Volume 92B, number 3,4 PHYSICS LETTERS 19 May 1980

NUCLEAR EXCITATION BY ELECTRON TRANSITION (NEET) IN 237Np

FOLLOWING K-SHELL PHOTOIONIZATION

Tadashi SAITO, Atsushi SHINOHARA and Kiyoteru OTOZAI Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan

Received 24 March 1980

Deexcitation 7-rays of 237 Np were detected during irradiation of 237 Np with the 7-radiation from 57 Co. This is ex- plained as a result of the nuclear excitation of the 103 keV level in 237Np induced by the KL 3 electronic radiationless transition following K-shell photoionization. The probability for nuclear excitation is found to be 2.1 × 10 -4 per created K-hole.

As is well established, muons in the muonic atoms of rather heavy elements may cascade to the inner shells by excitation of the nucleus as well as by ordina- ry radiative and radiationless transitions [ 1 ] . S u c h nuclear excitat ion can also occur in usual (electron) atoms. The process, which was first discussed by Morita [2] and named nuclear excitation by electron transition (NEET), should have a relatively large prob- ability if the nuclear and electronic transitions have nearly equal energies, E N ~ EA, and the same multi- polarity. Experimental evidence for such a process was given in 189Os [3,4], where NEET has a positive A-value (A = E A - -EN) and where it proceeds by E2 radiation.

The nuclide 237Np is another candidate which satisfies the NEET conditions: small A (negative) and common multipolarity. The nuclear excitat ion of the 102.95 keV level in 237Np proceeds by E1 as does the electronic transition between the K and L 3 shells with an energy of 101.072 keV. As shown in fig. 1, NEET operates by the exchange of virtual photons between these transitions.

The present letter reports the observation of NEET in 237Np by irradiating a 237Np sample with photons. The 7-rays from 57Co were used to ionize the K-shell of the 237Np atoms. These 7-rays are suitable by hav- ing energies (122 and 136 keV) slightly higher than the K-binding energy of Np (119 keV). Among sev- e rn 7-rays emitted in the decay of the 103 keV level in 237Np, the most intense is the 60 keV 7-ray which was chosen as an index of NEET. The probabil i ty for

K 1 1 8 . 6 8 0 7 / 2 - 102 .95

5 / 2 - ] 59 .53 (

(NEET) 7/ 3 3 . 2 0

L 3 1 7 . 6 0 8 5/2+] ~ ' ~ 0 . 0

(ELECTRON HOLE) ( NUCLEUS )

Fig. 1. NEET diagram for 237Np. The numerical values in the brackets indicate the photon intensities per 100 disinte- grations of the 103 keV level. The most intense 3,-ray at 60 keV was detected as an indication of NEET.

NEET, P, can be obtained in a simple way from the relative intensity of the 60 keV "r-ray and the fluor- escence K X-rays.

The oxide of 237Np supplied by ORNL was radio- chemically impure because it contained 238pu and 241 Am. These contaminants were removed by an anion exchange technique similar to that described in ref. [5]. Neptunium was precipitated as the tetra- fluoride and collected onto a 7.4 mg/cm 2 thick teflon filter, leaving its daughter 233pa in the filtrate. The precipitate, 2 cm 2 in area, together with the filter, was placed between 0.9 mg/cm 2 thick mylar films. This assembly containing 266 mg of 237NpF 4 was used as a target. A similar assembly containing the same amount of 238UF 4 was prepared for comparison pur- poses.

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Volume 92B, number 3,4 PHYSICS LETTERS 19 May 1980

The 4.7 mCi 57Co source and a detector were placed perpendicularly to each other at distances of 2 cm and 15 cm from the target, respectively. The detector was a hyperpure Ge low-energy photon spectrometer with an active volume of 1.4 cm 3. The output pulses were fed to a 4-kch pulse-height analyzer through a linear amplifier equipped with a pile-up rejector cir- cuit. The resolution of this system was typically 390 eV for 60 keV photons at count rates below 1500 s- 1. The photon spectrum was taken in the singles mode for 2.1 X 105 s (see fig. 2).

The photopeak areas were calculated by using the BOB75 code developed by Baba et al. [6] for analysis of T-spectra. The relative efficiency e of the detector, including correction for self-absorption in the target, was easily obtained by comparing the observed inten- sities to those given in the literature [7].

In the region near 60 keV, no peaks were recog- nized by the automatic peak-search routine of the BOB 75 code. Consequently, that region was analyzed separately in the following manner. The second deriv- ative of the data points clearly indicates the presence of a peak at 59.5 keV with a width close to that ex-

~06

105

lo 4

}03

10 2

pected for a single photopeak. The baseline counts in the peak region were interpolated by a cubic expres- sion fitted to the data points adjacent to this peak region, the points outside the full width at one tenth of the presumed maximum of the peak. The reduced X 2 value in this fit is 0.83 for 30 degrees of freedom, and the obtained expression has inflection points outside the fitted and peak regions. The remaining counts form a symmetrical distribution centered at 59.50 keV, the energy being extremely close to that of the T-ray to be detected, 59.536 keV. Its full width at half maximum is consistent with those of the near- est neighbors within a difference of 10%. From these facts, it may be judged that the 60 keV T-ray was ob- served in the photon spectrum of 237Np being irradi- ated with the 57Co T-rays.

Inelastic scattering of the 57Co ")'-rays at the target was checked by a measurement with a similar target replaced by 238U. Since no peaks could be extracted in the vicinity of 60 keV by the same procedure as above, the 60 keV peak is not ascribable to inelastic- ally scattered T-rays.

The interfering components, listed in table 1, were

' 5 0 0 ' '

237NpF4 .,. 57CO

5 9 . 5 k e Y r

I !

500 {

-500, J , L , ,,

910 930 950 I I i I L J I I J .

~ooo- ~5oo

Jp-X K~m

2o'0o '

CHANNEL NUMBER

Fig. 2. The photon spectrum of 237Np irradiated with 57Co ~,-rays with an accumulation time of 3 × 10 -4 s. After that time, counts in the peak regions overflowed the memory capacities of MCA. The results of analysis for the region near 59.5 keV is also shown.

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Volume 92B, number 3,4 PHYSICS LETTERS 19 May 1980

Table 1 Comparison of the estimated intensities for interfering com- ponents of various origin and the observed intensity for the 60 keV peak in the spectrum obtained with a combination of 237Np and SVCo.

Origin Count rate (s -1 )

Gamma-rays from 241Am (4.2 -+ 0.5) X 10 .4 Sum coincidence 1.3 X 10 .4 Pulse pile-up <1 X 10 -s Gamma-rays following

-/-resonance absorption <4 X 10 .6 Observed {2.2 ± 0.5) X 10 .2

estimated as follows. The alternative origin of the 60 keV y-ray is probably 241Am that may have remained in the target owing to insufficient removal of the element. The 241am content was assayed ef- ficiently by chemical isolation of 241 Am with the Nd carriers from the target.

In the decay of 237Np, sum peaks at 59.82 and 59.65 keV may appear if the 46.43 keV y-ray is added to the Pa L~ 1 and I ~ 2 X-rays emitted in a cascade transition, respectively. The intensity ratios of the sum peaks to the original y-ray peak are evaluated to be 10 -4 . In addition there is a possibility that pulse pile-up generates broad pseudopeaks at positions near the sum peaks. This effect was smaller by a factor of 10 than the sum effect in this experimental condition.

The main process competing with NEET is the res- onance absorption of y-rays by the 60 keV and 103 keV levels in 237Np. An upper limit for this contribu-

tion was set by assuming a somewhat extreme case. As is evident from table 1, the 60 keV peak cannot

be explained except by NEET in 237Np. The NEET probability can be obtained from the observed count rate for the 60 keV y-ray, C.y, and that for the fluor- escence Np K~ 1 X-ray at 101 keV, CK,~!, as

P _ C~. e 101 °°KI'R (Kcq) , 0)

CKc~ a e60B PR (K)

where B is the probability for emission of the 60 keV y-ray per disintegration of the 103 keV level, co K the Np K-shell fluorescence yield, PR (K%) the natural width of the K~ 1 line, PR(K) the total radiative width of the K shell, and e L. the photopeak efficiency of this detection system for photons o f E keV. With the measured value of (C-//CK~ a)(el01 / e60) = (1.6 + 0.4)

X 10 - 4 and the cited ones for co K [8], B [7], and PR(K%) /PR(K) [9], one obtains P = (2.1 -+ 0.6) X 10 -4 .

The Coulomb interaction energy between the nu- cleus and the orbital electron, E ' , is approximately related to P in this case as

lz" = --I AI (P/[1 + P(L3)/P(K)] } 1/2, ( 2 )

where P(L3) and P(K) are the total widths of the L 3 and K shells, respectively. The ratio P ( L 3 ) / P ( K ) can be evaluated as the product of PR(L3)/PR(K) [9] and COK/CO 3 [ I0 ] , where PR(L3) and 003 are the total radiative width and the fluorescence yield of the L 3 subshell, respectively. The value of E ' is thus de- termined experimentally as E' = - 2 6 + 4 eV.

A crude estimate by eq. (6) in ref. [4] gave E ' = - 2 . 5 4 f k e V , where f i s the correction factor due to the collective character of the nuclear transition. From the experiment, one obtains f ~ 10 . 2 as a reasonable value. This small f-value (P ocf2 ~ 10-4) is qualita- tively understandable by the fact that the rates of low- energy E1 transitions in odd-A nuclei of the actinide elements are retarded by an order of 4 - 5 with respect to that calculated by the sinde-oroton formula [11 ].

In summary, NEET in 237Np, subsequent to 189Os ' was successfully observed in the singles photon spec- trum.

The authors would like to acknowledge tile helpful advice and contribution of Prof. I. Fujiwara (Kyoto University) to the target preparation. They are indebted to Profs. N. Kunitomi and Y. Nakai (Osaka Univer- sity) for their making a 57Co source available for this study. The support of Prof. T. Nishi and Dr. H. Mor- iyama (Kyoto University) in the purification of 237Np is gratefully appreciated. This work was supported in part by a Grant-in-Aid from the Ministry of Educa- tion.

References

[1] See, e.g., S. Devons and I. Duerdoth, Adv. Nucl. Phys. 2 (1969) 295; J. Hiifner, F. Scheck and C.S. Wu, in: Muon physics, eds. V.W. Hughes and C.S. Wu, Vol. 1 (Academic Press, New York, 1977) p. 202.

[2] M. Morita, Prog. Theor. Phys. 49 (1973) 1574. [3] K. Otozai, R. Arakawa and M. Morita, Prog. Theor.

Phys. 50 (1973) 1771.

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[4] K. Otozai, R. Arakawa and T. Saito, Nucl. Phys. A297 (1978) 97.

[5] I. Fujiwara, N. Imanishi and T. Nishi, to be published. [6] H. Baba et al., J. Nucl. Sci. Tech. 8 (1971) 703;

H. Baba, T. Sekine, S. Baba and H. Okashita, JAERI 1227 (1972).

[7] Y.A. Ellis, Nucl. Data Sheets 23 (1978) 71; 24 (1978) 289.

[8] I. Ahmad, Z. Phys. A290 (1979) 1.

[9] J.H. Scofield, At. Data Nucl. Data Tables 14 (1974) 121.

[10] R.W. Fink and P.V. Rao, in: Handbook of spectro- scopy, ed. J.W. Robinson, Vol. 1 (CRC press, Cleveland, 1974) p. 219.

[11] E.K. Hyde, I. Perlman and G.T. Seaborg, The nuclear properties of the heavy elements, Vol. 1 (Prentice- Hall, Englewood Cliffs, NJ, 1964) p. 187.

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