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Alpha decay of Gd150
Nuclear Physics 66 (1965) i19--128; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or m i c r o f i l m without written permission from the publisher
A L P H A D E C A Y O F Gd Is° IWAO OGAWA, TADAYOSHI DOKE, MITSUHIRO MIYAJIMA Department of Physics, Rikkyo University, Toshima-ku, Tokyo, Japan and ATSUSHI NAKAMOTO Institute for Atomic Energy, Rikkyo University, Yokosuka-shi, Kanagawa-ken, Japan Received 29 October 1964 Abstract: The alpha particle energy and the half-life of Gd 15°have been measured with a double-grid ionization chamber. Backgrounds are minimized by the grid-collector coincidence technique. The alpha particle energy is determined as 2.7154-0.018 MeV by referring to that of Sm147 (Ea = 2.234-0.02 MeV) reported by Macfarlane. The half-life is estimated to be (1.44-0.4)× 10e y from the observed counting rate and the total number of Gd 16° nuclei contained in the specimen. The number is estimated from the observed beta-decay rate of the parent nuclei Eu 15°, which are produced by a photonuclear reaction Eu~Sl(y, n)Eu ~°. The experimental halflife thus obtained agrees within a factor of 4.4 with the theoretical half-life calculated from the observed alpha-particle energy on the basis of a conventional barrier-penetration theory. The results show that Gd ~5° behaves quite normally in the systematics of rare-earth alpha-decay. I E
RADIOACTIVITY Gd ~5° [from Eu1~1(7, n)EuXS°(fl-); measured T½~,E~. Natural target.
]
1. Introduction
In their extensive study o n the systematics o f alpha-radioactivity in the rare-earth region, T o t h and Rasmussen t) have c o m p a r e d the experimental half-lives o f a n u m ber o f nucleides with the theoretical calculations f r o m the observed alpha-particle energies using a simple barrier-penetration theory. They have noted that the experimental and theoretical half-lives o f all the even isotopes examined are in agreement within a factor o f 5 with the single exception o f G d 15°, whose theoretical half-life, 8.37 x 106 y as c o m p u t e d f r o m the observed alpha-energy o f 2.70_+0.15 M e V (refs. 2, 3)), is 28 times as large as its experimental half-life available at the time (,~ 3 x 105 y with an error factor o f 2). This discrepancy has been considered to be p r o b a b l y due to fairly large errors involved in the measurements. Indeed, there seems to be no particular reason for G d is° only to behave anomalously a m o n g the neighb o u r i n g even nucleides. These considerations b y T o t h and Rasmussen once appeared to be actually confirmed experimentally, when Siivola 4) reported in 1962 an experimental half-life as long as (2.1 _+0.3)x 106 y, together with a reasonable experimental alpha-energy o f 2.73-t-0.01 MeV. These data are fairly consistent a m o n g themselves f r o m the view119
I. OGAWA et al.
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point of conventional barrier-penetration theory. Recently, however, the confirmation again became not so conclusive. Namely, Friedman, Milsted and Harkness s) presented last year a value as small as (1.0+0.6)x l0 s y as the experimental half-life of Gd 15o. Since it is of primary importance in the alpha-decay systematics to establish a correct half-life energy correlation for even isotopes, we have attempted to remeasure the alpha-energy and the half-life of Gd is° as accurately as possible, by performing a low-background counting with a double-grid ionization chamber. The results are in good agreement with those of Siivola and indicate again that Gd is o does n o t constitute a special anomaly among the neighbouring isotopes. An account is given in sect. 2 of the experimental procedures and results. In sect. 3, a comparison is made between the theoretical and experimental half-lives, using the same formula as used by Toth and Rasmussen. 2. Experimental Procedures and Results
The Gd 15° nucleus was obtained as a beta-decay product of Eu ~5°, which was produced by the photoreaction Eu151(7, n)Eu 15°. The alpha-particle energy was measured with a low-background ionization chamber of the double-grid type 6, 7), using alpha particles from Sm 1*7, U 234 and U 23 s as energy standards. On the other hand, the half-life was determined from the observed counting-rate of alpha particles and the total number of Gd 15o nuclei in the specimen as estimated from the observed beta-decay rate of Eu 1s o. 2.1. P R O D U C T I O N
O F E u xS°
Powders of natural europium oxide (Eu2 03, purity 99.9 ~ ) were irradiated continuously for about 10 h with the bremsstrahlung X-rays from the 18 MeV electron linear accelerator at the Japan Atomic Energy Research Institute, Tokai-mura. The X-ray dose rate near the specimen was estimated to be of the order of l0 s R/h. Since natural europium is composed of two isotopes, Eu lsl (47.8 ~o) and Eu ls3 (52.2 ~), with nearly equal relative abundances and since the (7, n) cross section is not expected to dilfer appreciably between the both isotopes 8), we should obtain almost the same amount of Eu 15° and Eu ~s2 by the irradiation. No other isotopes were believed to be produced to such an amount that could affect appreciably the present measurement for the following reasons: (1)The products of (7, P) reactions in europium are stable isotopes. In addition, the cross sections for the reactions are very small. (2) The (% ~) reactions may be perfectly ignored since the cross sections are ten thousand times as small as those for (% n) reactions 8). (3)The isotope 015 produced from 0 26 by the (7, n) reaction may be ignored since the cross section for the reaction is only one twentieth of that for Eu ~s° or Eu 152 production s) and the half-life of O ~s is as short as 2.0 rain. (4) The possible (% n) products from the minor impurities such as Sm, Gd or Nd, which might have been present (less than
121
ALPHA DECAY OF Gd 1~0
0.2 %) in the specimen give rise to a negligible alpha or beta activity as compared to those for Eh is°, Eu 152 and their decay products. This may readily be expected from the magnitude o f cross sections for relevant (~,, n) reactions and of half-lives o f resuiting reaction products. (5) The (~, 2n) and (~, np) reactions are without importance because the great majority o f incident protons were o f quantum energies insu~eient to liberate two nucleons. It should be mentioned that the slow neutron flux that arose as a result of slowingdown of photoneutrons was fairly small ( ~ 10 5 n/cm 2 • sec) near the specimen, so that the number of Eu ls2 nuclei that could be produced by the Eu151(n, ~) reaction was estimated to be less than 1 % o f the number of those which were produced by the EulS3(~, n)Eu 152 reaction. Thus it was considered that Eu 15° and Eu lsz should have been produced in a proportion fairly close to the relative abundance of Eu ~51 and Eu 153 in natural europium t. The fact was used to check the results o f the analysis described in subsect. 2.3. 2.2. BETA DECAY OF THE IRRADIATED SPECIMEN The irradiated powders of europium oxide were made into a beta-source, about 1 mg/cm 2 thick, by suspending the powders in alcohol and by letting the solvent eva-
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I
50
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I Time
150 (h)
t'
I
I
200
250
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I
Fig. 1. Decay curve of the irradiated europium oxide. porate naturally. Fig. 1 gives the decay curve o f the source obtained by the measurement with a G M counter having a mica window 1.6 mg/cm 2 thick. It is seen from the figure that the decay curve consists of two parts, the one decaying with an average half-life of about 12 h and the other with a much longer half-life. t A possible competing reaction EulSS(~,n)Eu152mis tentatively neglected here.
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i. OGAWA et aL
The former may be ascribed to the short-lived beta decays of Eu 150 and Eu 152, while the latter presumably is due to the 13-y beta decay of Eu 152m together with other slower decays of unidentified reaction products. This assignment was partly confirmed by the gamma-ray spectrometry made with a 7.6 c m x 7.6 cm NaI(TI) crystal. Namely, the scintillation spectrum observed in the early period of decay showed a pattern characteristic of 9.2 h decay t of Eu 152, while the spedtrum for the slowly decaying period contained several peaks which may be regarded as transitions associated with the 13 y decay of Eu 152. 2.3. E S T I M A T I O N O F T H E T O T A L A M O U N T
O F G d 15° P R O D U C E D
The following two methods were adopted to estimate the activity of Eu is° just after the irradiation, from which the total number of Gd 15° nuclei, both produced and to be produced, was calculated. O) The method of decay curve. From the gross decay curve as given in fig. 1, we first subtract the long-lived component. The remainder will then be decomposed into two components, one decaying with a half-life of 13-14 h and the other with a 9.5 h half-life. The former value is in good agreement with the half-life of 13.7 h for Eu is° reported by Mack et al. 11), while the latter with 9.2 h for Eu 152 as measured by Butement 9). (ii) The method of absorption curve. If a specimen of natural Eu2 03 is irradiated with thermal neutrons, the beta activity to appear will be largely due to Eu 152, because the neutron capture (absorption) cross section for Eu lsl (9000+3000 b) is about 20 times as large compared to that for Eu 15a and more than 5 × 10~ times that for 016. Moreover, the activity of 017, if any, will decay in only a few minutes. Therefore, we can estimate the initial activity of Eu is o by subtracting the absorption curve in alnminium for the neutron-irradiated specimen (n, 7) products from that for the X-ray irradiated one ((7, n) products). In the present experiment, thermal neutrons were produced to irradiate a Eu2 03 specimen by moderating in water the d--d neutrons from the Cockcroft-Walton accelerator at the Rikkyo University. The absorption curve for the (n, 7) products is given in fig. 2 with the curve for the (7, n) products. In carrying out the subtraction, we brought both curves into approximate contact at the point (indicated by an arrow in fig. 2) where the absorber thickness corresponded to the maximum range ( ~ 400 mg/cm2 in aluminium) of beta particles from Eu is°. This was in order to take into account that the tails of both curves should not necessarily coincide with each other because of the possible effects of bremsstrahlung X-ray from the beta particles and of characteristic X-rays and gamma rays from the specimen. The above mentioned point near the maximum range may be regarded to be much less affected by these factors. t It h a s been reported that E u TM decays also by electron capture a n d p o s i t o n emission to S m x~° a n d emits s o m e g a m m a rays x0). However, the rate o f g a m m a emission is only a few percent o f t h e beta decay, so that the observed s p e c t r u m m a y be considered to be largely due to E u TM.
ALPHA
DECAY
OF
123
Gd ls°
The resulting difference curve, which is given also in fig. 2 by a dotted curve, should represent the absorption curve solely due to Eu 15°. The curve gives a half-value layer (HVL) of 51 + 7 m g / c m 2 for the beta-ray. The m a x i m u m energy Epmax o f the beta particles is thus assumed to be 1.07+0.10 MeV, using the well-known empirical relation between Epma~ and HVL. The last value is in excellent agreement with 1.07 MeV for Eu 15° reported by Mack etaL 11). Furthermore, the beta activity o f Eu 1s o just after the X-ray irradiation as estimated f r o m the curve is in good accordance with the previous results o f the decay curve method. IOz
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Fig. 2. Absorption curves in aluminium for the X-ray and thermal-neutron irradiated europium oxide. The arrow indicates the maximum range in aluminium of beta-particles from Eux6°.The dotted curve represents the difference between the two absorption curves.
In order to obtain the absolute disintegration rate for the Eu 15° beta decay, the overall counting efficiency of the G M counter system was estimated by using a reference source o f R a E with a known disintegration rate. Since the m a x i m u m energy of the beta particles f r o m R a E (Ep, ma, = 1.17 MeV) is fairly close to that for Eu is° and since the size, the thickness and the geometry of the R a E source used were made almost identical with those of the Eu 15 o source, the counting efficiencies for the both sources were safely presumed to be practically equal. On the basis of the beta disintegration rate of Eu 15 o thus obtained, the total number of G d 1~o nuclei produced was calculated to be (1.3___0.3)x 101° per mg of Eu2Oa. 2.4. ALPHA-SPECTRUM OF Gd15° The alpha-spectrum of G d 15 o was obtained with a double-grid ionization chamber by mounting a thin, wide sample of irradiated europium oxide on the high voltage electrode o f the chamber.
124
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al.
The chamber was filled to a pressure of 3 atm with a mixture o f argon and methane (5 70). In view o f the very low counting rate, backgrounds were minimized by counting only those collector pulses which were accompanied by a negative pulse from the first grid o f the chamber. Details o f this method o f background reduction have been described in ref. 7). As a result of this coincidence technique, a background level lower than 1.5 count per hour per 1 MeV interval was achieved in the energy region of a few MeV.
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.
.
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.
.
.
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.
.
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150
Chonnel Number
Fig. 3. Alpha spectrum of Gd1so. The Smx47 alpha particles were also observed simultaneously for comparison. The samples were prepared by the following procedures t: Irradiated powders o f europium oxide were first converted to nitrate and dissolved in alcohol. The solution was then spread over a backing disc of stainless steel, 12 cm in diameter and 0.8 m m thick, and was dried by natural evaporation. In the course o f evaporation, the disc was floated on a mercury surface to keep it horizontal. Finally, the disc was heated to convert the deposit to oxide (Eu2 03). The thickness of the sources thus obtained ranged f r o m 40 to 75 #g/cm 2, when estimated from the weight and the area (150-200 t The authors are indebted to Mr. H. Takahashi of our laboratory for the complete sample preparation described here.
ALPHA DECAY OF Gd~5°
125
cm 2) assuming a sufficient uniformity. The corresponding energy loss of an alpha particle traversing the source perpendicularly was found to be 50 to 80 keV from the observed alpha-spectra. Alpha-spectra of these samples were measured with a T M C model 404 pulse-height, analyser. Fig. 3 shows one o f the spectra thus obtained. In the energy measurement, alpha particles from a Sm 147 source were simultaneously observed for comparison so that the resulting spectra gave two separate peaks, one corresponding to alpha particles from G d ts° and the other to those from Sm 147. The peaks are apparently both asymmetric in shape, indicating the effect o f source thickness. It is seen that the Gd ~5° (irradiated europium) source was somewhat thicker than the Sm 147 source. The calibration of the energy scale for the pulse-height spectra was performed on the basis of known energy values of alpha particles from U 234", U 23 s and Sm 147. For U 234 and U 23s alpha particles, the values 4.768-t-0.003 and 4.195+__0.005 MeV, respectively, were adopted after Harvey et aL ~2). Proper account was taken of the broadening of peaks in the uranium spectra due to alpha transitions to low-lying excited states of daughter nuclei, For Sm 147 alpha particles, the energy value of 2.23 +__ 0.02 MeV as reported by Macfarlane 13, ~4) was employed. A variety of somewhat smaller values around 2.18 MeV have been proposed previously for Sm 147 alphaparticles, and are listed in ref. x4). The reasons that we deleted all these older values and chose instead Macfarlane's larger value as an energy standard are as follows: (1) It refers to energy standards closest to Sm 147 and it gives a theoretical half-life in essential agreement with the observed one. (2) It just coincides with the recent independent result (E~(Sm t47) = 2.23+__0.01 MeV) o f Siivola 4). (3) It has been used as an energy standard by Toth and Rasmussen, whom we attempted to follow as closely as possible in order to facilitate comparison of our data with theirs. When the effects o f the electronic noise (75 keV in terms of F W H M ) and the source thickness were appropriately taken into consideration, a sufficiently linear relationship was obtained between the pulse-height and the alpha-particle energy for most o f the operating conditions. Indications of the presence of an appreciable ( ~ 200 keV) ionization defect were sometimes noticed, but the values were considerably different from one another depending on the operating conditions, so that no definite conclusion was drawn about its magnitude. The drift of the pulse-height was confirmed to be less than 0.5 Yo during each run o f the measurements, which usually took more than 24 h. The channel width o f the pulse-height analyser was found to correspond to about 25 keV from the calibration just described. The broadening of the G d 15° peak due to shielding inetticiency of the double-grid system in the ionization chamber was estimated to be less than 6 keV by a formula presented in ref. 6). The shift in the mean pulse-height caused by this broadening is at most 3 keV. The net error due to this shift in the energy determination would have been further reduced in the present measurement, since the pulse-heights due to alpha particles from sources for energy standards must have been also shifted by nearly the same amount. No explicit correction was therefore made for the effect. Finally,
126
I. OGAWA et al.
the effect of the variation of pulse rise time (collection time) on the resolution was believed to be still smaller. 2.5. A L P H A - P A R T I C L E E N E R G Y
O F G d 15°
From an analysis of the observed alpha-spectra of Gd 15 o by using the pulse-height versus energy relation mentioned above, again taking into account the electronic noise and the source thickness, the alpha-particle energy of Gd is o was finally determined to be 2.715_+0.018 MeV as an average over the best six spectra obtained. The error indicates the standard deviation of the observed values from the mean value. The alpha-energy thus determined lies just midway between the previous value of 2.70 4- 0.15 MeV reported by Rasmussen et al. 2, a) and the recent one of 2.73 +_0.01 MeV by Siivola 4). Considering the error involved in each measurement, the agreement may be said to be fairly good. The effective decay energy (disintegration energy) Qeff, which is equal to the sum o f the alpha-particle energy E~ and the recoil energy 2?r in the laboratory system plus the orbital electron screening correction AEsc, can be calculated for the Gd 15° alpha decay from the present data of E~. By use of an approximate expression 1), A~'se = 65.3Z 7/s - 80Z 2/5 eV, where Z is the atomic number of the parent nucleus or Z = 64 for the present case, Qcff turns out to be 2.811-+0.019 MeV. It should be noted again that this result is based on Macfarlane's value of 2.23 MeV for Sm vr7 alpha particles. I f a typical older 15) value of 2.18 MeV is used instead, the alpha-energy of Gd 15 o assumes a value (2.667-+ 0.019 MeV) considerably smaller than the aforementioned result. 2.6. H A L F - L I F E O F G d 16°
In order to estimate the half-life of Gd is o, the disintegration rate was determined from the observed counting rate. In an observed alpha spectrmn, backgrounds were first subtracted from the gross counts for each channel corresponding to the main part of the G d is° peak, assuming a constant background level over the relevant energy region. The low-energy tail of the resulting spectra for the net counts was then extrapolated down to zero pulse-height, regarding the tail as due to the effect of source thickness. The integrated counts under the whole peak (including the extrapolated part) were considered to be very close to one half of the total number of alpha particles emitted from the source during the counting period. The counting loss due to backscattering was estimated to be less than a few percent and was neglected throughout considering the limited accuracy of the number of Gd ~s° in the specimen. The disintegration rate thus obtained was 0.74 +0.07 dis./h per mg of irradiated Eu20a. Combining the rate with the number of Gd is° produced per gramme in the specimen (see subsect. 2.3), the alpha-decay half-life T~ of Gd is o was eventually estimated to be (1.4 + 0.4) x 106 y. The indicated error takes into account the uncertainty in the number of G d 1s o together with the standard deviation of the observed disintegration rates. The half-life obtained here is 4.7 times as large as the value of 3 x 10s y reported
ALPHA
DECAY
OF G d 15°
127
by Rasmussen et al. 2.3), and 14 times the value presented by Friedman et al. 5). On the other hand, it agrees fairly well with the value (2.1_+0.3) x 10 6 y obtained by Siivola 4) within a factor of 1.5. 3. Discussion
It was mentioned in sect. 1 that the previous value of experimental half-life observed by Rasmussen et al. was 28 times as small as the theoretical one calculated from the observed alpha-particle energy on the basis of a simple barrier-penetration theory. With our present data, however, a considerably better agreement is found between the experimental and theoretical half-lives. In evaluating the theoretical half-life, we used the same approximate formula for the decay constant 2 = (ln2)/T½ as used by Toth and Rasmussen 1). The formula is 2* n 2 h 2 2 = M ~ R a ( B _ E ) ~ e x p [ - 2 y ( Z , R)7(x)],
(1)
where the barrier-height B = 2Ze2/R, the effective decay energy E = Qeff, # ( Z , R ) = ( 2 e / h ) ( M Z R ) -~, 7(x) = x -~ arccos ( x ~ ) - ( l - x ) ~, x - E/B, and M is the reduced mass of an alpha particle. The effective nuclear radius R was determined to be 8.02 fm from the formula R = (1.58A~-0.30) fm, again following Toth and Rasmussen 1). For the effective decay energy of 2.811 MeV as estimated in subsect. 2.5 from the observed alpha-particle energy, eq. (1) gives a theoretical half-life of T~(theor.) = 6.2 x 106 y. This is still longer than our experimental half-life, T~(exp) = 1.4 x 106 y, but the ratio T~(theor)/T~(exp) is now only 4.4. In comparison with all the other even rare-earth alpha emitters examined by Toth and Rasmussen 1), this ratio is of quite the same order of magnitude. Incidentally, the independent (and one of the most recent) measurement by Siivola 4) again gives a comparable T~(theor)/T~(exp) ratio. We are thus lead to conclude that Gd is o does not constitute an isolated anomaly in the systematics of rare-earth alpha-decay. It is not clear why all the previous measurements, except for Siivola's, resulted in considerably shorter experimental alpha half-lives, of the order of 10 s y. On the other hand, the alpha energy of about 2.70-2.73 MeV appears to be established fairly well. The authors gratefully acknowledge Professor H. Morinaga o f the University of Tokyo for suggesting the problem and for pointing out the applicability of the present method of half-life determination to G d 15°. They also express their deep gratitude to Dr. T. Kuryoanagi and to the crew of the linear accelerator laboratory, the Japan Atomic Energy Research Institute, for their kind help in the irradiation of specimens. Furthermore, they are greatly indebted to Dr. M. Okano of the Institute of Physical and Chemical Research for valuable remarks on the beta ray measurement.
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References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)
K. S. Toth and J. O. Rasmussen, Nuclear Physics 16 (1960) 474 J. O. Rasmussen, S. G. Thompson and A. Ghiorso, Phys. Rev. 89 (1953) 33 J. O. Rasmussen, Rollier and K. S. Toth, unpublished, quoted in ref. 1) A. Siivola, Ann. Acad. Sci. Fennicae A 6 0962) 34 A. M. Friedman, J. Milsted and A. L. Harkness, Bull. Am. Phys. Soc. 8 (1963) 525 I. Ogawa, T. Dolce and M. Tsukuda, Nucl. Instr. 13 (1961) 169 T. Dolce, Can. J. Phys. 40 (1962) 607 P. Erd6s, P. Scherrer and P. StoU, Helv. Phys. Acta 30 (1957) 639 F. D. S. Butement, Proc. Phys. Soc. 64A (1951) 395 Y. Yoshizawa et al., Nuclear Physics 46 (1963) 78 R. C. Mack, J. J. Neuer and M. L. Pool, Phys. Rev. 91 (1953) 903 B. G. Harvey, H. G. Jackson, T. A. Eastwood and (3. C. Hanna, Can. J. Phys. 35 (1957) 258 R. D. Macfarlane, Dissertation, Department of Chemistry, Carnegie Institute of Technology, NYO-7687 (May 1959) 14) R. D. Macfarlane and T. P. Kohman, Phys. Rev. 121 (1961) 1758 15) W. P. Jesse and J. Sadauskis, Phys. Rev. 78 (1950) 1
A L P H A D E C A Y O F Gd Is° IWAO OGAWA, TADAYOSHI DOKE, MITSUHIRO MIYAJIMA Department of Physics, Rikkyo University, Toshima-ku, Tokyo, Japan and ATSUSHI NAKAMOTO Institute for Atomic Energy, Rikkyo University, Yokosuka-shi, Kanagawa-ken, Japan Received 29 October 1964 Abstract: The alpha particle energy and the half-life of Gd 15°have been measured with a double-grid ionization chamber. Backgrounds are minimized by the grid-collector coincidence technique. The alpha particle energy is determined as 2.7154-0.018 MeV by referring to that of Sm147 (Ea = 2.234-0.02 MeV) reported by Macfarlane. The half-life is estimated to be (1.44-0.4)× 10e y from the observed counting rate and the total number of Gd 16° nuclei contained in the specimen. The number is estimated from the observed beta-decay rate of the parent nuclei Eu 15°, which are produced by a photonuclear reaction Eu~Sl(y, n)Eu ~°. The experimental halflife thus obtained agrees within a factor of 4.4 with the theoretical half-life calculated from the observed alpha-particle energy on the basis of a conventional barrier-penetration theory. The results show that Gd ~5° behaves quite normally in the systematics of rare-earth alpha-decay. I E
RADIOACTIVITY Gd ~5° [from Eu1~1(7, n)EuXS°(fl-); measured T½~,E~. Natural target.
]
1. Introduction
In their extensive study o n the systematics o f alpha-radioactivity in the rare-earth region, T o t h and Rasmussen t) have c o m p a r e d the experimental half-lives o f a n u m ber o f nucleides with the theoretical calculations f r o m the observed alpha-particle energies using a simple barrier-penetration theory. They have noted that the experimental and theoretical half-lives o f all the even isotopes examined are in agreement within a factor o f 5 with the single exception o f G d 15°, whose theoretical half-life, 8.37 x 106 y as c o m p u t e d f r o m the observed alpha-energy o f 2.70_+0.15 M e V (refs. 2, 3)), is 28 times as large as its experimental half-life available at the time (,~ 3 x 105 y with an error factor o f 2). This discrepancy has been considered to be p r o b a b l y due to fairly large errors involved in the measurements. Indeed, there seems to be no particular reason for G d is° only to behave anomalously a m o n g the neighb o u r i n g even nucleides. These considerations b y T o t h and Rasmussen once appeared to be actually confirmed experimentally, when Siivola 4) reported in 1962 an experimental half-life as long as (2.1 _+0.3)x 106 y, together with a reasonable experimental alpha-energy o f 2.73-t-0.01 MeV. These data are fairly consistent a m o n g themselves f r o m the view119
I. OGAWA et al.
120
point of conventional barrier-penetration theory. Recently, however, the confirmation again became not so conclusive. Namely, Friedman, Milsted and Harkness s) presented last year a value as small as (1.0+0.6)x l0 s y as the experimental half-life of Gd 15o. Since it is of primary importance in the alpha-decay systematics to establish a correct half-life energy correlation for even isotopes, we have attempted to remeasure the alpha-energy and the half-life of Gd is° as accurately as possible, by performing a low-background counting with a double-grid ionization chamber. The results are in good agreement with those of Siivola and indicate again that Gd is o does n o t constitute a special anomaly among the neighbouring isotopes. An account is given in sect. 2 of the experimental procedures and results. In sect. 3, a comparison is made between the theoretical and experimental half-lives, using the same formula as used by Toth and Rasmussen. 2. Experimental Procedures and Results
The Gd 15° nucleus was obtained as a beta-decay product of Eu ~5°, which was produced by the photoreaction Eu151(7, n)Eu 15°. The alpha-particle energy was measured with a low-background ionization chamber of the double-grid type 6, 7), using alpha particles from Sm 1*7, U 234 and U 23 s as energy standards. On the other hand, the half-life was determined from the observed counting-rate of alpha particles and the total number of Gd 15o nuclei in the specimen as estimated from the observed beta-decay rate of Eu 1s o. 2.1. P R O D U C T I O N
O F E u xS°
Powders of natural europium oxide (Eu2 03, purity 99.9 ~ ) were irradiated continuously for about 10 h with the bremsstrahlung X-rays from the 18 MeV electron linear accelerator at the Japan Atomic Energy Research Institute, Tokai-mura. The X-ray dose rate near the specimen was estimated to be of the order of l0 s R/h. Since natural europium is composed of two isotopes, Eu lsl (47.8 ~o) and Eu ls3 (52.2 ~), with nearly equal relative abundances and since the (7, n) cross section is not expected to dilfer appreciably between the both isotopes 8), we should obtain almost the same amount of Eu 15° and Eu ~s2 by the irradiation. No other isotopes were believed to be produced to such an amount that could affect appreciably the present measurement for the following reasons: (1)The products of (7, P) reactions in europium are stable isotopes. In addition, the cross sections for the reactions are very small. (2) The (% ~) reactions may be perfectly ignored since the cross sections are ten thousand times as small as those for (% n) reactions 8). (3)The isotope 015 produced from 0 26 by the (7, n) reaction may be ignored since the cross section for the reaction is only one twentieth of that for Eu ~s° or Eu 152 production s) and the half-life of O ~s is as short as 2.0 rain. (4) The possible (% n) products from the minor impurities such as Sm, Gd or Nd, which might have been present (less than
121
ALPHA DECAY OF Gd 1~0
0.2 %) in the specimen give rise to a negligible alpha or beta activity as compared to those for Eh is°, Eu 152 and their decay products. This may readily be expected from the magnitude o f cross sections for relevant (~,, n) reactions and of half-lives o f resuiting reaction products. (5) The (~, 2n) and (~, np) reactions are without importance because the great majority o f incident protons were o f quantum energies insu~eient to liberate two nucleons. It should be mentioned that the slow neutron flux that arose as a result of slowingdown of photoneutrons was fairly small ( ~ 10 5 n/cm 2 • sec) near the specimen, so that the number of Eu ls2 nuclei that could be produced by the Eu151(n, ~) reaction was estimated to be less than 1 % o f the number of those which were produced by the EulS3(~, n)Eu 152 reaction. Thus it was considered that Eu 15° and Eu lsz should have been produced in a proportion fairly close to the relative abundance of Eu ~51 and Eu 153 in natural europium t. The fact was used to check the results o f the analysis described in subsect. 2.3. 2.2. BETA DECAY OF THE IRRADIATED SPECIMEN The irradiated powders of europium oxide were made into a beta-source, about 1 mg/cm 2 thick, by suspending the powders in alcohol and by letting the solvent eva-
to g
10' .c G
g
| IO11
/
1O /
0
:=
I
I
50
tO0
I Time
150 (h)
t'
I
I
200
250
~ :
I
Fig. 1. Decay curve of the irradiated europium oxide. porate naturally. Fig. 1 gives the decay curve o f the source obtained by the measurement with a G M counter having a mica window 1.6 mg/cm 2 thick. It is seen from the figure that the decay curve consists of two parts, the one decaying with an average half-life of about 12 h and the other with a much longer half-life. t A possible competing reaction EulSS(~,n)Eu152mis tentatively neglected here.
122
i. OGAWA et aL
The former may be ascribed to the short-lived beta decays of Eu 150 and Eu 152, while the latter presumably is due to the 13-y beta decay of Eu 152m together with other slower decays of unidentified reaction products. This assignment was partly confirmed by the gamma-ray spectrometry made with a 7.6 c m x 7.6 cm NaI(TI) crystal. Namely, the scintillation spectrum observed in the early period of decay showed a pattern characteristic of 9.2 h decay t of Eu 152, while the spedtrum for the slowly decaying period contained several peaks which may be regarded as transitions associated with the 13 y decay of Eu 152. 2.3. E S T I M A T I O N O F T H E T O T A L A M O U N T
O F G d 15° P R O D U C E D
The following two methods were adopted to estimate the activity of Eu is° just after the irradiation, from which the total number of Gd 15° nuclei, both produced and to be produced, was calculated. O) The method of decay curve. From the gross decay curve as given in fig. 1, we first subtract the long-lived component. The remainder will then be decomposed into two components, one decaying with a half-life of 13-14 h and the other with a 9.5 h half-life. The former value is in good agreement with the half-life of 13.7 h for Eu is° reported by Mack et al. 11), while the latter with 9.2 h for Eu 152 as measured by Butement 9). (ii) The method of absorption curve. If a specimen of natural Eu2 03 is irradiated with thermal neutrons, the beta activity to appear will be largely due to Eu 152, because the neutron capture (absorption) cross section for Eu lsl (9000+3000 b) is about 20 times as large compared to that for Eu 15a and more than 5 × 10~ times that for 016. Moreover, the activity of 017, if any, will decay in only a few minutes. Therefore, we can estimate the initial activity of Eu is o by subtracting the absorption curve in alnminium for the neutron-irradiated specimen (n, 7) products from that for the X-ray irradiated one ((7, n) products). In the present experiment, thermal neutrons were produced to irradiate a Eu2 03 specimen by moderating in water the d--d neutrons from the Cockcroft-Walton accelerator at the Rikkyo University. The absorption curve for the (n, 7) products is given in fig. 2 with the curve for the (7, n) products. In carrying out the subtraction, we brought both curves into approximate contact at the point (indicated by an arrow in fig. 2) where the absorber thickness corresponded to the maximum range ( ~ 400 mg/cm2 in aluminium) of beta particles from Eu is°. This was in order to take into account that the tails of both curves should not necessarily coincide with each other because of the possible effects of bremsstrahlung X-ray from the beta particles and of characteristic X-rays and gamma rays from the specimen. The above mentioned point near the maximum range may be regarded to be much less affected by these factors. t It h a s been reported that E u TM decays also by electron capture a n d p o s i t o n emission to S m x~° a n d emits s o m e g a m m a rays x0). However, the rate o f g a m m a emission is only a few percent o f t h e beta decay, so that the observed s p e c t r u m m a y be considered to be largely due to E u TM.
ALPHA
DECAY
OF
123
Gd ls°
The resulting difference curve, which is given also in fig. 2 by a dotted curve, should represent the absorption curve solely due to Eu 15°. The curve gives a half-value layer (HVL) of 51 + 7 m g / c m 2 for the beta-ray. The m a x i m u m energy Epmax o f the beta particles is thus assumed to be 1.07+0.10 MeV, using the well-known empirical relation between Epma~ and HVL. The last value is in excellent agreement with 1.07 MeV for Eu 15° reported by Mack etaL 11). Furthermore, the beta activity o f Eu 1s o just after the X-ray irradiation as estimated f r o m the curve is in good accordance with the previous results o f the decay curve method. IOz
) - prOducts
104
102
IO /
i
T
i
I
0
i
I
T
I
I
1 I000
mg/cm
2
500
Thickness o f AI
in
I
I
I
Fig. 2. Absorption curves in aluminium for the X-ray and thermal-neutron irradiated europium oxide. The arrow indicates the maximum range in aluminium of beta-particles from Eux6°.The dotted curve represents the difference between the two absorption curves.
In order to obtain the absolute disintegration rate for the Eu 15° beta decay, the overall counting efficiency of the G M counter system was estimated by using a reference source o f R a E with a known disintegration rate. Since the m a x i m u m energy of the beta particles f r o m R a E (Ep, ma, = 1.17 MeV) is fairly close to that for Eu is° and since the size, the thickness and the geometry of the R a E source used were made almost identical with those of the Eu 15 o source, the counting efficiencies for the both sources were safely presumed to be practically equal. On the basis of the beta disintegration rate of Eu 15 o thus obtained, the total number of G d 1~o nuclei produced was calculated to be (1.3___0.3)x 101° per mg of Eu2Oa. 2.4. ALPHA-SPECTRUM OF Gd15° The alpha-spectrum of G d 15 o was obtained with a double-grid ionization chamber by mounting a thin, wide sample of irradiated europium oxide on the high voltage electrode o f the chamber.
124
i. OGAWAet
al.
The chamber was filled to a pressure of 3 atm with a mixture o f argon and methane (5 70). In view o f the very low counting rate, backgrounds were minimized by counting only those collector pulses which were accompanied by a negative pulse from the first grid o f the chamber. Details o f this method o f background reduction have been described in ref. 7). As a result of this coincidence technique, a background level lower than 1.5 count per hour per 1 MeV interval was achieved in the energy region of a few MeV.
40
S~ 4r
GdI~°
50
e-
20
.
.
~0
.
.
.
90
.
.
I00
I10
120
150
Chonnel Number
Fig. 3. Alpha spectrum of Gd1so. The Smx47 alpha particles were also observed simultaneously for comparison. The samples were prepared by the following procedures t: Irradiated powders o f europium oxide were first converted to nitrate and dissolved in alcohol. The solution was then spread over a backing disc of stainless steel, 12 cm in diameter and 0.8 m m thick, and was dried by natural evaporation. In the course o f evaporation, the disc was floated on a mercury surface to keep it horizontal. Finally, the disc was heated to convert the deposit to oxide (Eu2 03). The thickness of the sources thus obtained ranged f r o m 40 to 75 #g/cm 2, when estimated from the weight and the area (150-200 t The authors are indebted to Mr. H. Takahashi of our laboratory for the complete sample preparation described here.
ALPHA DECAY OF Gd~5°
125
cm 2) assuming a sufficient uniformity. The corresponding energy loss of an alpha particle traversing the source perpendicularly was found to be 50 to 80 keV from the observed alpha-spectra. Alpha-spectra of these samples were measured with a T M C model 404 pulse-height, analyser. Fig. 3 shows one o f the spectra thus obtained. In the energy measurement, alpha particles from a Sm 147 source were simultaneously observed for comparison so that the resulting spectra gave two separate peaks, one corresponding to alpha particles from G d ts° and the other to those from Sm 147. The peaks are apparently both asymmetric in shape, indicating the effect o f source thickness. It is seen that the Gd ~5° (irradiated europium) source was somewhat thicker than the Sm 147 source. The calibration of the energy scale for the pulse-height spectra was performed on the basis of known energy values of alpha particles from U 234", U 23 s and Sm 147. For U 234 and U 23s alpha particles, the values 4.768-t-0.003 and 4.195+__0.005 MeV, respectively, were adopted after Harvey et aL ~2). Proper account was taken of the broadening of peaks in the uranium spectra due to alpha transitions to low-lying excited states of daughter nuclei, For Sm 147 alpha particles, the energy value of 2.23 +__ 0.02 MeV as reported by Macfarlane 13, ~4) was employed. A variety of somewhat smaller values around 2.18 MeV have been proposed previously for Sm 147 alphaparticles, and are listed in ref. x4). The reasons that we deleted all these older values and chose instead Macfarlane's larger value as an energy standard are as follows: (1) It refers to energy standards closest to Sm 147 and it gives a theoretical half-life in essential agreement with the observed one. (2) It just coincides with the recent independent result (E~(Sm t47) = 2.23+__0.01 MeV) o f Siivola 4). (3) It has been used as an energy standard by Toth and Rasmussen, whom we attempted to follow as closely as possible in order to facilitate comparison of our data with theirs. When the effects o f the electronic noise (75 keV in terms of F W H M ) and the source thickness were appropriately taken into consideration, a sufficiently linear relationship was obtained between the pulse-height and the alpha-particle energy for most o f the operating conditions. Indications of the presence of an appreciable ( ~ 200 keV) ionization defect were sometimes noticed, but the values were considerably different from one another depending on the operating conditions, so that no definite conclusion was drawn about its magnitude. The drift of the pulse-height was confirmed to be less than 0.5 Yo during each run o f the measurements, which usually took more than 24 h. The channel width o f the pulse-height analyser was found to correspond to about 25 keV from the calibration just described. The broadening of the G d 15° peak due to shielding inetticiency of the double-grid system in the ionization chamber was estimated to be less than 6 keV by a formula presented in ref. 6). The shift in the mean pulse-height caused by this broadening is at most 3 keV. The net error due to this shift in the energy determination would have been further reduced in the present measurement, since the pulse-heights due to alpha particles from sources for energy standards must have been also shifted by nearly the same amount. No explicit correction was therefore made for the effect. Finally,
126
I. OGAWA et al.
the effect of the variation of pulse rise time (collection time) on the resolution was believed to be still smaller. 2.5. A L P H A - P A R T I C L E E N E R G Y
O F G d 15°
From an analysis of the observed alpha-spectra of Gd 15 o by using the pulse-height versus energy relation mentioned above, again taking into account the electronic noise and the source thickness, the alpha-particle energy of Gd is o was finally determined to be 2.715_+0.018 MeV as an average over the best six spectra obtained. The error indicates the standard deviation of the observed values from the mean value. The alpha-energy thus determined lies just midway between the previous value of 2.70 4- 0.15 MeV reported by Rasmussen et al. 2, a) and the recent one of 2.73 +_0.01 MeV by Siivola 4). Considering the error involved in each measurement, the agreement may be said to be fairly good. The effective decay energy (disintegration energy) Qeff, which is equal to the sum o f the alpha-particle energy E~ and the recoil energy 2?r in the laboratory system plus the orbital electron screening correction AEsc, can be calculated for the Gd 15° alpha decay from the present data of E~. By use of an approximate expression 1), A~'se = 65.3Z 7/s - 80Z 2/5 eV, where Z is the atomic number of the parent nucleus or Z = 64 for the present case, Qcff turns out to be 2.811-+0.019 MeV. It should be noted again that this result is based on Macfarlane's value of 2.23 MeV for Sm vr7 alpha particles. I f a typical older 15) value of 2.18 MeV is used instead, the alpha-energy of Gd 15 o assumes a value (2.667-+ 0.019 MeV) considerably smaller than the aforementioned result. 2.6. H A L F - L I F E O F G d 16°
In order to estimate the half-life of Gd is o, the disintegration rate was determined from the observed counting rate. In an observed alpha spectrmn, backgrounds were first subtracted from the gross counts for each channel corresponding to the main part of the G d is° peak, assuming a constant background level over the relevant energy region. The low-energy tail of the resulting spectra for the net counts was then extrapolated down to zero pulse-height, regarding the tail as due to the effect of source thickness. The integrated counts under the whole peak (including the extrapolated part) were considered to be very close to one half of the total number of alpha particles emitted from the source during the counting period. The counting loss due to backscattering was estimated to be less than a few percent and was neglected throughout considering the limited accuracy of the number of Gd ~s° in the specimen. The disintegration rate thus obtained was 0.74 +0.07 dis./h per mg of irradiated Eu20a. Combining the rate with the number of Gd is° produced per gramme in the specimen (see subsect. 2.3), the alpha-decay half-life T~ of Gd is o was eventually estimated to be (1.4 + 0.4) x 106 y. The indicated error takes into account the uncertainty in the number of G d 1s o together with the standard deviation of the observed disintegration rates. The half-life obtained here is 4.7 times as large as the value of 3 x 10s y reported
ALPHA
DECAY
OF G d 15°
127
by Rasmussen et al. 2.3), and 14 times the value presented by Friedman et al. 5). On the other hand, it agrees fairly well with the value (2.1_+0.3) x 10 6 y obtained by Siivola 4) within a factor of 1.5. 3. Discussion
It was mentioned in sect. 1 that the previous value of experimental half-life observed by Rasmussen et al. was 28 times as small as the theoretical one calculated from the observed alpha-particle energy on the basis of a simple barrier-penetration theory. With our present data, however, a considerably better agreement is found between the experimental and theoretical half-lives. In evaluating the theoretical half-life, we used the same approximate formula for the decay constant 2 = (ln2)/T½ as used by Toth and Rasmussen 1). The formula is 2* n 2 h 2 2 = M ~ R a ( B _ E ) ~ e x p [ - 2 y ( Z , R)7(x)],
(1)
where the barrier-height B = 2Ze2/R, the effective decay energy E = Qeff, # ( Z , R ) = ( 2 e / h ) ( M Z R ) -~, 7(x) = x -~ arccos ( x ~ ) - ( l - x ) ~, x - E/B, and M is the reduced mass of an alpha particle. The effective nuclear radius R was determined to be 8.02 fm from the formula R = (1.58A~-0.30) fm, again following Toth and Rasmussen 1). For the effective decay energy of 2.811 MeV as estimated in subsect. 2.5 from the observed alpha-particle energy, eq. (1) gives a theoretical half-life of T~(theor.) = 6.2 x 106 y. This is still longer than our experimental half-life, T~(exp) = 1.4 x 106 y, but the ratio T~(theor)/T~(exp) is now only 4.4. In comparison with all the other even rare-earth alpha emitters examined by Toth and Rasmussen 1), this ratio is of quite the same order of magnitude. Incidentally, the independent (and one of the most recent) measurement by Siivola 4) again gives a comparable T~(theor)/T~(exp) ratio. We are thus lead to conclude that Gd is o does not constitute an isolated anomaly in the systematics of rare-earth alpha-decay. It is not clear why all the previous measurements, except for Siivola's, resulted in considerably shorter experimental alpha half-lives, of the order of 10 s y. On the other hand, the alpha energy of about 2.70-2.73 MeV appears to be established fairly well. The authors gratefully acknowledge Professor H. Morinaga o f the University of Tokyo for suggesting the problem and for pointing out the applicability of the present method of half-life determination to G d 15°. They also express their deep gratitude to Dr. T. Kuryoanagi and to the crew of the linear accelerator laboratory, the Japan Atomic Energy Research Institute, for their kind help in the irradiation of specimens. Furthermore, they are greatly indebted to Dr. M. Okano of the Institute of Physical and Chemical Research for valuable remarks on the beta ray measurement.
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References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)
K. S. Toth and J. O. Rasmussen, Nuclear Physics 16 (1960) 474 J. O. Rasmussen, S. G. Thompson and A. Ghiorso, Phys. Rev. 89 (1953) 33 J. O. Rasmussen, Rollier and K. S. Toth, unpublished, quoted in ref. 1) A. Siivola, Ann. Acad. Sci. Fennicae A 6 0962) 34 A. M. Friedman, J. Milsted and A. L. Harkness, Bull. Am. Phys. Soc. 8 (1963) 525 I. Ogawa, T. Dolce and M. Tsukuda, Nucl. Instr. 13 (1961) 169 T. Dolce, Can. J. Phys. 40 (1962) 607 P. Erd6s, P. Scherrer and P. StoU, Helv. Phys. Acta 30 (1957) 639 F. D. S. Butement, Proc. Phys. Soc. 64A (1951) 395 Y. Yoshizawa et al., Nuclear Physics 46 (1963) 78 R. C. Mack, J. J. Neuer and M. L. Pool, Phys. Rev. 91 (1953) 903 B. G. Harvey, H. G. Jackson, T. A. Eastwood and (3. C. Hanna, Can. J. Phys. 35 (1957) 258 R. D. Macfarlane, Dissertation, Department of Chemistry, Carnegie Institute of Technology, NYO-7687 (May 1959) 14) R. D. Macfarlane and T. P. Kohman, Phys. Rev. 121 (1961) 1758 15) W. P. Jesse and J. Sadauskis, Phys. Rev. 78 (1950) 1