A bsorption of p ositron annihil ati on quanta in Ga and Bi 4 0 Sn60 alloy in the solid and the liqui d phase w as investi gated. I t w as found that the mechanism of absorption of annihil ati on quanta changes on passing from the solid to the liqui d state. Results of measurements for Ga and Bi 40 Sn60 alloy show that for metals changing their density at the melting p oint the photop eak pro Ùles are mo diÙed: their centers of gra vity in liquid phase are displace d w ith resp ect to those in

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... he solid phase and the numb er of counts under the photop eak changes, to o. PACS numb ers: 64. 70. Dv I n t r o d u ct io n For man y year s t he p ositro n anni hi l ati on spectro scopy ha s been successful l y used fo r studyi ng the structure and pro p erti es of m etal l ic m ateri a ls. Am ong others, resul ts of num erous i nvesti gati ons of the tem p erature dep endence of vari ous anni hi l ati on pa ra meters: F ( T ) i n angul ar correl ati on (A C), S ( T ) i n the D oppl er bro adeni ng (D B) and the mean p ositro n l if eti me § ( T ) i n positro n l i feti m e spectra (P L S) metho d i n a wi de tem p erature ra ng e incl udi ng the m elti ng po int were rep orted. An i nspecti o n of these results shows tha t, as a rul e, the anni hi l ati on para m eters mentio ned ab ove, i ncrease on passing fro m the sol i d to the l iqui d sta te. For exam pl e, Tri ftsh auser [1] rep orted a 2% increa se i n the val ue of the F ( T ) pa ra meter at the m elti ng p oi nt for In. Al so Rud zi¥ska et al . [2] made som e i nv esti gati ons for In and observed ab out a 1% i ncrease i n the F ( T ) para m eter at T m . The speciÙc vol ume of In i ncreases on mel ti ng by a bout 2.7% and thi s i s why the autho rs of the pa p ers [1, 2] interpreted an i ncrease i n the F pa ra meter at T m as a resul t of the speciÙc vol um e i ncrease and / or as an e˜ect of a chang e i n the concentra ti on and a typ e of positro n tra ppi ng centers. Thi s i s an i m p orta n t pro bl em Ê corr esp on din g au t h o r; e-m ail : klu za@m ax .i fd .un i.wr oc.p l (4 07)