54?C)576 -
47 30 84 27 46 0 27 46 27 0 30
639.4639.4698.4698.4789.8789.8874.7863.5874.7863.5874,7-
27 0 46 27 30 0 46 27 27 0 0
E, (23’Pa) per 104cf 283.6710.06 286.6610.10 3~.08~0.~ 302.67&O&6 310.15f0.10 313.~~0.08 327.20&0,10 330.0710.06 340.8 110.07 351.61~0.10 354.5910.08 357.16~!=0.07 359.47zto.10 363.96hO.10 375.13&0.10 379.33kO.08 384.94kO.10 387.25hO.10
169 1.0 244 252 0.15 10.6 3.2 I40 17.8 0.38 10.2 18.6 0.97 0.80 0.50 5.3 0.44 0.05
391.75*0.10 395.75*0.10
0.73 0.28
398.7QO.08 407.97kO.06
1.00 3.9
410.76fO.l
0.20
435.37kO.l 438.17&0.1 438.97&0.1
0.36 0.44 0.16
487.0 491.3 501.9 509.3 516.5 535.6
1, per 103#1
*er:Yo*p
f0.3 10.6 10.5 *1 10.6 kO.7
0.19 0.05 0.06 0.03 0.14 0.05
546.9 f0.7 572.4 ho.8
0.06 0.05
l) From intensity balance with a-feeding.
I t01
175
283.68f0.10
b,
34
36
371 264
300.09*0.10 302.68+0.10
b,
51 48
77 SO
17 3.3 217 19
327.2 &-0.2 330.08&0.10
3.0 30
3.1 46
341.1 -f;O.l
2.2
3.0
354.6 j110.2
7.5
9.0
379.4 rto.1
4.7
5.8
390.4 +0.06
0.78
398.4 kO.4
1.8
0.4 12.2 28 1.3 1.0 6.6 0.6 0.05
1.2 4.8
407.97 &to.10 b)
0.93 2.5
428.4 40.2 435.4 &to.1
468.5 10.5 471.3 *o.s 478.4 kO.4 486.98f0.10 490.5 *0.5 501.40*0.10 510.0 *to.2 516.2 10.2 535.6 f0.2 543.1 &-0.1 611.4 639.4 652.2 671.1 760.3 789.8 828.9 836.4 846.1 863.5 874.7
24
b,
b, b) b) b, b,
ho.2 b, kO.2 kO.2 10.2 kO.2 ho.3 kO.3 ho.3 kO.3 kO.3 kO.3
2.7 2.7 0.9 2s lb: 1: 4:;
13 0.5 2.4 ::: :*z 1:o < 0.6 A:$
b, Coincident with 27.4 keV transition. “) This level is not shown in the Ievel scheme, as there is only one line to support it.
344
W. LOURENS et al.
table 1. Between the X-rays and the strong 186 keV transition no lines could be observed except for a weak peak at about 165 keV due to background radiation of 139Ba. Above 186 keV several lines have been observed; they are also listed in table 1. The spectrum above 125 keV has been investigated with the X-ray detector, up to 400 keV, (see fig. 2a, b) and with the 4 % Ge(Li) detector (fig. 3a, b). The energy calibration of the latter detector was obtained in the following way. In the spectrum of fig. 3a, b are lines at 241, 352, and 611 keV. Their energies are close to the very well-known transitions in the decay chain of 226Ra, but yet they are due to the decay of 227Ra. After about l$h, the 226Ra lines become dominant, but some strong lines in the 22‘Ra are also present. The 226Ra lines were used as internal energy calibrations for the 227Ra lines. The other spectra were calibrated with the energies, obtained in this way. In table 1 we have summarized all reliable data obtained from analysis of the y-spectra. The highest energy peak due to the 227Ra decay has been found at 874.7 keV. Lines with higher energies in the spectra of the irradiated sources were attributed to the contaminants 41Ar, 56Mn and 24Na. 4.2. GAMMA-GAMMA
COINCIDENCE
MEASUREMENTS
A coincidence measurement was performed between the 27.3 keV y-line recorded with the Si(Li) X-ray detector and the spectrum taken with the largest Ge(Li) detector. The coincidence window of 1 keV width was placed on the 27.3 keV peak. The window did not include the bump on the front slope of this line, which was also present in the Si(Li) spectrum. The coincidences from the background, about 30 times lower than the peak, were monitored by a second window selected behind the 29.9 keV peak in the spectrum. During the measuring time nearly no coincidences were observed with this window. The chance coincidences collected during the measurement were negligible, which is shown in the spectrum of fig. 4, where the strong 330 keV line is nearly absent. Gamma rays that are in coincidence with the 27.3 keV line have been marked by “) in table 1. In Ge(Li)-Ge(Li) coincidence measurements between higher energy lines, no coincidence relations could be observed for transitions above 300 keV. However the 258 keV line was seen to be strongly coincident with the 232 keV line, and the 228 keV line was seen to be in coincidence with the 243 keV lines. Evidently the 243 keV and 272 keV lines are not coincident with each other. The coincidences have been collected from two sources which were alternately irradiated for half an hour and measured for an hour. 4.3. THE HALF-LIFE
OF 227Ra
Butler and Adams ‘) established a half-life for the decay of 227Ra, on the basis of the measurement of the decay rate of the P-rays. We measured the decay rates of the 330 keV and 284 keV y-rays for three hours. The measurement of the half-life was performed by using the pulse-height analyzer
aaTRa DECAY
345
in the multi-analyzer mode; 18 spectra of 128 channels each, containingthey-spectrum,
between 200 and 350 keV were recorded for 300 s. Between the successive measurements 300 s of waiting time were included. The 18 spectra were analyzed with the PDP-9 and corrected for dead-time and background radiation. By the least-square method a linear curve was fitted to the points in the diagram which displays the logarithm of the intensity against the time. The half-life was established as 42.2kO.5 minutes.
:OUNTS
200
LOO CHANNEL
600 NUMBER
Fig. 4. Coincidence spectrum, gating channel on 27.3 keV, the coincident transitions are indicated in table 1. 4.4. THE @-SPECTRUM OF zz7Ra
The ~-s~t~rn was recorded during 20 min with the Si(Li) electron detector. The measurements were started about 15 min after the irradiation so that the shortlived activities in the backing would be eliminated. During the measurements we also have recorded a y-spectrum in order to see whether any strong contamination has been taken place. No other lines than these belonging to the 227Ra spectrum could be observed. After 2 h we performed another measurement also of 20 min with the same source and subtracted that spectrum from the first one in order to reduce the contribution of the longer lived activities. (e.g. 24Na). The residue was analyzed with a special program on the PDP-9. The analyzing method has been applied earlier in our laboratory and is described elsewhere “). In fig. 5 the Fermi-Kurie plots of the highest branches are displayed. Beside these wo other groups of branches could be determined. The energies and intensities of the
W. LOURENS et al.
346
four groups are as follows: &(keV) 1300(+15) 1026(+20) 745( + 50) 550(_+80)
Is 100 45+ 5 20* 10 104 5
We believe that all four branches in fact are groups of branches which will be understood from our decay scheme (fig. 6). The highest energy branch will be a
28
24 ENERGY-l.55
+CH.NUMBER
t 22.3
20
1E
12
8
4
0 CHANNEL
NUMBER
-I, Fig. 5. Fermi-Kurie plots of the two highest energy p-branches.
mixture of transitions to the ground state and 30 keV state (both weak) and the 27.4 and 46.4 state (both strong). We therefore propose a Q-value for the /I-decay of 227Ra of 1335+20 keV. The second group of branches will mainly feed the two states at 330 and 354 keV and also the rather strong populated 304 keV level. The energies and intensities of the two last groups are only given as indication because of their heavy mixing and bad accuracy,
i 8 : iI ;g -
_ _ _ _ _ _
-
_
-
-
_
-
_
I_
_
-
_
-
” _ _ _ -
-
_
-
_
-
_
-
_
-
_
-
--
-.
-
_
_
-
_
-
_
-
_
-
” _ _ _ _ _
-
_ _ _ _
-
” _ _ _ _ _ _ _ _ _ _
-
E
2
348
W. LOURENS
5. Decay scheme of 22’Ra@-)227Ac;
et al.
comparison with 231Pa(a)22’Ac
The level scheme given in fig. 6 has been constructed with the present y-ray energy and intensity values and the present yy coincidence results. Furthermore we compared our results with 231Pa data on cr-y-ray and electron energies and intensities as given by Baranov ef al. “) and, mainly, de Pinho et al. ‘) and with cry coincidence results of Leang “). We accept the interpretation of the states in 221A~ below 273 keV as belonging to K” = $- and 4’ bands ‘, “). In the decay of 227Ra the +-, %-, $-, 3’ and 3’ levels in these bands occur, whereas a rather low limit can be set on the occurrence of the 3’ level. It may be mentioned here, that, in contradiction to theconclusion ofde Pinho et al. 7), we are of the opinion that the decay of the 46.38 keV level can be understood from the y-rays as given schematically in fig. 6 (de Pinho invokes decay to another intermediate level at about 30 keV). With intensities and transition assignments as given in table 1 for the 231Pa decay, total intensity feedings to the levels at 27.36,46.38 and 84.56 keV agree within 10 y0 and those to 29.95 and 74.14 keV within 20 ‘A with those intensities TABLE 2 Branching ratios in the p-decay of 227Ra Level
Calculated intensity per lo3 /?-
874.7 863.5 789.8 698.4 639.4 562.85 537.0 514.3 501.2 469.51 “) 438.13 “) 435.35 428.4 425.73 387.17 ‘) 354.58 330.04 304.70 273.2 84.56 74.14 46.38 29.95 27.36 0
1.3 2.6 2.9 4.0 14 23 10 35 26
“) Known from 231Pa(a).
lot3 ft
7.3 7.1 7.2 7.4 7.0 7.0 7.2 6.8 7.0 > 8.3 > 8.0 7.1 8.1 7.5 > 7.9 6.6 6.3 6.7 8f0.5 > 7.7 > 7.4 6.6 $8 6.5 2 7.5
227Ra DECAY
349
depopulating these levels (for the last levels this would also be within about 10 % if the intensity of the 44 keV line would be half as large). We also point to the fact that the present energy values require extensive reinterpretation of the energy assignments of the electron lines as reported by Baranov et al. ‘). Among others, they interpret L,, M, and Mz conversion lines of the 19.0 keV transition as 19.7-L,, 33.9-L, and 16.4-Mr lines and also their adoption of an energy value of 29.5 keV instead 29.95 keV causes some misassignments. Our measurements confirm the existence of the 273.10 keV level tentatively assigned by de Pinho. The intensity ratio of the 273 and 243 keV lines is exactly the same as in de Pinho’s “‘Pa measurement. Furthermore coincidences with the 228 keV y-radiations strongly support the existence of this level. The great strength of the upper component of the 245.4-245.9 keV y-ray doublet, for which de Pinho ‘) could only derive an upper limit not seriously contradicting the present results, strongly suggests that it belongs to the decay of the same level. The existence of a new level at 304.70 keV is proved by the coincidences between the 258 keV y-ray and the 209 and 232 keV ones. The 258 keV y-ray was seen by de Pinho ‘) but not assigned a place in the decay scheme. The present results on the 330.04 and 354.58 keV levels agree quite well with those obtained in 231Pa(a) [refs. 61‘)I. It should be noted, however, the 245.4 keV y-ray would fit quite well between the 330 and 84 keV levels. Only theoretical reasons (M2 strength expected) would make this assignment somewhat less probable. The 387.17 keV level, strongly excited in 231Pa(a), is conspicuously absent in 22‘Ra (B-J The strong line at 379 keV from the level at 425.73 keV proposed by de Pinho ‘) is found in 22‘Ra in coincidence with the 27 keV line, in agreement with the proposed assignment. Of the other lines depopulating this level in the 231Ra(a) decay, only the strongest could be seen in the 227Ra(p). The 398.1 keV line may be double. It is observed that the 398.1 keV line in 227Ra is stronger relative to the 379 keV one than in 231Pa(a). Part of it may come from a level at 428.4 keV not fed in the last decay. Of the lines decaying from the 438.13 keVleve1 of de Pinho, none is seen in 227Ra. The strong 407.97 keV line observed in both 22‘Ra@-) and 231Pa(a) cannot be assigned to this level. In 227Ra, it is observed in coincidence with the 27 keV line, so that it cannot feed the 29.9 keV one. Even in de Pinho’s data, the energy value 407.71 kO.06 keV is 0.22 keV lower than required for assignment to decay to the 29.9 keV level. Also, no other y-rays assigned to decay from the 438 keV level are seen in the decay of 227Ra. The further observations that the intensity ratio of the 435 and 408 keV transitions is nearly the same in both decays and that the 435 keV line is not in coincidence with the 27 keV transition make it quite certain that these transitions depopulate this 435.35 keV level. The y-rays originating in the 469.51 keV level of de Pinho ‘) are not observed in 227Ra.
350
W. LOURENS et al.
The level at 501 keV proposed by de Pinho ‘) and Leang ‘> is offered additional evidence by the coincidence measurements on the 228 keV transition, which transition is not yet seen in the 231Pa(a) decay. Instead of the levels at 509.2 and 516.5 keV proposed by de Pinho ‘) we suggest levels at 514.3 and 537.0 keV. Contrary to de Pinho ‘) we want to assign the 435 keV transition as decaying from the 435.37 keV level as mentioned above, which abolishes the 509 keV level, that was only supported by the 435.37 keV transition. The 487 and 516 keV lines are both coincident with the 27.4 keV y-ray. This leads us to introduce levels at 514.3 and 537.0 keV as the 516 keV level of de Pinho was supported only by these two transitions we are compelled to remove this level. The higher levels in the decay scheme are entirely based on sum rules. Using the present y-ray intensities combined with conversion coefficients as measured in 231Pa (a) [refs. 3*4r 9)], with th eoretical conversion coefficients in several other cases, and with a few intensity ratios for transitions between low-lying levels in 227Ra as observed again in 231Pa(a), we calculated /?-feedings of the low-lying levels.The results are presented in table 2. As for spin-parity assignments, those to the two lowest bands and to the levels at 330 and 354 keV are considered to be certain from a-hindrance factors and from conversion-electron data as observed in 231Pa(a). The parities of the levels at 273, 387 and 425 keV are considered to be well documented by conversion-electron data. The other spin-parity assignments given in fig. 6 are based entirely on consideration of relative y-ray transition probabilities. 6. Nitsson model assignments to 227Ra and to levels in “‘AC (fig. 7) Consideration of levels in isotonic or nearly isotonic nuclei leads to the prediction that the ground state of 227Ra should belong to one of the following Nilsson levels: j* [631], 3’ [633], and 4- [752]. On the other hand, it appears that the $- [530] level at 354.58 keV is definitely fed by a direct p-transition which could be allowed hindered or more probably, first forbidden unhindered (log ff = 6.6). This agrees only with the first one of the three assignments mentioned. The somewhat remarkable facts that the negative-parity ground state band is fed so much less (logft 2 7.5) than the +- [530] band but that the low-lying 3’ [651] band is fed relatively strongly (log ft s 6.5) find a natural explanation in the 4’ [6311 Nilsson assignment for the 227Ra ground state. The transitions are then first-forbidden hindered and allowed hindered, respectively. A possible difficulty for the above Nilsson model assignment for the 227Ra ground state is the apparent B-decay to the level at 425.73 keV, which, according to its y-ray decay, most probably has spin-parity 3’. Yet, feeding of this level through weak 88.47 and 111.17 keV y-rays originating in the levels at 514.3 and 537.0 keV cannot be excluded. It would make the 425.73 keV level have rather similar properties to the level at 304.70 keV. When we take into
**‘Ra DECAY
351
account that in other nucleides with 89 or 91 protons and neutrons the Nilsson levels, 3’ [660], 3’ [651] and 3’ [642] occur involved in a Coriolis coupling and, for neutrons, in a AN = 2 coupling with 3’ [400] and 2’ [402], it is attractive to interpret the levels at 304.70,425.73 and 469.51 keV as the spin $, 3 and 3 levels of the 3’ [642] band. The relatively strong B-feeding of the lowest of these levels [logft = 6.71 is in accord with the allowed-hindered character following from this interpretation. 5+
436.1 e $[530]
.I-
i-
't'
469.6 307.2 364.6 330.0
’ B
271.3
112
196.6
j-
126.6
7T
14.1
f
29.9
1-
0
$- [632]<
425.1 f
I $
5+
++@I’ $
++[660]
I
537.0 511.3
F
1+
T
435.b
210.9 161.4 304.1
110.0
T
84.6
,g
46.4 27.3
,2
Fig. 7. Nilsson levels in **‘AC.
The evident candidate for the ground state of the 3’ [660] band is the 435.35 keV level, excited with medium intensity (logft = 7.1, F. = 50) in both 227Ra(b-) and 231Pa( a). Interpreting the levels at 514.3 and 537.0 keV as spin 3’ and 3’ levels of this band would explain their feeding and their decay to the 3’ [642] band and to the +’ [651] one. This proposition implies a rather small value for the decoupling parameter (u x 0.7). However also in 233Pa [ref. lo)], the same three bands mentioned before are mixed and the decoupling parameter for the 3’ [660] band is also low (Q = 0.67). The level at 273.10 keV probably has spin-parity 3- and is fed by a weak /I-transition (log ft = 8 f0.5); its first rotational state should nearly coincide with the 304.70 keV level and is therefore difficult to observe. No easy Nilsson level assignment presents itself for this level: it may be a combination of the 3’ [651] level with octupole (1 -) core excitation which in near-by doubly even nucleides is known to occur at nearly this energy. We do not wish to suggest assignments for the levels at 501.2 and 562.85 keV and for the higher ones. One curious difficulty remains: for some unknown reasons, no gamma rays are observed decaying from the 438.131 keV level which is thought to be 3 spin level of the +- [530 ] band. The authors are indebted to Mr. K. Smit for his careful preparation radium solution.
of the pure
352
W. LGURENS
et al.
References 1) J. P. Butler and J. S. Adams, Phys. Rev. 91 (1953) 1219 2) F. S. Stephens, private communication reported by E. K. Hyde er al. in: Nuclear properties of the heavy elements (Prentice Hall, New Jersey, 1964) p. 689 3) A. Baranov, V. M. Kulakow, P. S. Samoilov, A. G. Zelenkov, Y. R. Rodionov and S. V. Pirozkokov, ZhETF (USSR) 41 (1961) 1475 (translation, JETP (Sov. Phys.) 14 (1962) 1053) 4) A. G. de Pinho, Thesis, Paris (1963); H. Abou-Leila, R. Foucher, A. G. de Pinho, N. Perring and M. Valadares, J. Phys. Rad. 24 (1963) 857 5) R. C. Lange and G. R. Hagee, Nucl. Phys. Al24 (1968) 412 6) Chin-fan Leang, thesis (Paris, 1969) 7) A. G. de Pinho, E. F. da Silveira and N. L. da Costa, Phys. Rev. C2 (1970) 572 8) W. Lourens, B. 0. ten Brink and A. H. Wapstra, Nucl. Phys. Al52 (1970) 463 9) G. R. Hagee, R. C. Lange, A.C.Barnett, A. R. Campbell, C. R. Cothern, D. D. Griffing and H. J. Nennecke, Nucl. Phys. All5 (1968) 157 10) W. Hoekstra and A. H. Wapstra, Phys. Rev. Lett. 22 (1969) 859
Received 4 March 1971
(Revised 10 May 1971) Abstract: The radioactive decay of 227Ra has been investigated with Ge(Li) and Si(Li) detectors Fifty-six new y-rays have been detected. The endpoints of four groups of &branches have been determined with a Si(Li) detector. The Q-value for the decay of 227Ra to 227Ac has been established as 1335&20 key. The results of yy coincidence experiments have been used in constructing a level scheme for 227A~. In this construction data known from the a-decay of’ 23’Pa have also been used. Some levels known from these studies have been reinterpreted and replaced. Seven new levels at 304.70, 428.4, 639.4, 698.4, 789-8, 863.5 and 874.7 keV only showing up in the @-decay of az7Ra have been added to the tevel sheme. The half-life of 2z7Ra was remeasured as 42.210.5 min. E
The &disintegration of (40 min) “%a has been studied before by Butler and Adams “). They reported a D-branch with an endpoint of about 1250 keV, three y-rays at 1300, 500 and 27.5 keV. Gamma rays have been reported by Stephens “)_ The level scheme of the daughter nucleus 227Ac is very weil studied by several authors from the a-decay of 231Pa. The early studies have been carried out mainly by means of magnetic ~~~~r~~~~e~s,both for electron- and aipha-radiation spectroscopy, and by NaI detectors in y-spectroscopy studies 2-4). In a more recent publication, Lange and Hagee “) describe y-spectroscopy studies with the aid ofGe(Li) and Si(Li) detectors. They also describe an q coincidence experiment. Chin-Fan Leang “> performed e~per~me~ts on the a-spectrum, especially on the very weak tra~sitio~s, as well as high~r~solutio~ y-ray and q coincidence spectroscopy studies. Very recently, de Pinho et al. ‘) published results from experiments with Ge(Li) detectors with very high energy resolution. From the last three mentioned experiments it is certain that in 227A~ at least two rotational bands, constructed on the Nilsson orbit& -j” 15301 and $-[532], can be identified. A third rotational band belonging to the -$* [65i ] orbital is quoted in refs. ‘* ‘1. These authors suppose a very strong Cariolis interaction for this band with possible 1)+[MO] or +* [642] bands. The present work is intended to correlate these data with those to be obtained from a study of the radiations following @-decay of 227Ra. 337
338
W. LOURENS et al.
2. Source preparation The 227Ra activity was obtained by the exposure of 226Ra to neutrons in the 2 MW Swimming pool reactor of the “Interuniversitair Reactor Instituut” (IRI) in Delft. The reported ‘) rather high cross section for neutron capture (20 b) permitted us to use samples that contained as little as about 30 pg of 226Ra nitrate. The irradiation time of the sample varied from 30 to 45 min in a neutron flux of IOX neutrons * cmd2 * s-l. With these conditions, sources of about 30 @i were obtained. The commercially available 226Ra contains stable Ba isotopes which must be removed since the 13’Ba isotope induced by (n, y) reactions cause a troublesome background during the measurements. The carrier-free 226Ra was produced in the following way: radium chloride was brought onto a Dowex-50 EDTA column. Ba and Ra are then both fixed. The fractions are then washed out by means of a solution of 0.01 N EDTA + NH,NO, (5.8 < pH < 6.0). The barium leaves the column first. After this the radium is removed by adding more NH4N03 to the eluting agent. The Ra fraction is brought onto a smaller Dowex-50 column in order to remove the EDTA from the solution by means of O.lN HNO, as eluting agent. The purified radium is then obtained by eluting with 8N HNO,. The solution is then evaporated to dryness in a quartz container. During the separation, the daughter products of 226Ra were also removed. But some of them grow into the sample in about 3 hafter the chemical separation to an extent that cannot be tolerated in the measurements. Fortunately the first daughter of 226Ra 9 222Rn, is a gaseous product which can be easily removed from an aqueous solution by bubbling nitrogen through it at a slow rate. In about three hours the shortlived daughters ” 4Pb (RaB) and 214Bi (RaC) will decay to a negligible amount, and only the primary cl-radiation and 186 keV y-radiation of 226Ra is left in the sample. During the chemical separation the original radium chloride is also converted into nitrate because this radical does not contribute to the background after irradiation. For the y-ray measurements, aqueous solutions of radium-nitrate were sealed into a polyethylene tube and irradiated in the reactor by the use of a transfer system. After the irradiation the samples were put into another polyethylene tube, as the activated argon in the air left in the irradiated tube caused a very troublesome background. A further reduction of the background was achieved by the use of a quartz container in which the radium is kept and in which purification by means of the “bubbling” method takes place. Normal glass contains too much sodium and manganese which dissolve into the liquid and contribute to the background after irradiation. Beta-sources were made by drying a drop of the purified radium solution deposited on a mylar tray. This sample was then exposed to the neutron flux for 40 min. The backing material caused only some very short-lived background. The radium solution cannot be used many times because the traces of Na, Mn and Cl become too strong, and another chemical separation has to be performed.
339
227Ra DECAY
In the measurements of the y-spectra and the yy coincidence spectra, two Ge(Li) detectors were used with efficiencies‘of 2.5 % and 4 % (compared with 7.5 cm x 7.5 cm NaI) and 2.8 keV resolution for the 1.33 MeV line of “*Co. The low-energy y-rays were recorded with a Ge(Li) X-ray detector (resolution 640 eV at 122 keV). A Si(Li) X-ray detector was used in a coincidence experiment. The relative detector efficiencies were calibrated by means of the known relative intensities of y-lines in the isotopes f82Ta; s7Co-f 241Am*, 75Se and 226Ra.
t
I
I
8
200
LW
600
600
CHANNEL
I
1000 NUMBER
Fig. 1. Low-energy spectrum taken with Ge(Li) X-ray detector.
The &spectrum was recorded with a cooled (77” K) Si(Li) detector with a depletion depth of 3 mm. The coincidence measurements were performed with a commercially available ~‘~ro~ross-uver detection” type of coincidence arrangement (2r = 100 ns). The singles and three coincidence spectra ~102~hannels each) were recorded sim~taneuosly by the use of a home-made routing system for our 4096channel Laben pulse-height analyzer. The coincidence gates were selected by three modular-type single-channel analyzers. The analysis of /?- and y-spectra was carried out by means of FORTRAN IV programs, running on our PDP-9 and the IBM 360/65 of the university. 4. Experimental results 4.1. GAMMA
SPECTRUM
The gamma spectrum of the low-energy region below 125 keV, taken with theX-ray detector is shown in Sg. 1. It is difficult to distin~ish any ~-transitions between the close-lying L X-rays.
W. LOURENS et al.
340
The origin of the bump on the front slope of the 27.3 keV transition is mainly caused by the detector response. This was made sure by measuring y-lines of 2J’Am and lo9Cd under the same circumstances. We therefore believe that the detection of the 24.6 keV transition is problematic. The line at 22 keV shown in fig. 1 is a result of background radiation as is the 58.3 keV line. In some spectra, traces of peaks at about 37 keV and 57 keV were observed. Probably, they do not belong to the decay of 227Ra. The shape of the front slope of the K,, line of Bi, caused by background of the 226Ra daughter shows some evidence for the existence of a 74.2 keV line. The peaks due to the decay of 227Ra are listed in
COUNTS
I 1600
8
/
mu0
2000
r
--
2200
2;oo CHANNEL
NUMBER
COUNTS
CHANNEL
NUMBER
Fig. 2a, b. Gamma spectrum below 400 keV taken with Ge(Li) X-ray detector.
227Ra DECAY
342
W. LOURENS et al.
TAB&El Gamma lines in z31Pa(er) (de pinho) and az7Ra(B-) (present work) (y-ray energies above 200 keV of de Pinho corrected for systematic diEerence) Transition
4630 4627 211-187 351-330 IlO85 270 300
IlO85-
74 46
1430 460 438-387 llO30 8430 330-273 8427 187-127 llO46 199-127 271- 199 740 187-110 127- 30 211-110 187- 85 199- 74 271-127 501-355 27?74 514-30s
Ev(231Fa) per 10% 14.1 &O.l 16.5 +0.1 18.9 23.6 24.5 fO.1 25.54f0.06 27.35kO.02 29.95 kO.02 31.00~0.05 31.54kO.05 35.82j20.03 38.20&0.02 39.57&0.04 39.97&0.02 42.48 IO.05 43.05 f0.05 44.16&0.05 46.3710.02 50.98 ho.05 52.74f0.02 54.61 f0.02 56.76f0.04
I, per lO=+p
bOt
30 84 27 74 46
387-121 2730 387-110 30527
ZY per IO38
W W
s 1 w 10 1000 9.9 1.0 0.7 1.7 16.0 0.15 1.3 0.6 0.7 6.5 22.4 0.15 9.1 :;
ww9
(95P)
(~~)*) 2180
!%3
24.9 +0.1
174 0.36
27.3710.02 29.95&O&4
5 1130
14 43 (4) (228) 13.6
< 0.2
46.38fO.M
(54.6)
199
+1
:: 514 0.7 0.2 0.4 2.7 7.3 9.5 3.4 B2 0.5 1.3 0.6
680 1.0 490 (2) (4)
1.0
242.2S;tO.lO 243.05&0.10 24X45&0.10 246.05f0.20 255.85&0.07 258.47&0.10
0.9 3.7 0.8 c 0.1 10.9 0.25
< 0.1 13.8 0.43
260.22;tO.OS 273.17&0.09 277.09&0.09
18.6 6.2 7.2
34 9.4 7.4
6.3
< 0.1
< 1.6
W
<3
(146.9) AO.5 198.9 &to.2 209.6 kO.2 218.19f0.10 219.9 kO.15 226.6 AO.1 228.00&0.10 232.20&O-l 2.42.10~0.2 243.15&0.10 245.9 &O.l 255.76&0.10 258.30&0.10 259.7 kO.2
0.3 1.1 2.1 2.1 0.3 4.2 3.0 0.3 5.4
“)
277.39&0.10
0.3 2.0 20 0.3 9.6
273.15f0.10 a}
< 14
< 1.5 3.6
74.2 kO.2
$)a) 133 39 % 22 2.2 3.6
870 79
< 2.7 1.9
0.24 < 0.2
57.1 k0.1 57.19;to*o3 60.5010.03 63.67&0.03 70,50j$.05 71.9 fO.1 72.5 SO.1 74.18&0.04 77.3610.03 96.88 10.03 100.92,tO.O4 102.6 124.6 fO.l 144.5 kO.1
I 1(11
w
30585 27346 501-273 537-305 273330273330305-
E, (227Ra)
29
5 < 32
7(if E2)
0.5 3.6 zG.5 0.3 11.7 8.4 9 0.3 3Y 14.6 43
343
227Ra DECAY TABLE 1 (continued) Transition 330
-
46
330 330
-
0 27
387 - 74 354 - 27 330 0 387 - 46 426 84 426 74 355 0 387 - 30 469 -110 438 - 74 426 469 387 435 438 426 426 428 43s 438 438 428 43s 438 -
46 84 0 46 46 30 27 30 27 30 27 0 0 0
514 501 563 514 537 501 537 563 563 54?C)576 -
47 30 84 27 46 0 27 46 27 0 30
639.4639.4698.4698.4789.8789.8874.7863.5874.7863.5874,7-
27 0 46 27 30 0 46 27 27 0 0
E, (23’Pa) per 104cf 283.6710.06 286.6610.10 3~.08~0.~ 302.67&O&6 310.15f0.10 313.~~0.08 327.20&0,10 330.0710.06 340.8 110.07 351.61~0.10 354.5910.08 357.16~!=0.07 359.47zto.10 363.96hO.10 375.13&0.10 379.33kO.08 384.94kO.10 387.25hO.10
169 1.0 244 252 0.15 10.6 3.2 I40 17.8 0.38 10.2 18.6 0.97 0.80 0.50 5.3 0.44 0.05
391.75*0.10 395.75*0.10
0.73 0.28
398.7QO.08 407.97kO.06
1.00 3.9
410.76fO.l
0.20
435.37kO.l 438.17&0.1 438.97&0.1
0.36 0.44 0.16
487.0 491.3 501.9 509.3 516.5 535.6
1, per 103#1
*er:Yo*p
f0.3 10.6 10.5 *1 10.6 kO.7
0.19 0.05 0.06 0.03 0.14 0.05
546.9 f0.7 572.4 ho.8
0.06 0.05
l) From intensity balance with a-feeding.
I t01
175
283.68f0.10
b,
34
36
371 264
300.09*0.10 302.68+0.10
b,
51 48
77 SO
17 3.3 217 19
327.2 &-0.2 330.08&0.10
3.0 30
3.1 46
341.1 -f;O.l
2.2
3.0
354.6 j110.2
7.5
9.0
379.4 rto.1
4.7
5.8
390.4 +0.06
0.78
398.4 kO.4
1.8
0.4 12.2 28 1.3 1.0 6.6 0.6 0.05
1.2 4.8
407.97 &to.10 b)
0.93 2.5
428.4 40.2 435.4 &to.1
468.5 10.5 471.3 *o.s 478.4 kO.4 486.98f0.10 490.5 *0.5 501.40*0.10 510.0 *to.2 516.2 10.2 535.6 f0.2 543.1 &-0.1 611.4 639.4 652.2 671.1 760.3 789.8 828.9 836.4 846.1 863.5 874.7
24
b,
b, b) b) b, b,
ho.2 b, kO.2 kO.2 10.2 kO.2 ho.3 kO.3 ho.3 kO.3 kO.3 kO.3
2.7 2.7 0.9 2s lb: 1: 4:;
13 0.5 2.4 ::: :*z 1:o < 0.6 A:$
b, Coincident with 27.4 keV transition. “) This level is not shown in the Ievel scheme, as there is only one line to support it.
344
W. LOURENS et al.
table 1. Between the X-rays and the strong 186 keV transition no lines could be observed except for a weak peak at about 165 keV due to background radiation of 139Ba. Above 186 keV several lines have been observed; they are also listed in table 1. The spectrum above 125 keV has been investigated with the X-ray detector, up to 400 keV, (see fig. 2a, b) and with the 4 % Ge(Li) detector (fig. 3a, b). The energy calibration of the latter detector was obtained in the following way. In the spectrum of fig. 3a, b are lines at 241, 352, and 611 keV. Their energies are close to the very well-known transitions in the decay chain of 226Ra, but yet they are due to the decay of 227Ra. After about l$h, the 226Ra lines become dominant, but some strong lines in the 22‘Ra are also present. The 226Ra lines were used as internal energy calibrations for the 227Ra lines. The other spectra were calibrated with the energies, obtained in this way. In table 1 we have summarized all reliable data obtained from analysis of the y-spectra. The highest energy peak due to the 227Ra decay has been found at 874.7 keV. Lines with higher energies in the spectra of the irradiated sources were attributed to the contaminants 41Ar, 56Mn and 24Na. 4.2. GAMMA-GAMMA
COINCIDENCE
MEASUREMENTS
A coincidence measurement was performed between the 27.3 keV y-line recorded with the Si(Li) X-ray detector and the spectrum taken with the largest Ge(Li) detector. The coincidence window of 1 keV width was placed on the 27.3 keV peak. The window did not include the bump on the front slope of this line, which was also present in the Si(Li) spectrum. The coincidences from the background, about 30 times lower than the peak, were monitored by a second window selected behind the 29.9 keV peak in the spectrum. During the measuring time nearly no coincidences were observed with this window. The chance coincidences collected during the measurement were negligible, which is shown in the spectrum of fig. 4, where the strong 330 keV line is nearly absent. Gamma rays that are in coincidence with the 27.3 keV line have been marked by “) in table 1. In Ge(Li)-Ge(Li) coincidence measurements between higher energy lines, no coincidence relations could be observed for transitions above 300 keV. However the 258 keV line was seen to be strongly coincident with the 232 keV line, and the 228 keV line was seen to be in coincidence with the 243 keV lines. Evidently the 243 keV and 272 keV lines are not coincident with each other. The coincidences have been collected from two sources which were alternately irradiated for half an hour and measured for an hour. 4.3. THE HALF-LIFE
OF 227Ra
Butler and Adams ‘) established a half-life for the decay of 227Ra, on the basis of the measurement of the decay rate of the P-rays. We measured the decay rates of the 330 keV and 284 keV y-rays for three hours. The measurement of the half-life was performed by using the pulse-height analyzer
aaTRa DECAY
345
in the multi-analyzer mode; 18 spectra of 128 channels each, containingthey-spectrum,
between 200 and 350 keV were recorded for 300 s. Between the successive measurements 300 s of waiting time were included. The 18 spectra were analyzed with the PDP-9 and corrected for dead-time and background radiation. By the least-square method a linear curve was fitted to the points in the diagram which displays the logarithm of the intensity against the time. The half-life was established as 42.2kO.5 minutes.
:OUNTS
200
LOO CHANNEL
600 NUMBER
Fig. 4. Coincidence spectrum, gating channel on 27.3 keV, the coincident transitions are indicated in table 1. 4.4. THE @-SPECTRUM OF zz7Ra
The ~-s~t~rn was recorded during 20 min with the Si(Li) electron detector. The measurements were started about 15 min after the irradiation so that the shortlived activities in the backing would be eliminated. During the measurements we also have recorded a y-spectrum in order to see whether any strong contamination has been taken place. No other lines than these belonging to the 227Ra spectrum could be observed. After 2 h we performed another measurement also of 20 min with the same source and subtracted that spectrum from the first one in order to reduce the contribution of the longer lived activities. (e.g. 24Na). The residue was analyzed with a special program on the PDP-9. The analyzing method has been applied earlier in our laboratory and is described elsewhere “). In fig. 5 the Fermi-Kurie plots of the highest branches are displayed. Beside these wo other groups of branches could be determined. The energies and intensities of the
W. LOURENS et al.
346
four groups are as follows: &(keV) 1300(+15) 1026(+20) 745( + 50) 550(_+80)
Is 100 45+ 5 20* 10 104 5
We believe that all four branches in fact are groups of branches which will be understood from our decay scheme (fig. 6). The highest energy branch will be a
28
24 ENERGY-l.55
+CH.NUMBER
t 22.3
20
1E
12
8
4
0 CHANNEL
NUMBER
-I, Fig. 5. Fermi-Kurie plots of the two highest energy p-branches.
mixture of transitions to the ground state and 30 keV state (both weak) and the 27.4 and 46.4 state (both strong). We therefore propose a Q-value for the /I-decay of 227Ra of 1335+20 keV. The second group of branches will mainly feed the two states at 330 and 354 keV and also the rather strong populated 304 keV level. The energies and intensities of the two last groups are only given as indication because of their heavy mixing and bad accuracy,
i 8 : iI ;g -
_ _ _ _ _ _
-
_
-
-
_
-
_
I_
_
-
_
-
” _ _ _ -
-
_
-
_
-
_
-
_
-
_
-
--
-.
-
_
_
-
_
-
_
-
_
-
” _ _ _ _ _
-
_ _ _ _
-
” _ _ _ _ _ _ _ _ _ _
-
E
2
348
W. LOURENS
5. Decay scheme of 22’Ra@-)227Ac;
et al.
comparison with 231Pa(a)22’Ac
The level scheme given in fig. 6 has been constructed with the present y-ray energy and intensity values and the present yy coincidence results. Furthermore we compared our results with 231Pa data on cr-y-ray and electron energies and intensities as given by Baranov ef al. “) and, mainly, de Pinho et al. ‘) and with cry coincidence results of Leang “). We accept the interpretation of the states in 221A~ below 273 keV as belonging to K” = $- and 4’ bands ‘, “). In the decay of 227Ra the +-, %-, $-, 3’ and 3’ levels in these bands occur, whereas a rather low limit can be set on the occurrence of the 3’ level. It may be mentioned here, that, in contradiction to theconclusion ofde Pinho et al. 7), we are of the opinion that the decay of the 46.38 keV level can be understood from the y-rays as given schematically in fig. 6 (de Pinho invokes decay to another intermediate level at about 30 keV). With intensities and transition assignments as given in table 1 for the 231Pa decay, total intensity feedings to the levels at 27.36,46.38 and 84.56 keV agree within 10 y0 and those to 29.95 and 74.14 keV within 20 ‘A with those intensities TABLE 2 Branching ratios in the p-decay of 227Ra Level
Calculated intensity per lo3 /?-
874.7 863.5 789.8 698.4 639.4 562.85 537.0 514.3 501.2 469.51 “) 438.13 “) 435.35 428.4 425.73 387.17 ‘) 354.58 330.04 304.70 273.2 84.56 74.14 46.38 29.95 27.36 0
1.3 2.6 2.9 4.0 14 23 10 35 26
“) Known from 231Pa(a).
lot3 ft
7.3 7.1 7.2 7.4 7.0 7.0 7.2 6.8 7.0 > 8.3 > 8.0 7.1 8.1 7.5 > 7.9 6.6 6.3 6.7 8f0.5 > 7.7 > 7.4 6.6 $8 6.5 2 7.5
227Ra DECAY
349
depopulating these levels (for the last levels this would also be within about 10 % if the intensity of the 44 keV line would be half as large). We also point to the fact that the present energy values require extensive reinterpretation of the energy assignments of the electron lines as reported by Baranov et al. ‘). Among others, they interpret L,, M, and Mz conversion lines of the 19.0 keV transition as 19.7-L,, 33.9-L, and 16.4-Mr lines and also their adoption of an energy value of 29.5 keV instead 29.95 keV causes some misassignments. Our measurements confirm the existence of the 273.10 keV level tentatively assigned by de Pinho. The intensity ratio of the 273 and 243 keV lines is exactly the same as in de Pinho’s “‘Pa measurement. Furthermore coincidences with the 228 keV y-radiations strongly support the existence of this level. The great strength of the upper component of the 245.4-245.9 keV y-ray doublet, for which de Pinho ‘) could only derive an upper limit not seriously contradicting the present results, strongly suggests that it belongs to the decay of the same level. The existence of a new level at 304.70 keV is proved by the coincidences between the 258 keV y-ray and the 209 and 232 keV ones. The 258 keV y-ray was seen by de Pinho ‘) but not assigned a place in the decay scheme. The present results on the 330.04 and 354.58 keV levels agree quite well with those obtained in 231Pa(a) [refs. 61‘)I. It should be noted, however, the 245.4 keV y-ray would fit quite well between the 330 and 84 keV levels. Only theoretical reasons (M2 strength expected) would make this assignment somewhat less probable. The 387.17 keV level, strongly excited in 231Pa(a), is conspicuously absent in 22‘Ra (B-J The strong line at 379 keV from the level at 425.73 keV proposed by de Pinho ‘) is found in 22‘Ra in coincidence with the 27 keV line, in agreement with the proposed assignment. Of the other lines depopulating this level in the 231Ra(a) decay, only the strongest could be seen in the 227Ra(p). The 398.1 keV line may be double. It is observed that the 398.1 keV line in 227Ra is stronger relative to the 379 keV one than in 231Pa(a). Part of it may come from a level at 428.4 keV not fed in the last decay. Of the lines decaying from the 438.13 keVleve1 of de Pinho, none is seen in 227Ra. The strong 407.97 keV line observed in both 22‘Ra@-) and 231Pa(a) cannot be assigned to this level. In 227Ra, it is observed in coincidence with the 27 keV line, so that it cannot feed the 29.9 keV one. Even in de Pinho’s data, the energy value 407.71 kO.06 keV is 0.22 keV lower than required for assignment to decay to the 29.9 keV level. Also, no other y-rays assigned to decay from the 438 keV level are seen in the decay of 227Ra. The further observations that the intensity ratio of the 435 and 408 keV transitions is nearly the same in both decays and that the 435 keV line is not in coincidence with the 27 keV transition make it quite certain that these transitions depopulate this 435.35 keV level. The y-rays originating in the 469.51 keV level of de Pinho ‘) are not observed in 227Ra.
350
W. LOURENS et al.
The level at 501 keV proposed by de Pinho ‘) and Leang ‘> is offered additional evidence by the coincidence measurements on the 228 keV transition, which transition is not yet seen in the 231Pa(a) decay. Instead of the levels at 509.2 and 516.5 keV proposed by de Pinho ‘) we suggest levels at 514.3 and 537.0 keV. Contrary to de Pinho ‘) we want to assign the 435 keV transition as decaying from the 435.37 keV level as mentioned above, which abolishes the 509 keV level, that was only supported by the 435.37 keV transition. The 487 and 516 keV lines are both coincident with the 27.4 keV y-ray. This leads us to introduce levels at 514.3 and 537.0 keV as the 516 keV level of de Pinho was supported only by these two transitions we are compelled to remove this level. The higher levels in the decay scheme are entirely based on sum rules. Using the present y-ray intensities combined with conversion coefficients as measured in 231Pa (a) [refs. 3*4r 9)], with th eoretical conversion coefficients in several other cases, and with a few intensity ratios for transitions between low-lying levels in 227Ra as observed again in 231Pa(a), we calculated /?-feedings of the low-lying levels.The results are presented in table 2. As for spin-parity assignments, those to the two lowest bands and to the levels at 330 and 354 keV are considered to be certain from a-hindrance factors and from conversion-electron data as observed in 231Pa(a). The parities of the levels at 273, 387 and 425 keV are considered to be well documented by conversion-electron data. The other spin-parity assignments given in fig. 6 are based entirely on consideration of relative y-ray transition probabilities. 6. Nitsson model assignments to 227Ra and to levels in “‘AC (fig. 7) Consideration of levels in isotonic or nearly isotonic nuclei leads to the prediction that the ground state of 227Ra should belong to one of the following Nilsson levels: j* [631], 3’ [633], and 4- [752]. On the other hand, it appears that the $- [530] level at 354.58 keV is definitely fed by a direct p-transition which could be allowed hindered or more probably, first forbidden unhindered (log ff = 6.6). This agrees only with the first one of the three assignments mentioned. The somewhat remarkable facts that the negative-parity ground state band is fed so much less (logft 2 7.5) than the +- [530] band but that the low-lying 3’ [651] band is fed relatively strongly (log ft s 6.5) find a natural explanation in the 4’ [6311 Nilsson assignment for the 227Ra ground state. The transitions are then first-forbidden hindered and allowed hindered, respectively. A possible difficulty for the above Nilsson model assignment for the 227Ra ground state is the apparent B-decay to the level at 425.73 keV, which, according to its y-ray decay, most probably has spin-parity 3’. Yet, feeding of this level through weak 88.47 and 111.17 keV y-rays originating in the levels at 514.3 and 537.0 keV cannot be excluded. It would make the 425.73 keV level have rather similar properties to the level at 304.70 keV. When we take into
**‘Ra DECAY
351
account that in other nucleides with 89 or 91 protons and neutrons the Nilsson levels, 3’ [660], 3’ [651] and 3’ [642] occur involved in a Coriolis coupling and, for neutrons, in a AN = 2 coupling with 3’ [400] and 2’ [402], it is attractive to interpret the levels at 304.70,425.73 and 469.51 keV as the spin $, 3 and 3 levels of the 3’ [642] band. The relatively strong B-feeding of the lowest of these levels [logft = 6.71 is in accord with the allowed-hindered character following from this interpretation. 5+
436.1 e $[530]
.I-
i-
't'
469.6 307.2 364.6 330.0
’ B
271.3
112
196.6
j-
126.6
7T
14.1
f
29.9
1-
0
$- [632]<
425.1 f
I $
5+
++@I’ $
++[660]
I
537.0 511.3
F
1+
T
435.b
210.9 161.4 304.1
110.0
T
84.6
,g
46.4 27.3
,2
Fig. 7. Nilsson levels in **‘AC.
The evident candidate for the ground state of the 3’ [660] band is the 435.35 keV level, excited with medium intensity (logft = 7.1, F. = 50) in both 227Ra(b-) and 231Pa( a). Interpreting the levels at 514.3 and 537.0 keV as spin 3’ and 3’ levels of this band would explain their feeding and their decay to the 3’ [642] band and to the +’ [651] one. This proposition implies a rather small value for the decoupling parameter (u x 0.7). However also in 233Pa [ref. lo)], the same three bands mentioned before are mixed and the decoupling parameter for the 3’ [660] band is also low (Q = 0.67). The level at 273.10 keV probably has spin-parity 3- and is fed by a weak /I-transition (log ft = 8 f0.5); its first rotational state should nearly coincide with the 304.70 keV level and is therefore difficult to observe. No easy Nilsson level assignment presents itself for this level: it may be a combination of the 3’ [651] level with octupole (1 -) core excitation which in near-by doubly even nucleides is known to occur at nearly this energy. We do not wish to suggest assignments for the levels at 501.2 and 562.85 keV and for the higher ones. One curious difficulty remains: for some unknown reasons, no gamma rays are observed decaying from the 438.131 keV level which is thought to be 3 spin level of the +- [530 ] band. The authors are indebted to Mr. K. Smit for his careful preparation radium solution.
of the pure
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et al.
References 1) J. P. Butler and J. S. Adams, Phys. Rev. 91 (1953) 1219 2) F. S. Stephens, private communication reported by E. K. Hyde er al. in: Nuclear properties of the heavy elements (Prentice Hall, New Jersey, 1964) p. 689 3) A. Baranov, V. M. Kulakow, P. S. Samoilov, A. G. Zelenkov, Y. R. Rodionov and S. V. Pirozkokov, ZhETF (USSR) 41 (1961) 1475 (translation, JETP (Sov. Phys.) 14 (1962) 1053) 4) A. G. de Pinho, Thesis, Paris (1963); H. Abou-Leila, R. Foucher, A. G. de Pinho, N. Perring and M. Valadares, J. Phys. Rad. 24 (1963) 857 5) R. C. Lange and G. R. Hagee, Nucl. Phys. Al24 (1968) 412 6) Chin-fan Leang, thesis (Paris, 1969) 7) A. G. de Pinho, E. F. da Silveira and N. L. da Costa, Phys. Rev. C2 (1970) 572 8) W. Lourens, B. 0. ten Brink and A. H. Wapstra, Nucl. Phys. Al52 (1970) 463 9) G. R. Hagee, R. C. Lange, A.C.Barnett, A. R. Campbell, C. R. Cothern, D. D. Griffing and H. J. Nennecke, Nucl. Phys. All5 (1968) 157 10) W. Hoekstra and A. H. Wapstra, Phys. Rev. Lett. 22 (1969) 859