Volume 107A, number 9 PHYSICS LETTERS 4 March 1985

STRONG ANISOTROPIC NUCLEAR PHOTON SCATTERING FROM PYROLYTIC BN

Raymond MOREH Nuclear Research Centre, Negev, Beer-Sheva, lsrael and Ben-Gurion University of the Negev, Beer.Sheva, lsrael

Received 21 June 1984

Strong anisotropy in the nuclear resonance scattering of photons from a is N target in the form of pyrolytic boron ni- tride (PBN) was observed. This is explained quantitatively in terms of the high anisotropy in the atomic binding properties of the BN lattice.

In an earlier paper [1 ] we studied the dependence of the nuclear resonance fluorescence (NRF) signal from 11B levels on the molecular orientation of the target using bremsstrahlung photons. A partially oriented 11B target in the form of pyrolytic boron nitride (PBN) was used and the scattered intensity was measured using two different geometries. In the first, the photon beam was parallel to the hexagonal planes of the PBN target and in the second it was per- pendicular to the PBN hexagonal planes. In addition, a thick self-absorber was placed in the incident bremsstrahlung beam to enhance the dependence of the scattered radiation on the molecular orientation.

In the present work, we report the results of a sim- ilar study on the other partner of BN namely on 15 N occurring in the same PBN target. Here however, we used an entirely different technique [2] in which the photon beam is generated from the Cr(n, 7) reaction. In this method one of the emitted 3' lines overlap by chance the 6.324 MeV nuclear level in 15N. The pho- toexcitation process is such that both the incident 7 line and the resonance level in 15N are Doppler- broadened and the scattering intensity 1 s depends on the extent of the overlap of the two lines. It was noted [2] that 1 s depends on the effective temper- ature T e of the I5N scatterer and hence on the direc- tional binding properties of 15N. Thus 1 s will be highest when the hexagonal planes of the PBN target coincides with the 7-beam direction because of the increased kinetic energy of the N atom contributed by the zero-

468

point vibrational energy of the atomic lattice [2,3]. Further I s is lowest when the beam direction coincides with the PBN c-axis because of the weaker lattice binding forces of the N atom along the c-direction.

The present measurement was motivated by the fact that it can provide some information about the energy sharing between the two partners constituting the hexagonal lattice of BN. It also illustrates experimen- tally that the effective temperature T e of B can be vastly different from that of N where both partners occur in the BN lattice.

In principle, it would have been possible to carry out the same measurement on 14N or 15N using the bremsstrahlung technique [1 ]. However, the widths of the bound 14N nuclear levels and the scattering signals were not favourable to observe any statistically significant effect. A similar reasoning applies to 15N where its low natural abundance reduces drastically the intensity of the scattered signal using a brems- strahlung beam. The fact that the ~,-source used in the present work consists of discrete monochromatic lines, made it possible to observe a relatively strong NRF signal.

Experimentally, the photon beam is obtained from the (n, 7) reaction on some chromium disks placed along a tangential beam tube and near the core of the IRR-2 reactor [2]. The intensity of the 6.324 MeV T line emitted by the 53Cr(n, 3~) reaction is ~104 pho- tons/era 2 s on the target position; its energy spread is determined by the Doppler width, being A

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Volume 107A, number 9 PHYSICS LETTERS 4 March 1985

= E3,(2kTs/Mc2)I/2 = 9.6 eV where T s = 670 K is the chromium effective temperature [4] at T = 650 K (which is the temperature of the chromium during the operation of the reactor). The PBN target (manu- factured by Union Carbide) has a hexagonal planar layer structure resembling that of graphite, and it has only some partial orientation [5]. The mosaic spread of the crystallites (as determined by us via a rocking curve measurement using X-ray diffraction) has a FWHM ofA¢ = 81 + 3 °. This last figure is widely dif- ferent from A~ = 69* mentioned in a previous paper and specified by the manufacturer. The BN was of natural N (0.366% 15N, 99.634% 14N); thus out of 28 g of the BN sample, the weight of the 15N reso- nant scatterer was only 102 mg. The target consisted of 28 identical PBN plates each weighing 1.0 g with dimensions 0.28 X 0.50 X 3.7 cm 3 and oriented with its c-axis perpendicular to the 0.5 X 3.7 cm 2 plane. This geometry made it possible to pack the target as shown in fig. 1. It can be seen that this form is verynearly symmetric around the perpendic- ular axis allowing the sample to be rotated from a position in which the photon beam is perpendicular to the c-axis of the BN-plates (shown in fig. 1) to a second position where the beam is parallel to the c-axis. With this arrangement there is no need to in- troduee any correction for photon attenuation when

D

300

z 200

. . . . . i i i m i t , , i J I

50 100 150

CHANNEL NO

Fig. 1. Scattered spectrum from lSN in the form of pyrolytic BN target (0.366% lSN) as measured by a 30 cm 3 Ge(Li) de- tectoL The peaks denoted D and F refer to double and first escape peaks o f the 6,324 MeV ,y line; the photopeak is not shown. The photon beam is obtained from the C~(n, ~) reac- tion. The geometry of the target is such that the photon beam is perpendicular to its c-axis.

the intensities from the two scattering geometries are compared.

Fig. 1 shows the spectrum of the resonantly scat- tered 6.324 MeV T line from the 15N occurring in the PBN target (at T = 295 K) taken with the parallel geometry. A 30 cm 3 Ge(Li) detector was used. The measured ratio of the scattering intensities with the photon beam perpendicular Gp) and parallel (1-c) to the c-ads was found to be - - m

Ip/I c = 1.23 f 0.03.

The calculation of the scattered intensities was ex- plained in ref. [2] in which the incident 7 line is rep- resented by a gaussian and the Doppler-broadened resonance level by a ~-function, separated from each other by 6 = 29.5 eV. Thus the scattering cross section which is directly proportional to the overlap integral of the two line shapes is strongly dependent on the broadening of the 6.324 MeV nuclear level and hence on the effective temperature T e. It should be noted that the lSN occurring in a highly oriented PBN sam- ple is characterized by two effective temperatures Tp and T c related to the lSN motion parallel and perpen- dicular to the hexagonal planar layers of PBN. In order to calculate T_ and T e we should first consider the value of the e~fective temperature T e of 15N for a powdered (non-oriented) hexagonal BN sample. This was measured at T = 295 K and reported [3] to be T o = 561 + 28 K. Here, we follow ref. [3] and adopt the value T e = 560 K which is within the error limits and is consistent with the experimental data of 11B. The three temperatures are related to each other by

T e = (2Tp + To)/3. (1)

The factor 2 appearing in this equation accounts for the two degrees of freedom of the planar motion of the 15N atom. Since T e is related to the weak van der Waals forces between the BN planes, we may use T¢

300 K (at room temperature) from which the value Tp = 660 K is deduced using eq. (1). For the partially oriented sample used in the present work, the values of Tp and T c should be corrected to account for the fact that the mosaic spread has a gaussian distribution with a FWHM of ~ = 81 + 3 °. This is done in the same manner as described in eq. (2) of ref. [1] where the effective temperature of an atom whose axis of vibration is oriented at an angle O was taken to con- tribute a cos20 component along the direction of the

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Volume 107A, number 9 PHYSICS LETTERS 4 March 1985

incident photon beam. Thus the corrected values Te and Tp may be obtained by summing over all contri- butions of the crystallites of the PBN sample. The result is

,/2 L = re + - to) f sin30 exp(-O2/o2)dO

0

,r/2 x ( f sinOexp(-02/o2)d0) -1 , ( 2 )

0

Tp = (3T e - Te) /2 , (3)

yielding Te = 495 K and Tp- = 593 K with an uncer- tainty of 4%. This corresponds to a calculated inten- sity ratio for the scattering from 15N ofl"_fi" e = 1.20

. . . . l J /

whmh is in excellent agreement w~th the measured value. It is important to note that the effective temper-

ature Tp of 11 B occurring in BN is different from that o f 15Nand seems to obey a relation of the form

I

Tp/Tp = 15/11 which follows from the assumption that the kinetic energies o f B and N in the normal vi- brational modes o f the BN lattice are distributed in- versely as their mass ratio. Thus, the effective temper-

, ' 11 atures Tp , T e of B (at T = 295 K) for a highly oriented PBN sample are

Tp( l lB) = 900 K , T~(11B) ~ 300 K ,

t yielding T e = 700 K. Here again the,value of T~ at room temperature is determined by the weak van der Waals forces acting between the weak BN hexagonal

P t planes. It may be seen that the values of Tp and T e for 11 B are much higher than the corresponding values for 15N and show that the effective temperatures of different elements constituting the same chemical compound can be entirely different. For the partially oriented sample used here, the values of Tp and ire of l l B can be corrected to account for the mosaic spread of the BN crystallites, by using eqs. (2) and (3) above.

This yields

T'c = 600 K , Tp = 750 K .

The present values o f Tc and Tp yield nice agree- ment with all known measured scattering intensities from l lB , but are different from those quoted in ref. [ 1 ] because they were obtained after correcting two errors which occurred in that paper. First, the exper- imental value A¢ = 81 ° is different from A¢ = 69 ° used in ref. [1] and has a relatively large effect on the

~ t

effective temperatures Tp and T ' c. Second, eq. (3) o f the present paper corrects eq. (2) o f ref. [1] which is in error. The overall effect of the two errors seems to nearly compensate each other in the sense that the calculated ratio lp/1 e for scattering from l i B obtained using the present values of Tp - ' " and T e is nearly equal to the same predicted ratios given in table 1 of ref. [1], and denoted lafltl. It may thus be seen that while the present values of ~ ' and T e introduce numerical changes in the results o f ref. [ 1 ], it leaves intact all conclusions and main features. It should further be noted that the calculated ratio of scattering intensities (from 15N) for a highly oriented PBN sample is pre- dicted to be

Ip/I c = 2 .2 .

Thus it is of great interest to prepare a highly oriented sample which will enable us to obtain a more refined experimental test o f the above assumptions.

References

[1] R. Moreh, W.C. Sellyey and R. Vodhanel, Phys. Lett. 92B (1980) 286.

[2] R. Moreh, O. Shahal and V. Volterra, Nucl. Phys. A262 (1976) 221.

[3] O. Shahal, R. Moreh and M. Pazi, Nucl. Phys. A339 (1980) 157.

[4] W.E. Lamb, Phys. Rev. 55 (1939) 190. [5] A.W. Moore, Nature 221 (1969) 1133.

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