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237Np EMISSION SPECTRA IN 241Am : AmO2,
AmAs and AmBi SOURCES
J. Friedt, R. Poinsot, J. Rebizant, W. Muller
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au na 12, Tome 37, Décembre 1976, page C6-935
237
Np EMISSION SPECTRA IN
241Am : Am0
2, AmAs
and AmBi SOURCES
J. M. FRIEDT, R. POINSOT
Centre de Recherches Nucleaires, 67037 Strasbourg Cedex, France J. REBIZANT and W. MULLER
Institut Europeen des Transuraniens, 75 Karlsruhe, R. F. A.
Résumé. — Les spectres d'émission de 237Np dans des sources de 241Am : Am02, AmAs et AmBi révèlent plusieurs états de charge des ions de recul ainsi que des atomes isovalents de neptunium en substitution dans les sites d'américium. Cette situation représente les conséquences des processus de recul et d'ionisation déclenchés par la désintégration a de 241Am. L'observation d'états ioniques définis et bien distincts indique un caractère non métallique pour tous ces composés. L'étude détaillée de la variation des spectres de AmC>2 en fonction de la température permet de confirmer l'apparition d'un ordre magnétique dans ce composé vers 8 K ; aucune évidence pour l'existence d'une transition magnétique n'est obtenue dans AmAs et AmBi.
Abstract. — The 237Np emission spectra in 241Am sources of Am02, AmAs and AmBi reveal several ionized configurations of the recoil species in addition to isovalent neptunium species substituted in the americium sites. These are the consequences of the recoil and electronic shake-off processes triggered by the a-decay of 241Am. The observation of well defined discrete charge states suggests a non-metallic character for these compounds. The detailed temperature dependence of the spectra of AmO 2 confirms the magnetic ordering at ~ 8 K in this compound whereas no evidence for a magnetic transition is obtained in AmAs and AmBi.
1. Introduction. — The electronic, structural and
magnetic properties of americium compounds can in principle be investigated using either the 59 keV resonance of 2 3 7Np fed by a-decay of 2 4 1Am or the 84 keV resonance in 2 4 3Am. The former technique of emission spectroscopy suffers however both from the fact that one investigates impurity atoms in the host and from the possibility of severe recoil-and electronic after effects expected to be associated with the nuclear decay. The emission of the 5.5 MeV a-particle indeed imparts to the decayed 2 3 7Np nucleus a recoil energy (ER ~ 93 keV) greatly
exceed-ing the energy required to displace an atom in a solid (Eu ~ 20 eV). The a-disintegration itself implies
a decrease by two units of the charge state of the neptunium atom in comparison to that of the emitting 2 4 1Am atom. In addition, further ionization of the decayed atom is triggered by the electronic shake-off associated with the sudden change of the nuclear potential. Physical evidence for the existence of both recoil-and ionization processes has been obtained from a variety of experiments and in particular from charge spectroscopy data [1] ; one may point out that the decrease by two units of charge corresponding to a emission has practically never been observed ; on the other hand the particular efficiency of the ionization processes has been generally confirmed.
We report in the present work the Mossbauer emission spectra of 2 3 7Np in several 241Am-americium sources (Am02, AmAs and AmBi). A m 02 and AmAs have been recently reported to order antifer-romagnetically with Neel temperatures of 8.5 [2] and 13 K [3] respectively. We have attempted to confirm the magnetic behaviour of these materials from the investigation of the temperature dependence of their Mossbauer spectra. In addition, we have analysed the consequences of the nuclear decay in these systems.
2. Experimental. — Three independent samples of A m 02 have been investigated. One was used as supplied by Oak-Ridge N. L., one has been refired in air, whereas the last one has been prepared by oxidation of the metal [4]. AmAs and AmBi were obtained by the reaction of americium hydride with the pure metals [3, 5]. The compounds were checked from their X-ray diffraction patterns before as well as after the Mossbauer measurement. There was in particular no evidence for any oxide phase in AmAs and AmBi [4, 5]. The sample of AmAs used in the present work was of the same batch than that used for the magnetic susceptibility investigation [3].
Sources containing 30 to 50 mg Am (corresponding to an activity of 80 to 120 mCi) were encapsulated
C6-936 J. M. FRIEDT, R. POINSOT, J. REBIZANT AND W. MULLER
in hermetically sealed aluminium holders welded by an electron beam. They were measured from 1.2 to
77 K against a moving absorber of NpO, (60 or
120 mg/cm2). The resonance width of our N p 0 2 absorber was found to broaden from 2.50 to 4.00 mm/s between 77 and 4.2 K due to the magnetic transition
occuring in this compound at 22 K [6]. The broaden- ing was shown to be reproducible from a number of experiments against single line americium metal sources. All isomer shifts reported in this paper refer to N p 0 2 ; in the case of source experiments, the sign convention of velocities has been reversed in order to allow a direct comparison with the absorption data. The detection of the 59 keV y-rays of 237Np was performed either with a 5 mm thick NaI-TI scintillation counter associated with a fast single channel analyser or with a 25 mm thick tin-loaded plastic scintillator connected to a low-level discriminator. Typical counting rates accessible under our geometrical conditions were respectively 150 x lo3 s - l and
400 x lo3 s-' whereas the resonance effects were identical with both detection systems. The experi- mental data were fitted on an 1108 Univac Com- puter to proper theoretical line shapes 171.
3. Experimental results.
-
The 237N p emission spectra in sources of 241Am02 reveal in the whole temperature range from 1.2 to 77 K two single lines in addition to a quadrupole split pattern superposed on the peak at-- -
25 mm/s (Fig. 1, Table I). The resonance pattern at-
-
25 mm/s cannot be attri- buted to a quadrupole spectrum alone in view of the iarge excess intensity of its central part as compared to the theoretical relative intensities. Furthermore, the linewidths of the quadrupole spectrum are noti- ceably broader than the single lines (Table I). The systematic dependence of the isomer shift on the neptunium charge state [8] allows one to attribute the resonances at-
-
1 and-
-
25 mm/s to Np4+I I I I I P I I I I
I
80 60 4 0 20 0 -20 - 4 0 -60 -80
VELOCITY l m m l r )
FIG. 1. - 237Np emission spectra of 241Am02 at 77 and 4.2 K. The best computer fit to the experimental points js represented as a solid line. Notice that the sign convention of the velocity scale has been reversed in order to allow for a comparison
with absorption data.
and Np5+ ions respectively. One may thus assign the Np4' line to normal Np ions substituting for the Am ions in a cubic lattice site of the CaF, type structure [4] ; the Np5+ ions are observed as well in non-perturbed substitutional lattice sites and in a distribution of defect sites as revealed by the broad quadrupole spectrum. This situation is attributed to the stabilization of the Np5+ recoil species in a variety of defect configurations such as interstitial or vacancy associated sites.
The temperature dependence of the spectra reveals varying relative intensities of the two Np5+ com-
Hyperfine interaction parameters and relative intensities for the various sites observed in the 237Np emission spectra in AmO,.
The meanings of the headings are the following : IS, the isomer shifts with respect to NpO,, e2qQ and q qua- drupole coupling constant and asymmetry parameter ; WQ and WSL the resonance widths for the quadrupole pat- tern and the single lines respectively ; INT the relative intensities with respect to the base line. Experimental errors are given in brackets.
237Np EMISSION SPECTRA IN 241Arn : AmO2, AmAs AND AmBi SOURCES C6-937
ponents : at lower temperature, the relative intensity of the quadrupole component increases. The relative intensity of the Np4+ state in comparison to the total NpSf contribution does not depend on the tempe- rature. The Np4+/Npsf intensity ratio depends slightly (less than 10
%)
on the sample ; this may arise from differing stoichiometries although such differences could not be established by X-ray diffrac- tion on our samples [4].The detailed investigation of the temperature dependence of the Np4+ resonance width reveals an abrupt increase below
-
8 K (Fig. 2). In spitematrices [6, 91. The 243Am absorption data in AmO, did not provide any evidence for the magnetic state at 4.2 K [lo, 111. This may be due as well to an anomalously small value of the hyperfine field at the Am nucleus in the oxide matrix ; an investigation of the detailed temperature dependence of the 243Am resonance in AmO, would be required for further confirmation of this behaviour.
The spectra of 237Np in the AmAs sources were fitted at both 77 and 4.2 K as a sum of two single lines and a quadrupole pattern (Fig. 3, Table 11). The
most intense line at 16.5 mm/s is identified from its
of the lack of spectral resolution, this anomalous FIG. 3. - 237Np emission spectra of AmAs at 77 and 4.2 K. temperature dependence may be attributed to the Notice the reversed sign convention for the velocity scale.
5
-
3
2
magnetic ordering of AmO, at
TN
--
8 K, in agree- ment with the results from magnetic susceptibilityinvestigations 121. The magnetic hyperfine field at isomer shift as arising from Np atoms substituting in the 2 3 7 ~ p nucleus in AmO, is estimated to be around the cubic sites of the Am in AmAs [3, 141. It is indeed
60 kOe. This value is of the same order of magnitude consistent with the IS of Np in NpAs (22.1 mm/s) [12], as those reported for 237Np in both NpO, and UO, although significantly lower. The decrease of the isomer
I
':i
't
I
'trt
7 , . , . , . 2 ,-
-
I I I I I I I IHyperJne interaction parameters and relative intensities for the various sites observed in the 237Np emission spectra in AmAs
0 20 40 60 80 T ( I 0
FIG. 2. - Temperature dependence of the line width of the nor- mal Np4+ site. The point at 77K is corrected for the broadening of the NpOz absorber between 77 and 4.2 K. For all the other
measurements, the absorber was held at 4.2 K.
C6-938 J. M. FRIEDT, R. POINSOT, J. REBLZANT AND W. MULLER
shift of Np impurities in americium matrices as compa- red to those observed in the neptunium compounds may tentatively be attributed to an apparent pressure effect implied by the smaller ionic volume of an Am ion in comparison to that of the corresponding Np ion ;
by comparing with rare-earth systems, the trend of the IS is indeed consistent with a pressure effect 1151. The other two absorptions observed in AmAs sources are attributed to the stabilization of Np4+ and Np5+ ions as a consequence of the nuclear decay. The change of linewidth on cooling both source and absorber from 77 to 4.2 K (Table 11) is attributed to a broaden- ing in the absorber alone since it is equal (within the error limits) to that observed on cooling the NpO, absorber with respect to a single line metal source. There is thus no evidence for the occurrence of a
magnetic transition in AmAs from the present source experiments.
The emission spectra of AmBi are rather similar to those of AmAs (Fig. 4). The most intense line at
VELOCITY tmrn/s )
FIG. 4. - 237Np emission spectra of AmBi at 77 and 4.2 K. Notice the reversed sign convention for the velocity scale.
29 mm/s is again attributed to Np atoms in normal substitutional sites from the systematics of the IS in the Np monopnictides 1121. The central pattern (IS = - 2.) is attributed to Np4+ species whose resonance is broadened by paramagnetic relaxation effects (it is fitted in figure 4 with the assumption of
a static hyperfine field and allowing for a broadening in comparison to the single line widths). The single line at IS =
-
28 mm/s arises from the stabilization of Np5* ions as a consequence of the nuclear decay.4. Discussion and conclusion. - The fact that a Mossbauer resonance is observed after the a decay of 241Am signifies that the recoil nuclei become stabilized in, possibly metastable, rest positions on a time scale much shorter than the nuclear half-life ,z (- 70 ns). This behaviour is in agreement with a theoretical discussion of the slowing down of the recoil nuclei 1161. Several ionic configurations of neptunium are revealed by the emission spectra. This indicates the importance of the electronic shake-off consecutive to the a emission. The observation of well defined ionic species in the compounds under investigation suggests a non-metallic character since in conducting materials the charge neutralization usually proceeds much faster than
z
, and single emission lines (possibly broadened) are observed. This conclusion may be connected to the unexpected variation of the lattice constants along the series of Am monopnictides 1171. Notewor- thy, however, the emission spectrum of AmSb did not reveal several oxidation states 1131. The distri- bution of the recoil atoms in several lattice configura- tions and defect environments is in agreement with the results of model calculations for the slowing down of high energetic atoms in solids : the recoil energy is progressively dissipated in the lattice whereas the final steps of energy loss involve the creation of vacancy-and defect clusters. The rest positions have a high probability of being associated with some kind of defects [18]. This probability increases theore- tically with decreasing mass of the atoms in the lattice, in agreement with a larger number of defect sites in AmO, as compared to AmBi. The increased proportion of Np5' defect-associated species at lower temperatures observed in AmOz is also in agreement with the theoretical description of the recoil energy dissipation and the expected annealing behaviour.
237Np EMISSION SPECTRA IN 241Am: AmOz, AmAs AND AmBi SOURCES C6-939
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