Polarization studies of the 16O(γ, n)15O reaction

Volume

PHYSICS

30B, number 2

POLARIZATION

STUDIES

G. W. COLE Jr., Electron

Accelerator

Laboratory,

OF

15 September

LETTERS

THE

lSO(r,

.)I5

0

1969

REACTION*

F. W. K. FIRK and T. W. PHILLIPS Yale

New Haven,

University,

Received

Connecticut

06520,

USA

21 July 1969

The polarization of photoneutrons from the reaction 160(y,n)150 has been studied at angles of 450 and 900 with the 12C(n, n)12 C reaction as an analyser. The neutron energies were determined with a nanosecond time-of-flight spectrometer. Appreciable polarization is observed at 45’ throughout the giant dipole resonance region.

Recently, the neutron and proton decay widths of the giant electric dipole states in 160 have been estimated by several authors [l-4]. The

polarization of the photonucleons has also been calculated [ 1, 21. An important aspect of all the models is the

* Work supported by the United States Atomic

mixing portant

of particle-hole states; it is clearly imto test the predicted admixtures exper-

Energy

( MeV)

Energy

Commission.

67

I

6

I

Neutron

I

oll”“““‘l”““‘l 100

“““’

200 Channel

300

Number

Fig. 1. Observed spectra of 160 photoneutrons scattered f 500 from the l2 C analyser, for a reaction angle of 45O ( A indicates left-hand side yield, + 500). Measured background have been subtracted. The inset shows the placement of the bremsstrahlung target, the oxygen target and the polarization analyser.

91

Volume

36B. number 2

PHYSICS

LETTERS

15 September 1969 Table 1

Values of the polarization of photoneutrons from ‘60 at 45O. The energies correspond to those at which the values of P2 are known. Estimates of the ratio as/ad are also listed. Ext. Energy in I60 (MeV)

160

Pal. at 450

PlP2

=1

c”s ad

22.17

0.133-10.024

0.23 i 0.05

o~23-o.04 +o. 06

22.51

0.092 i: 0.025

0.28 f 0.10

+0.33 025-o.06

23.06

0.020i0.040

0.04 & 0.08

23.27

0.108-tO.037

0.19 i 0.07

o.20-o.05 +0.05

23.45

0.088 * 0.036

0.09 I 0.04

0.08 -to.05 -0.03

23.68

0.043i

0.035

0.08 i 0.07

o*07-o.03 +0.05

24.47

0.050 &0.025

0.29 & 0.25

+2.9 0.26-0.11

24.70

- O.OZl& 0.028

0.08 rt 0.14

24.90

- 0.037+0.032

0.07 1 0.09

- 0.1

- 0.1 +0.05 o*07-o.04

do _~ ,., R,” { 2(ai + a;) + (3asad cos A- a;) P2 (Cos 0))

Fig. 2. (a) Measurements of the analysing power of 12~ (P2) for neutrons from 4 to 16 MeV, at a 50’ scattering angle [ 81. (b) 9P2, the product of the 160 photoneutron polarization and the 12C analysing power, for a reaction angle of 45O. The error bars reflect only the statistical = 30 MeV, t indicates uncertainty. ( 4 indicates E Yo = 60 MeV). fiyP P for a reaction angle of 90°. where for pure El transiti!ns PI should be identically zero at all energies CFyO = 60 MeV) imentally. It purity of the

is also necessary to determine the El absorption process, since this is basic to all the calculations. The standard test is to measure angular distributions of the photoarticles as a function of energy [ 5,6]. Unfortunately, these results alone exare not definitive as shown in the followin pression which assumes El absorption in ii60 and both s- and d-wave ground state particle emission: 92

(1) where as and ad are the magnitudes of the s- and d-wave neutron emission amplitudes, and A is their relative phase. However, the polarization of the photonucleons is also related to the three unkowns [7]: dP

ds2 - -

AX: { asadSinA}

sin2

B

(2)

where i; is a unit vector normal to the scattering plane. We thus have two linearly independent relations which give values for the ratio of a,,‘ud and for A. (Note that any observed polarization at 90’ would be evidence of interference between opposite parity transitions ). The inset of fig. 1 shows the arrangement of the photo-neutron time-of-flight polarimeter. The Yale University electron linear accelerator provides a pulsed beam of electrons: the energy can be varied between about 10 and 80 MeV. The peak current is 1 A contained in a pulse 10 ns wide; the repetition rate is 209~~l. Photoneutrons are produced at reaction angles of 45O or 90°, depending on the location of the 160(y, n)

PHYSICS

Volume 30B, number 2

target (7.5 cm dia. cylinder

25 m flight path a resolution

of water).

LETTERS

With a

of 140 keV for

6 MeV neutrons is achieved. The neutron polarization is measured by right-left scattering from 12.7 cm dia. X 2.5 cm thick 12C analyser. A comparison between the time-of-flight spectra, obtained in scattering to the right and to the left of the analyser, is shown in fig. 1. The reaction angle is 45O and measured backgrounds have been subtracted. Measurements were made at bremstrahlung end-point energies of Em = 30 and 60 MeV. At the lower energy, the photoneutrons were therefore predominantly due to ground state transitions in the region of interest between 22 and 25 MeV. At 45’, appreciable right-left asymmetries are observed. The right (R) - left (L) ratios yield the product of the incident neutron polarization (PI) and the carbon analysing power (P2) [ PlP2= = (32 - l)/(% + 1) where 32 = R/L ] so that the desired result, PI, can only be obtained if the analysing power is known independently. As fig. 2a shows, the measurements of P2 are sparse [8,9] particularly in the region between 4.5 and 6.0 MeV and at energies above 9 MeV. Although the structure in the I2C(n, n)12C reaction makes it difficult to determine the asymmetry with precision at some energies, it has the advantage of providing a built-in check on the absolute magnitude of PlP2, since known zeros occur in p2 at 3.9, 7.8 and 8.4 MeV. Any systematic errors would therefore be obvious in the failure of the product PIP2 to be zero at these energies. The measured values of PIP2 for reaction angles of 45’ and 90° are shown in fig. 2(b) and fig. 2(c), respectively. Values of the 160 photoneutron at 45’ have been deduced at energies where P2 is known and the results listed in table 1 The only previous measurement of the polarization of photoneutrons from 160 was made by Hanser [lo] using a helium recoil polarimeter

15 September

1969

with low resolution (7 1 MeV). He obtained a positive, almost energy-independent value of Pl = 50% for photoneutrons between 5 and 9 MeV at 45’. The present work, however, indicates a strongly energy-dependent positive value of P1 at 45’ in the region of the main dipole resonances between 22 and 25 MeV [5]. If one assumes pure El absorption (not inconsistent with the very small value of PI observed at 90’ between 22 and 25 MeV), tentative values of as/ad are obtained (see table 1; the angular distribution data given in [6] are used in deriving these estimates). Values of A range from fn to $rr. Such information will be necessary in more realistic calculations of the properties of the giant dipole states in 160. The experiment is being refined to provide more reliable results over a much wider energy range. it should be noted that the Final1 I6C(y,n) Pp 50 reaction at 45’ could provide a useful source of polarized neutrons in the energy range from a few MeV to about 50 MeV, where little work has been reported as yet.

References 1. B.Buck and A.D.Hill, Nucl. Phys. A95 (1967) 271. 2. M.S. Weiss, Phys. Letters 19 (1965) 393. and W.M.MacDonald, Nucl. Phys. 3. W. P.Beres A91 (1967) 529. M.Danos and W.Greiner, Phys. 4. H.G.Wahsweiler, Letters 23 (1966) 257. and N.W.Tanner, Nucl. Phys. A95 5. E.D.Earle (1967) 241. private com6. J.E. E. Baglin and M.N.Thompson, munication. V.I.Goldanskii and I.L.Rozental, 7. A.M.Baldin, Kinematics of nuclear reactions (Oxford University Press, London, 1961). 8. C.A. Kelsey, S. Kobayashi and A. S. Mahajan, Nucl. Phys. 68 (1965) 413. 9. M. Zombek, private communication. thesis, M.I.T., 1967, (unpublished). 10. F.Hanser,

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