AN EVALUATING SYSTEM FOR POSITRON ANNIHILATION

30 Pages·2009·0.44 MB·English

KFKI-1978-79
A.G. BALOGH
1. FARAGÓ
POSITRON - AN EVALUATING SYSTEM FOR
POSITRON ANNIHILATION MEASUREMENTS
ON R-W COMPUTERS
Hungarian academy of Sciences
CENTRAL
RESEARCH
INSTITUTE FOR
PHYSICS
BUDAPEST
KFKI-197B-79
POSITRON - AN EVALUATING SYSTEM FOR
POSITRON ANNIHILATION MEASUREMENTS
ON R-m COMPUTERS
A.G. Balogh and I. Faraqó*
Nuclear Physics Department
"Software Department
Central Research Institute for Physics
ll-lr>25 Budapest Р.О.В.19, Hungary
IUI ISSN 0368 5.Í30
A b s t r a ct
POSITRON is a proeгаи package for evaluating positron annihilation
Measurements on R-40 computers. The evaluating system endeavours to use
the services of the operating system to the utmost extent. The main
feature of its structure is its Modularity and the advantage of this is
obvious when a fitting program needs to be changed or new measuring
equipment is to he interfaced.
А н н о т а ц ия
ла ыБ на адзос ая нскелпмок nporpa«**ia по иш иавэан POSITRON,ая ндогирп ля д ра бо
ки тоб ов татьлузер ий неремзи с ей цялигинкаов нортизоп с ью щомоп ВМ Э па ит
R-40. ма етсиС ки тобарбо но ьламискам ет узьлопси ти сонжомзов ой нноицарепо
мы етсис машины. на Она еортсоп иэ й,елудом что но боду в ую вреп дь еречо
тогда, да гок ся тидохирп ть атобарбо ые ннад ой вон ой ньлетиремзи ы,рутараппа
ли и ть идовв ую вон му маргорп и.ктобарбо
К 1 у о и a t
POSITRON néven olyan prograncsoportot hoztunk létre, amely alkalmas a
pozitron annihilációs mérések R-'iO-ея számítógépen való kiértékelésére.
A kiértékelő rendszer igyekszik maximálisan kihasználni az operációs
rendszer adta szolgáltatásokat. Szerkezetét a modular!tás Jellemzi, mely­
nek előnyel elsősorban akkor mutatkoznak neg, mikor egy ú,) mérőberendezés
adatait kell feldolgozni, illetve amikor új kiértékelő programot kívánunk
illeszteni a rendszerbe.
1. I ti t rodln: t i t»it
Of late, positron annihilation experiments have become «ore anil «ort»
frequently used in researchilie solid state structures. This «elhod is
available for studying the electron structure, phase transformation, or
defect structure of different Materials. The information is always obtained
in the for» of easily detectable photons.
In positron research, usually two experimental Methods are used, vi/,
lifetime measurements and angular correlation measurements.
In the case of lifetime measurements we study, that the 0.r,'i7 MeV
maximum enerey positrons which are emitted by a *""Na radioactive isotope,
and pent-1 ra t. i nc into the sample, how lone are liviiie befon- annihilation,
and what Kind of lifetime components / / and relative intensities /l/
arise.
Kor such measurements, a fast-slow coincidence circuit is used where
the first of the two detectors - which are in coincidence - detects those
1.2S MeV 'Y photons indicatine the "birth" of the positrons /the "~Na
sourse also emits such a photon practically at the same time as the
position/; the second detector detects one of the two O.^l MeV annihila­
tion 'У photons, which indicate the "death" of the positrons. The time
In terv.» 1 between the detection of the two different energy photons is the
lifetime of the positrons. The lifetime spectrum is collected by a multi­
channel analyser. /Kig. I./
In angular correlation measurements, we study the annular distribu­
tion of those ^ photons which occur following the annihilation of the
positron-electron pair. Under the law of the momentum conservation
the angle between the two-photons is:
</ =
where -Pi. is the I component of the resultant momentum of the
positron-electron pair /Fig. 2/,
The measurement is performed by the use of a conventional coincidence
circuit where the two detectors - which are in coincidence - detect the
two annihilation photon». One of the two detectors is fixed, the other is
Movable around the /го е axis of the equipment into the vertical direction.
If we »ensure the coincidences during the same period at different angular
positions, we can obtain the curve of the angular distribution and thus the
momentum distribution of the positron-electron pair. Because the momentum
of positrons, due to their quick thermali/ation, is very low compared
with that of the electrons, we can neglect it. Therefore we in fact measure
the moment urn distribution of the electrons in the samp If- /Pi;,. 3/.
11. Evaluation of lifetime measurements
lict us assume that the time resolution function of the equipment is
^(w and furthermore assume that, the lifetime spectrum "•(&/ is the sum
of *fl exponential decay curves with different lifetimes and relative
intens!ties.
Thus
4Ш - 0 if i < 0
(
In this case the measured spectrum will be the convolution of 4111 and Ql«/:
Fit)-/ 1tlWU-t')M
- J -
It is wi-ll known from experience that th« time resolution can easily be
fitted with one or recently m*»re Gaussians:
, ) ф -рле ^Г,
í4
•>r in the case of more liaussians:
where: / , ^f} == \ .To is the zero channel number and t-1 ^f» is
4»M
a given displacement.
The task of the evaluating program is to fit the F/l/ curve on the
measured spectrum. This task is complicated by the source correction term,
namely the measured spectrum contains not only those annihilation events
which come from the sample, but also those which arise in the source it­
self. This part of the events can amount to 10-20 %, dependent upon the
source and the sample /Fig. '•/.
The evaluating program which is used is the POSITRONFIT EXTENDED
/I'OSr-ITE/ £l, zj .
The features of this program are as follows:
- it makes the fit of the time resolution curve of the equipment
with one or more /max. 7/ Oaussians possible;
- before the fitting of the measured spectrum we can perform a source
correction, namely the program subtracts a source correction spectrum which
is defined by the lifetime and relative Intensities of its components;
- the program analyses the lifetime spectra by an iterative least-
squares technique of the semi-linear Mnrquardtf^J type, which is the
developed version of the r>aiiss-Nnwf on technique;
- 4 -
- the program was originally written for Burroughs «»700 or 1W 37»/l6?
computers and the typical running times were about H1; sec and 11! sec,
respectively.
III. Evaluation of angular correlation Measurements
When Measuring Metal» and alloys the angular correlation curve is
always the sua of two components. It can he distinguished by a narrow peak
which is centred around the *tr = 0 angle and which is placed in the low
momentum range, and a broader one with lower intensity which is extended to
the higher momenta.
The first arises fro« the rnnihilation events among the positrons and
the valence electrons of the metal and the annihilations froa the positron-
defect bound states. The second, broader coaponent arises froa the annihila­
tion aaong the positrons and the core electrons.
In a few cases, naaely in the case of the staple aetals when the Perai
surface is a sphere, the central peak could be well fitted with a parabola.
In this case:
Usually the Fermi surface is auch aore complicated and then the central
peak is a Gaussian. Although the core component is always a Gaussian, the
angular correlation curve in most cases can he analysed as the sua of
two Gaussians /Pig. V.
Our model function is
Fit)- £*«*-(*£*/
- ъ -
where the intensities of the peaks is
h - ff1 Víi .
The evaluating program which is used is the PAACF1T£1J .The features
i>r this program are as follows:
- it can analyse a spectrue as a SUM of sore than two /мах. 7/ «iauss—
itins, therefore it is advisable to fit the time resolution function of our
lifetime equipment which is usually the sua of Ъ fiaussians;
- it can analyse the spectra by an iterative least-squares technique
of the seMi-linear Marguardt type;
- it vas written for a W»70O computer, its typical running time was
about lr) sec.
IV. Features of the POSITRON program package
The experiments are performed by two independent measuring set-ups;
the lifetime and the angular correlation equipment. The program group has
three fundamental functions:
- to evaluate the lifetime measurements by using the POSITRONFIT
EXTENDED /POSFITE/;
- to evaluate the angular correlation measurements by using the
1КСЛЛРТ;
- to evaluate the time resolution curve /or "PROMPT" curve/ of the
lifetime measuring equipment by using the PAACFIT.
Our computer is an EC-IO'iO /H-'tO/, which is compatible with the IBM
S/V>0/CP1J: 1024 Kbyte, background: 12 disks of 7.25 Mbyte/. The operating
system, under which POSFITE and PAACFJT were adapted, is S/360 OS MVf.
Efforts were mad«, to utilize to the greatest possible extent the data
management services offered by the operating system, and to accomplish the
- 6 -
Modularity »Г the prop DU» package.
This modularity can кияrantee the easy de. el «patent of the Measuring and
the evaluating syste» because the use of a new evaluating propra« or any
other measuring equipment needs only the changing of one propra« Module in
the system.
.he Modularity ?f the ам грогр package:
Measuring
equipacnt ^»Л Е
tine output
data «anageMent
'DATA"
propra« «odules
]
storing syste«
data preparation ,щр-п|.»
propra« «odules
J creation of the whole
^У\ input data set
evaluating „p „
IT
propra«»
The "DATA" and "PRF.I*" propra« «odules are written in PL/1 programming
language. The "PREP" Modules work with a free-format input and can check nil
of the control data so the evaluating ам грогр hae to he run only in the
case of a perfect input data set. This function is advantageous with regard
to the use of the CPU time.
- / -
MI..rh sch-mt- »Г r»SITK»»N:
I.iff tine »i-asiin'wnl Angular corn-1 at ion
wasiiroMPiit "С"ЛЛР
-|4>SK"
"i-chaniM-l pap«»Г lapi» rt—chnnnrl paper tap«»
output <>u t pu t
// TEI.EX Dl) // TEI.EX IH) // TKI.EX 1)1)
POSFDATA PHINTAPE PAACDATA
// SPECTRA DD // SYSPRINT IH» // SPECTRA 1)1)
// COMMENT 1)1) // COMMENT Dl)
LIST FROM PAPF.R
// SVSPHINT 1)1» TAPE // SVSPRINT DD
'PROMPT"
// SPECTRA DD // SPECTRA DD
// COMMENT Dl) // COMMENT DD
POSFITE PAACFIT
Con t rolDnt Hn»ntrol Dati
CONTROL 1)1) // CONTHCL DD
1-1
PARM Fli»Ы PARM Field
POSPREP PAACPREP
// TRANSIT DD // TRANSIT DD
// SYSPRINT DD // SYSPRINT DD
A TRANSIT
// FT0rjH)Ol DD // FWJFOOl DD
POSFITF, PAACFIT
// FTOÍVF0O1 DD // FTOÍ>FOOt Dl)