Bremsstrahlung Beam Studies of the Electron-Positron Pairing

- 221 - BREMSSTRAHLUNG BEAM STUDIES OF THE ELECTRON-POSITRON PAIRING

Cs.A.Gutliy, A.I.Guthy, T.l.Szabolcsy

i urn linn UA0000799 Uzhgoroci State University, 294000 Uzhgorod, Ukraine

Pair production processes in the field of atomic nuclei discovered more then 60 years ago, is assumed to be well-studied experimentally as well as theoretically. The measured cross-sections are in good agreement with the theoretical calculations performed in the Born-approximation using Bethe- Heitler amplitudes [I]. It should be noted that the majority of experimental investigations for this process were earned out only at small departure angles of electrons (positrons) regarding to the direction of the incoming y- ray. There are only a few works concerning with angular distribution of eV -pair production. One of them [2} treat an effective method for eV-pair registration (practically 100% effectivity of registration in the chosen solid angle). This investigation was carried out for 27Al and UC nuclei at departure angtes in the range between 40" and 140" for the energy range of y-rays 4-30 MeV. *7AI and "(WPb were chosen to be the objects of our measurements. The investigations were carried out utilising bremsstrahlung of our Betatron in the energy range 10-25 MeV with the step 5 MeV. We measured the angular distribution of eV-pair yields in the interval from 40" to 140° with (he step 10". We used the registration method similar in principle to the one described in [2]. This method consists in that one registrate a single-particle (electron or positron) with a plastic scintillation detector having low y-ray sensitivity. This detector was situated in the lead chamber with a hole playing the role of the collimator of the secondary particles that are within the solid angle equal to 5". The distance between the target and the detector was equal to 30 cm. - The results of our experiment are plotted in Figs. 1-2 (marked with solid circles). The theoretical calculations (solid lines) and the results of [2] (open triangles) are also presented. An unambiguous conclusion follows from our investigations, i.e. that there is a visible difference between the behaviour of the experimental and theoretical data at angles larger than 80". On the other hand our experimental data are in good agreement for 27A1 nucleus with - 222 -

10 r O

27AI

6 9 (0 A TJ 0 O 0,1 9 V Si \ * 25 MeV 1 ^- 20MeV \ ^15 MeV \ 10 MeV 0,01 r -4 I.I.I t t 1 • .I. 40 60 80 100 120 140 160 180

Departing angle, grad.

Fig. 1

results of [2]. For the heavy nucleus 2"liPb the behaviour of tlie angular distribution is approximately the same. Though our experimental data are smoothed with bremsstrahlung spectrum one can speak about some structural peculiarities of these yield curves, li should be noted the fact that on the basis of yield curves the differential cross sections can be obtained as functions of the photon-impact energy and the above mentioned effect will be more unambiguous. In [2] such a calculation was performed for i2C nucleus. It attracts attention the fact that a - 223 -

slight break in the yield curve for this nuclei results very large structural change in the effective cross-section.

100

3 •e

2 10

25MeV 20MeV 15 MeV 10MeV

40 60 80 100 120 140 160 160

Departing angle, grad.

Fig. 2

Thus it is necessary lo perform investigations for a wider class of nuclei with belter statistics to clarify the nature of the effect producing discrepancies between theory and experiment at large angles.

1. H.A.Helhe, W.lleitler. Proe.Roy.Soc. AI46, 83 (1934)

2. Yu.A.Novikov. Yademaja Fiz. 37, 1073 (1983)