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Proton channeling in uranium mononitride
JOURNAL OF NUCLEAR MATERIALS 54 (1974) 143-145.0
PROTON CHANNELING S. NASU, T. KURASAWA,
IN URANIUM MONONITRIDE
K. OZAWA,
Japan A totnic Energy Research institute,
College
ofGeneralEducation,
NORTH-HOLLAND PUBLISHING COMPANY
K. SHIOZAWA
and K. KAWATSURA
Tokai, Naka-gun, Ibaraki-ken 319-I
1, Japan
K. KOMAKI University of Tokyo, Komaba 3-8-1,
Tokyo 153, Japan
T. TAKADA department
of Nuclear ~n~.~eering, Nagoya Uni~,ersit~~hrfgoya 464, Japan Received 12 July 1974 Revised manuscript received 19 August 1974
Channeling studies on nuclear fuels such as UO, [ 11, U409 [2] and UC [3,4] have been performed. In addition to these studies, the channeling technique was used to study the formation of oxide layers on UC single crystals ]5] and the position of rare gas atoms in UC single crystals [6]. The purpose of the present investigation is to present channeling data on uranium mononitride, UN, of the NaCl-type structure with partly covalent and partly metallic bonds. The measurements were carried out on the JAERI 2 MV Van de Graaff using a technique similar to that described elsewhere [7]. The goniometer rotation and tilt angles could be set reproducibly to 0.02”. For all experiments the (100) direction was chosen. The beam current was 8 nA and the beam cross section was about 1 mm2, The scattered beam was measured at an angle of 150”. The energy analysis was performed by a 256channel analyser. The energy resolution of the solid-state detector was approximately 40 keV full width at half maximum for 1 MeV helium ions. The target was always kept at room temperature. UN single crystals were purchased from Battelle Memorial Institute, Columbus, Ohio. The crystals of UN were cut parallel to the (100) plane using an electro-spark machine, polished mechanically, and annealed in vacuum at 1500°C for 5 h to remove uranium sesquinitride (U2N3) 181. After annealing the crystals were polished electrochemically. Typical energy spectra of the backscattered particles are shown in fig. 1. The random spectrum Is obtained by orienting the crystal so that the incident
beam is not aligned with any crystal axis or plane. The aligned spectrum shows the large reduction in backscattered yield when a crystal axis ((100) in this case) is parallel to the beam direction. Particles with the highest energy (i.e. at the spectrum edge) correspond to scattering from the U atoms in the surface region. The small peak at the spectrum edge is due to random scattering from uranium atoms in a disordered surface layer, as indicated by the position of the peak.
UN, P
CO’1
0
15MeV.
25°C
I
50
150
100
CHANNEL
2M I
NUMBER
Fig. 1. Energy spectra of 1.5 MeV protons backscattered from UN single crystats. The lower curve is for aligned incidence along the (100) axis. The upper curve is for random incidence. The small surface peak in the aligned spectra indicates the existence of a disordered surface layer of uranium atoms.
144
12
S. Nasu et a(., Proton channeling in uranium mon~nitride
UN,
P
15MeV,
25°C
UN P
10
z
1.5 MeV,25”C
08
a,
b
p”
06
Y 0.4
0.2
IO
.S E “(’
09
Fig. 2. Orientation dependence of the normalized Rutherford scattering yield of 1.5 MeV protons along the (100) axis in UN single crystals obtained from the energy regions scattered from 0.6-1.0 Mm (191-195 channels) and 1.6-2.0 ctrn (181-185 channels) beneath the surface respectively.
Since the energy loss during scattering for a given projectile decreases with the mass of the scattering atoms, and since the scattering cross section is proportional to the square of the atomic number of the scattering atoms, Rutherford scattering mainly reflects heavy atoms. Therefore, the energy spectra for the backscattered protons do not show any clear indication of a contribution from the nitrogen atoms. The protons lose energy as they move through the crystal, so that the continuum at lower energies corresponds to scattering from progressively larger depths. Detailed orientation scans from two different depths are shown in fig. 2. These were obtained by recording the yield in the narrow energy regions scattered from 0.6- 1.O pm (19 1- 195 channels) and 1.6-2 .O I.trn (18 l-l 85 channels) beneath the surface, respectively, while tilting the QOO>axis through the beam direction. Since the 1.5 MeV protons backscattered from light nitrogen atoms will be 180 keV, the yield curve of fig. 2 will be due to uranium atoms in UN. The critical angle is determined by measuring the angular halfwidth (Gl12) at a level midway between the aligned and random levels. The observed critical angle ($ l,2) and minimum
I
05 depth
/
10 beneath
I
1.5 surface
I
I 2.0
,
25
30
pm
Fig. 3. Depth dependence of Jig and xmin for I.5 MeV protons along the (100) axis in UN single crystats expressed in terms of energy loss and depth of the scattering zone.
yield (Xmi,) both depend on the depth beneath the surface at which the measurements are made. The minimum yield Xmin is defined as the ratio of the yield in the perfectly aligned direction to that in a random direction. It is therefore a direct measure of the unchanneled fraction of the beam. The effects of the depth of the scattering zone (or of the energy loss of the particle) on 4 l,Z and Xrnin are shown in fig. 3. The depth scale was calculated from the data on stopping cross sections (e) for various elements that have been collected by Whaling [9]. A plot of e versus atomic number for various elements and 1.5 MeV protons was made to extrapolate the e values for U and N. With a simple calculation the stopping power ti1d.x is deduced from E. The value of Ji 1iZ can be cornpared with Lindhard’s theory on the directional effects. According to Lindhard’s theory [lo], the critical angle for axial channeling is given by $I/2 =cti1
9
(1)
S. Ntrsu et al., Proton channeling in uranium ~~no~it~~e
provided J/I
J/I =(2Z,Z,e2/E#/2,
Acknowledgements
where ZI and .Z2 are the atomic numbers of the projected and target atoms respectively, e the electronic charge, a the Thomas-Fermi screening distance (= 0.1 A for U atoms and = 0.2 8, for N atoms), d the lattice spacing along the row, and E the projectile energy. The condition $ I < a/d is always satisfied under the experimental conditions of the present study. The axial critical-angle measurement for UN is compared with theory, and the constant C is estimated to be 0.899 or 0.90. According to Barrett [ 111, the minimum yield, Xmin, just below the surface is expressed rather well by the form Xmin = 3Ndnu; U2
= 2115
,
145
(2)
1 ’
where N is the atomic density and u 1 is the r.m.s. displacement of uranium atoms. Though there exists a surface peak on the aligned spectrum in fig. 1, the r.m.s. displacement at room temperature is calculated to be 0.13 A from eq. (2) and the experimentally observed xmin of 0.016. This value seems to be more reasonable by comparison with the value of 0.08 A obtained from the neutron diffraction study [ 121.
We would like to thank Professor F. Fujimoto (University of Tokyo) for constructive comments on this work, Dr. ‘I. Kikuchi (JAERI) for his support and continuous interest in this work, and Mr. C. Kobayashi and Mr. S. Kanda for making the 2 MV Van de Craaff accelerator available. One of us (T.T.) would like to thank Professor T. Kirihara (Nagoya University) for his kind and helpful encourageyylent .
References [ 1] Hj. Matzke and J.A. Davies, J. Appt. Phys. 38 (1967) 805. [2] Hj. Matzke, J.A. Davies and N.G.E. Johansson, Canad. J. Phys. 49 (1971) 2215. [ 31 Hj. Matzke, J. Mater. Sci. 5 (1970) 777. (41 Hj. Matzke, Whys. Stat. Sol. (a) 8 (1971) 99. [5] Hj. Matzke, J. Appl. Phys. 40 (1969) 3819. [6] Hj. Matzke, J, Nucl. Mater. 30 (1969) 110. [ 7] K. Ozawa, F. Fujimoto, K. Komaki, M. Mannami and T. Sakurai, Phys. Stat. Sol. (a) 9 (1972) 323. [8] M.J. Sole and CM. van der Walt, Acta Meta~iurgjca 16 (1968) 501. [9] W. Whaling, Handbuch der Physik, Vol. 34, ed. S. Fliigge (Springer-Verlag, Berlin, 1958) p. 193. [lo] J. Lindhard, Mat.-Fys. Medd. Dan. Vid. Selsk. 34 (1965) No. 14. [ 111 J.H. Barrett, Phys. Rev. B3 (1971) 1527. [ 121 M.H. Mueller and H.W. Knott, Acta Cryst. 11 (1958) 751.
PROTON CHANNELING S. NASU, T. KURASAWA,
IN URANIUM MONONITRIDE
K. OZAWA,
Japan A totnic Energy Research institute,
College
ofGeneralEducation,
NORTH-HOLLAND PUBLISHING COMPANY
K. SHIOZAWA
and K. KAWATSURA
Tokai, Naka-gun, Ibaraki-ken 319-I
1, Japan
K. KOMAKI University of Tokyo, Komaba 3-8-1,
Tokyo 153, Japan
T. TAKADA department
of Nuclear ~n~.~eering, Nagoya Uni~,ersit~~hrfgoya 464, Japan Received 12 July 1974 Revised manuscript received 19 August 1974
Channeling studies on nuclear fuels such as UO, [ 11, U409 [2] and UC [3,4] have been performed. In addition to these studies, the channeling technique was used to study the formation of oxide layers on UC single crystals ]5] and the position of rare gas atoms in UC single crystals [6]. The purpose of the present investigation is to present channeling data on uranium mononitride, UN, of the NaCl-type structure with partly covalent and partly metallic bonds. The measurements were carried out on the JAERI 2 MV Van de Graaff using a technique similar to that described elsewhere [7]. The goniometer rotation and tilt angles could be set reproducibly to 0.02”. For all experiments the (100) direction was chosen. The beam current was 8 nA and the beam cross section was about 1 mm2, The scattered beam was measured at an angle of 150”. The energy analysis was performed by a 256channel analyser. The energy resolution of the solid-state detector was approximately 40 keV full width at half maximum for 1 MeV helium ions. The target was always kept at room temperature. UN single crystals were purchased from Battelle Memorial Institute, Columbus, Ohio. The crystals of UN were cut parallel to the (100) plane using an electro-spark machine, polished mechanically, and annealed in vacuum at 1500°C for 5 h to remove uranium sesquinitride (U2N3) 181. After annealing the crystals were polished electrochemically. Typical energy spectra of the backscattered particles are shown in fig. 1. The random spectrum Is obtained by orienting the crystal so that the incident
beam is not aligned with any crystal axis or plane. The aligned spectrum shows the large reduction in backscattered yield when a crystal axis ((100) in this case) is parallel to the beam direction. Particles with the highest energy (i.e. at the spectrum edge) correspond to scattering from the U atoms in the surface region. The small peak at the spectrum edge is due to random scattering from uranium atoms in a disordered surface layer, as indicated by the position of the peak.
UN, P
CO’1
0
15MeV.
25°C
I
50
150
100
CHANNEL
2M I
NUMBER
Fig. 1. Energy spectra of 1.5 MeV protons backscattered from UN single crystats. The lower curve is for aligned incidence along the (100) axis. The upper curve is for random incidence. The small surface peak in the aligned spectra indicates the existence of a disordered surface layer of uranium atoms.
144
12
S. Nasu et a(., Proton channeling in uranium mon~nitride
UN,
P
15MeV,
25°C
UN P
10
z
1.5 MeV,25”C
08
a,
b
p”
06
Y 0.4
0.2
IO
.S E “(’
09
Fig. 2. Orientation dependence of the normalized Rutherford scattering yield of 1.5 MeV protons along the (100) axis in UN single crystals obtained from the energy regions scattered from 0.6-1.0 Mm (191-195 channels) and 1.6-2.0 ctrn (181-185 channels) beneath the surface respectively.
Since the energy loss during scattering for a given projectile decreases with the mass of the scattering atoms, and since the scattering cross section is proportional to the square of the atomic number of the scattering atoms, Rutherford scattering mainly reflects heavy atoms. Therefore, the energy spectra for the backscattered protons do not show any clear indication of a contribution from the nitrogen atoms. The protons lose energy as they move through the crystal, so that the continuum at lower energies corresponds to scattering from progressively larger depths. Detailed orientation scans from two different depths are shown in fig. 2. These were obtained by recording the yield in the narrow energy regions scattered from 0.6- 1.O pm (19 1- 195 channels) and 1.6-2 .O I.trn (18 l-l 85 channels) beneath the surface, respectively, while tilting the QOO>axis through the beam direction. Since the 1.5 MeV protons backscattered from light nitrogen atoms will be 180 keV, the yield curve of fig. 2 will be due to uranium atoms in UN. The critical angle is determined by measuring the angular halfwidth (Gl12) at a level midway between the aligned and random levels. The observed critical angle ($ l,2) and minimum
I
05 depth
/
10 beneath
I
1.5 surface
I
I 2.0
,
25
30
pm
Fig. 3. Depth dependence of Jig and xmin for I.5 MeV protons along the (100) axis in UN single crystats expressed in terms of energy loss and depth of the scattering zone.
yield (Xmi,) both depend on the depth beneath the surface at which the measurements are made. The minimum yield Xmin is defined as the ratio of the yield in the perfectly aligned direction to that in a random direction. It is therefore a direct measure of the unchanneled fraction of the beam. The effects of the depth of the scattering zone (or of the energy loss of the particle) on 4 l,Z and Xrnin are shown in fig. 3. The depth scale was calculated from the data on stopping cross sections (e) for various elements that have been collected by Whaling [9]. A plot of e versus atomic number for various elements and 1.5 MeV protons was made to extrapolate the e values for U and N. With a simple calculation the stopping power ti1d.x is deduced from E. The value of Ji 1iZ can be cornpared with Lindhard’s theory on the directional effects. According to Lindhard’s theory [lo], the critical angle for axial channeling is given by $I/2 =cti1
9
(1)
S. Ntrsu et al., Proton channeling in uranium ~~no~it~~e
provided J/I
J/I =(2Z,Z,e2/E#/2,
Acknowledgements
where ZI and .Z2 are the atomic numbers of the projected and target atoms respectively, e the electronic charge, a the Thomas-Fermi screening distance (= 0.1 A for U atoms and = 0.2 8, for N atoms), d the lattice spacing along the row, and E the projectile energy. The condition $ I < a/d is always satisfied under the experimental conditions of the present study. The axial critical-angle measurement for UN is compared with theory, and the constant C is estimated to be 0.899 or 0.90. According to Barrett [ 111, the minimum yield, Xmin, just below the surface is expressed rather well by the form Xmin = 3Ndnu; U2
= 2115
,
145
(2)
1 ’
where N is the atomic density and u 1 is the r.m.s. displacement of uranium atoms. Though there exists a surface peak on the aligned spectrum in fig. 1, the r.m.s. displacement at room temperature is calculated to be 0.13 A from eq. (2) and the experimentally observed xmin of 0.016. This value seems to be more reasonable by comparison with the value of 0.08 A obtained from the neutron diffraction study [ 121.
We would like to thank Professor F. Fujimoto (University of Tokyo) for constructive comments on this work, Dr. ‘I. Kikuchi (JAERI) for his support and continuous interest in this work, and Mr. C. Kobayashi and Mr. S. Kanda for making the 2 MV Van de Craaff accelerator available. One of us (T.T.) would like to thank Professor T. Kirihara (Nagoya University) for his kind and helpful encourageyylent .
References [ 1] Hj. Matzke and J.A. Davies, J. Appt. Phys. 38 (1967) 805. [2] Hj. Matzke, J.A. Davies and N.G.E. Johansson, Canad. J. Phys. 49 (1971) 2215. [ 31 Hj. Matzke, J. Mater. Sci. 5 (1970) 777. (41 Hj. Matzke, Whys. Stat. Sol. (a) 8 (1971) 99. [5] Hj. Matzke, J. Appl. Phys. 40 (1969) 3819. [6] Hj. Matzke, J, Nucl. Mater. 30 (1969) 110. [ 7] K. Ozawa, F. Fujimoto, K. Komaki, M. Mannami and T. Sakurai, Phys. Stat. Sol. (a) 9 (1972) 323. [8] M.J. Sole and CM. van der Walt, Acta Meta~iurgjca 16 (1968) 501. [9] W. Whaling, Handbuch der Physik, Vol. 34, ed. S. Fliigge (Springer-Verlag, Berlin, 1958) p. 193. [lo] J. Lindhard, Mat.-Fys. Medd. Dan. Vid. Selsk. 34 (1965) No. 14. [ 111 J.H. Barrett, Phys. Rev. B3 (1971) 1527. [ 121 M.H. Mueller and H.W. Knott, Acta Cryst. 11 (1958) 751.