Lifetime measurements with magnetically analyzed positron beams were made in condensed media with uniform and non-uniform properties. As expected, the lifetime values with magnetically analyzed positron beams in uniform targets are similar to those obtained with conventional positron sources. The lifetime values with magnetically analyzed beams in targets which have non-uniform properties vary with positron energy and are different from the conventional positron source derived lifetime values in these targets

Holt, W. H.

Mock, W., Jr.

Singh, J. J.

English

NASA Technical Reports Server

NASA Technical Memorandum 84535
r NASA-TM-84535 19820025559
POSITRONA NIHILATIONSPECTROSCOPYWITHMAGNETICALLY
ANALYZEDBEAMS
JAGJ,SINGH,WILLIAMH,HOLT,ANDWILLISMOCK,JR,
kJBIAIYIIPY
AUGUST1982 ,.:ii,?!.J !982
J.ANGLEY RESEARCH CENTER
LIB:_ARY, NASA
HAMPTON, VIRGINIA
f
NASA
NationalAeronautics and
Space Adrninistration
Langley ResearchCenter
Hampton, Virginia 23665 _"-....
"_-......
https://ntrs.nasa.gov/search.jsp?R=19820025559 2020-03-21T06:08:21+00:00Z

POSITRON ANNIHILATION SPECTROSCOPY WITH
MAGNETICALLY ANALYZED BEAMS
By
Jag J. Singh
NASA Langley Research Center
Hampton, VA
and
William H. Holt and Willis Mock, Jr.
NSWC Dahlgren Laboratory
Dahlgren, VA
ABSTRACT
Lifetime measurements with magnetically analyzed positron beams have
been made in condensed media with uniform and non-uniform properties. As
expected, the lifetime values with magnetically analyzed beams in uniform
targets are similar to those obtained with conventional positron sources.
However, the lifetime values with magnetically analyzed beams in targets which
have non-uniform properties vary with positron energy and are different from
the conventional positron source-derived lifetime values in these targets.
INTRODUCTION
Positron Annihilation Spectroscopy (PAS) is usually conducted with posi-
trons having a continuous energy spectrum. (I-3) This is quite satisfactory when
the medium properties of interest are uniform throughout the medium. Positrons
of all energies encounter the same kind of molecular environment and, hence,
undergo common decay processes. However, if the medium properties are spatially-
dependent, the positron annihilation characteristics will vary depending on
where the positrons come to rest. For example, if the defects in a material
are confined to the surface layers, (4) only those positrons which stop in the
surface regions will get trapped and exhibit damage-dependent decay. The posi-
trons which penetrate deeper will not be affected by the surface defects and
will exhibit normal positron decay features. The resultant lifetime spectrum
will be a mixture of the two types of annihilation spectra. Similarly, if the
moisture in an organic matrix is not uniformly distributed, the positron
annihilation characteristics will vary depending on where the positrons are
thermalized in it. Those positrons which stop in the drier regions of the
target will have different lifetimes from those which stop in the regions
where more moisture is present.(5, 6) The resultant lifetime spectrum will be
¥
A/gA-Jj CJ5
/quite complex, being a mixture of spectra characterized by different lifetimes.
Thus, positrons of appropriate, well-defined, energies are needed for studying
spatially-dependent properties of the test media. The only practical means,
currently feasible, for producing monoenergetic positron beams of various
energies is to magnetically analyze positrons emitted from appropriate radio-
active sources. In this paper, we describe an experimental procedure for
producing monoenergetic positron beams. These monoenergetic positron beams have
been used for investigating spatial dependence of moisture concentration in
selected target media. The results of these studies are presented in the
following sections.
EXPERIMENTAL PROCEDURE
Production of Monoenergetic Positron Beams
A specially prepared i0 mc Na22 source was used as the positron emitter.
A 0.39 cm diameter, 7.62 cm long stainless steel tube placed over the source
produced a well-collimated positron beam. This narrow, collimated, beam of
positrons entered an 8.84 cm radius circular magnetic analyzer and, after
bending through 90° , exited through a specially designed collimated port.
Figure 1 shows the experimental arrangement for producing magnetically analyzed
positron beams. Figures 2(a) and 2(b) show the source holder and the collimator
used in the entrance channel. Figure 2(c) shows the geometrical details of
the collimator through which the magnetically analzyed positrons had to pass
before arriving at the test target. The exit-channel collimator was covered
with a 0.25 cm thick carbon annular disc in order to minimize Bremsstrahlung _
production at it. Figure 3 and 4 show computed trajectories of the positron
beams through the magnetic analysis system at two different energies. (7)
One of the important considerations in using magnetically-analyzed
positron beams is the survival of the positrons in the magnetic analyzer.
This is not expected to be a problem if the analyzer box is pumped down to
10-5 torr. Another important factor in using magnetically-analyzed positron
beams for PAS is the time reference for positron lifetime measurement. If one
is to continue to use the 1.28 MeV gamma ray as the signal for the birth of the
Na22 positrons, it is essential that the transit time for their passages through
the analyzer be small and--more importantly--the spread in it be much less than
the resolution time of the lifetime measurement system. Computed values for
the transit times and the energy resolution of the magnetic analysis system for
two energies are given in Tables I and II. (7) It is apparent that the energy
resolution, &E/E, is quite satisfactory (< 10%) and the spread in the positron
transit time is much less than the system resolution of _450 picoseconds. Thus,
it may be expected that conventional PAS can be conducted successfully with
magnetically-analyzed positron beams derived from a Na22 source or similar
sources where the positron emission is followed by a prompt gamma emission. For
positron sources without associated gamma emission, it will be necessary to
generate a reference time marker before the positrons entered the test medium.
One technique for producing such a reference marker _vld entail the use of a
Cerenkov counter placed in front of the test target.
kuj
Preparation of the Test Targets
Three different types of targets were used in the present study. A
2.54 cm diameter, 0.25 cm thick lucite disc which had been stored in a controlled
environment (relative humidity = 50%; temperature = 75°F) for several years
was used as the target with uniform moisture distribution in it. Two specially
designed assemblies served as targets with non-uniform properties: one of
them, made up of two dry, 2.54 cm diameter, 381 _m thick nylon-6 discs placed
back-to-back served as target assembly # 2. The other one, made up of one dry
2.54 cm diameter, 381 _m thick nylon-6 disc placed next to an identical disc
saturated with water served as target assembly # 3. The water-saturated disc
in target assembly # 3 was sealed in a 2.54 _m thick aluminized mylar to pre-
vent moisture loss during the positron lifetime measurement.
T_get assemblies 2 and 3 were designed for use with 250 keY positron
beams." " The thicknesses of the two discs comprising these two target
assemblies are such that 250 keV positrons pass through the first disc and
come to rest in the second. Thus, the 250 keV positrons will come to rest in
the dry disc in target assembly # 2 and water-saturated disc in target
assembly # 3.
EXPERIMENTAL RESULTS
• A conventional, fast-slow, constant fraction positron lifetime spectro-
meter, shown in figure 5, was used in the present study.(8) The spectrometer
had a full width at half maximum resolution of about 450 picoseconds. Each
full source spectrum was accumulated until it contained at least 106 counts
and was repeated several times to obtain average values of the lifetime for
the long-life component. The spectra with magnetically-analyzed beams were
accumulated until a statistical accuracy of at least i0 percent was achieved
in the short lifetime peak. All measurements were made at the room temperature
(75o). The lifetime spectra.were analyzed using a two-component exponential
least-squares fit program. (5)
Figures 6(a) and 6(b) show the lifetime spectra in lucite observed with
magnetically-analyzed 250 keV positron beams and the full Na22 positron
spectrum, respectively. Even though the statistical accuracy of the data
shown in figure 6(a) is rather poor, the measured values of the long-component
lifetimes indicated in these figures are in agreement with each other within
the accuracy of the measurements. Figures 7(a) and 7(b) illustrate the life-
time spectra observed with 250 keV positron beams striking target assembly # 2
and target assembly # 3, respectively. Again, the statistical accuracy of the
data shown in these figures is not very high, thus producing rather large
uncertainties in the long-component lifetime values.
(*)250 keV was selected as the test energy for the analyzed positron beams
_or reasons of workable beam intensity from a Na22 source. It is also
appropriate for investigating typical composite structural components
whose thickness is _762 _m.
Figures 8(a) and 8(b) show the positron lifetime spectra obtained with
the full Na22 positron spectrum in dry and water-saturated nylon-6 targets,
respectively. It is noted that the positron lifetime in target assembly # 2
(Fig. 7(a)) is in agreement with the value in dry nylon-6 target whereas the
lifetime in target assembly # 3 (Fig. 7(b)) is in agreement with the value
in wet nylon-6 target within the accuracy of the respective measurements.
CONCLUSIONS
The following general conclusions have been drawn on the basis of the
data presented in this report:
i. Positron annihilation spectroscopy can be conducted with magnetically-
analyzed positron beams using a conventional, fast-slow, constant fraction
positron lifetime spectrometer.
2. Magnetically-analyzed positron beams can be used to monitor non-
uniform properties of certain targets--such as non-uniform moisture distribu-
tion in molecular substances. This feature is especially useful in measuring
moisture profiles in polymeric materials.
However, a much stronger positron source (_ 25 mc) and longer data
recording times would be needed for improved statistical accuracy with
magnetically-analyzed positron beams.
REFERENCES
i. J. W. Wilkenfeld and J. John: In "Nondestructive Evaluation of Materials"
(eds.: J. J. Burke and V. Weiss; Plenum Press, New York, 1976) p. 479.
2. "Positrons in Solids" (ed.: P. Hautojarvi; Springer Verlag, Berlin, 1979).
3. J. R. Stevens: In "Methods of Experimental Physics, Vol. 16 (Part A),
Polymers (Molecular Structures and Dynamics);" (ed.: R. A. Fava; Academic
Press, New York, 1980) p. 371.
4. P. G. Forrest: Fatigue of Metals (Pergammon Press, New York, 1970);
Stress Analysis and Growth of Cracks/ASTM Special Technical Publication
# 513 (American Society for Testing and Materials, Philadelphia, 1972);
R. W. Siegel: Positron Annihilation - A Localized Probe of Lattice Defects
in Metals. Scripta Metallurgica, Vol. 14, 15, 1980; R. W. Siegel:
Positron Annihilation Spectroscopy. Annual Rev. Materials Science, IO,
393, 1980.
5. J. J. Singh, W. H. Holt, and W. Mock, Jr.: Moisture Determination in
Composite Materials Using Positron Lifetime Technique. NASA TP-1681, 1980.
6. J. J. Singh, W. H. Holt, and W. Mock, Jr.: Measurement of Moisture Depth
Distribution in Composite Materials Using Positron Lifetime Technique.
Proceedings of Fourth International Conference on Nuclear Methods in
Environmental and Energy Research, held at Columbia, MO, April 14-17, 1980,
pages 258-267 (Available from Technical Information Center, U.S. Department
of Energy as CONF-800433).
7. J. J. Singh: Computational Methods for Analyzing the Transmission
Characteristics of a Beta Particle Magnetic Analysis System. NASA
Technical Memorandum 80179, 1979.
8. W. H. Holt and W. Mock, Jr.: Positron Lifetime Technique for Nondestruc-
tive Evaluation of Materials. NSWC/DL-TR-3573, U. S. Navy, December 1976.
(Available from DTIC as AD A038 305).
Table I. Summary of Critical Parameters for Positrons Arriving at
Target for Magnetic Field Set for 200 keV (Ref. 7)
_onvex source configuration; i000 positrons emitted at the sourc_
Target Diameter
Parameter
i cm 2.54 cm
Y = 9 cm
c
Eo+ (min), keV 175.83 154.91
E_+ (max), keV 230.75 263.40
Tp (min), ps 996.10 982.49
T (max), ps 1085.64 1118.87
No. of hits 68 174
Y = 12 cm
c
Eo+ (min), keV 187.34 165.20
E_+ (max), keV 218.70 247.30
Tp (min), ps 1143.53 1123.25
T (max), ps 1215.44 1241.45
No. of hits 44 140
Y = 16 cm
c
E + (min), keV 187.94 174.86
ES+ (max), keV 211.70 230.75
TB (min), ps 1350.55 1320.40
T (max), ps 1411.32 1436.76
No. of hits 38 103
Y = 20 cm
c
L
E + (min), keV 191.72 179.44
E + (max), keY 208.06 224.71
_ (min), ps 1549.43 1518.31
T (max), ps 1587.12 1610.38
No. of hits 28 77
Table II. Summary of Critical Parameters for Positrons Arriving at
Target for Magmetic Field Set for 300 keV (Ref. 7)
_onvex source configuration; I000 positrons emitted at the sourc_
Target Diameter
ParameL=_
i cm 2.54 cm
Y = 9 cm
c
E + (min), keV 272.96 235.58
E_+ (max), keV 347.05 398.60
T_ (min), ps 900.02 894.96
(max), ps 952.90 972.04
No. of hits 74 161
Y = 12 cm
c
E + (min), keV 277.95 250.92
E_+ (max), keV 328.33 365.64
_ (min), ps 1030.65 1025.05
T (max), ps 1081.73 1101.24
No. of hits 58 127
Y = 16 cm
c
E_+ (min), keV 285.76 262.36
E_+ (max), keV 317.88 347.05
Tp (min), ps 1210.39 1192.08
(max), ps 1253.52 1260.10
No. of hits 44 98
Y = 20 cm
c
E + (min), keV 288.78 272.24
• E_+ (max), keV 315.12 332.66
T_ (min), ps 1381.06 1367.96
(max), ps 1425.30 1433.93
No. of hits 31 84

NOT TO SCALE
TARGET
EXIT PORT I MIL KAPTONWINDOWCOLLIMATOR
Y
POSITRON
SOURCE
(0,0) -. - --_ -,.. x\
" LC MAG.FIELD
REGION
RF = 8.839 cm
Lc = 7.620 cm
FIGUREI- SCHEMATICDRAWINGSHOWINGSOURCECOLLIMATORAND MAGNETIC
FIELD REGION.
NOT TO SCALE
Dc=0,39 cm
Dc =
22
(a) PROFILE OF No SOURCEDEPOSITED AT BOTTOMOF
COLLIMATOR SELECTED FOR DETAILED STUDY.
No22SOURCE
J 0.08 cm
°ctr I Oc=Oc
Lc = Lc=7.62 cm
(b) SCHEMATICDIAGRAMOF SOURCECOLLIMATOR
1.02 cm
0.25cm
0.16 cm
5.59 cm
1.89 cm
2.59 cm =
(C) GEOMETRICAL DETAILS OF THE EXIT PORT COLLIMATOR
FIGURE 2.- GEOMETRICAL DETAILS OF THE SOURCE PROFILE , SOURCE
COLLIMATOR AND THE EXIT PORT COLLIMATOR.
20.0 cm= Yc
SOURCE _"
MAGNETICFIELD
REGION
(o) I-cm- DIAMETER TARGET.
II.-
TARGET T
20.0 cm = Yc
SOURCE =
MAGNETICFIELD
REGION
(b) 2.54- cm- DIAMETER TARGET.
FIGURE3.-TRAJECTORIESOF POSITRONSSTRIKINGTARGET
FROMCONVEXSOURCECONFIGURATION.MAGNETIC
FIELDSET FOR Eft.= 200 keV
TARGET _,.. l
20.0cm =Yc
SOURCE
(o) I-cm- DIAMETER TARGET.
TARGET --- [
20.0cm =Yc
SOURCE
MAGNETICFIELD
REGION
(b) 2.54-cm-DIAMETER TARGET,
FIGURE 4.- TRAJECTORIESOF POSITRONS STRIKING,TARGET
LOCATED 20cm FROM CENTER OF MAGNETIC
FIELD SET FOR Era.= 300 keV. CONVEX SOURCEHCONFIGURATION.
( 051 MeV /
__\CHANNEL/
LEGEND
I I0 mCi COLLIMATEDNoz2SOURCE
5 I_ 2 PLASTIC SCINTILLATOR
3 PHOTOMULTIPLIERDETECTOR
, 4 PREAMPLIFIER
__o__ 5 0.025 mm KAPTONWINDOW6 AMPLIFIER/DISCRIMINATOR
"/ I _, J ---/ : ", I I_ _1 7 25.4 mm x 25.4mm x 3.2ram
I / ) -\\ I I-" _L COMPOSITE TESTTARGET
--....---" ',1 0T,MTO.,GHT.TO,9 TiMiNGDISCRIMINATOR
e* /I / I0 VARIABLEDELAY
MAGNETIC / I I
I.28 MeV/
CHANNEL/
FIGURE5.- SCHEMATICDIAGRAMOF THE POSITRONLIFETIME MEASUREMENTSYSTEMUSING
MAGNETICALLYANALYZEDPOSITRONBEAMS.
u_
Z
Lu> _o102 - LONG LIFETIME
Z _0 COMPONENT
0 o _,q)/ T2= (1883_69T)ps
__1 o \x_ Ocb"r
oOOoO_o°o° _ o oo o o
e oo °°o oo °oo ° \_o°__oSoooooOO
z o oO \ o \ OoOO_oO_ooOo_o_ oOo
,0,-°°°°°° °o
SHORT LI FETIME
_'- COMPONENT
ZERO TIME--,- _ r I = (374± 63) ps
I0° --4. i i i \ i a i i
Y
0 40 60 80 I O0 120 140 160
CHANNEL NUMBER (ONE CH.= 117.35ps)
FIGURE 6(o)- POSITRON LIFETIME SPECTRUM OBTAINED WITH 250
KEV POSITRONS INCIDENT ON A LUCITE
SPEClMEN.
,0_-
Q
105 _
o
LONG LIFETIME
o COMPONENT
T2 = (1945-*-25)ps
_ 104 _
ZU.I o
b.I
Z
0
_J
-_ ,d-
z
z AVERAGE BACKGROUND
o COUNTS- I00 COUNTS/CH.
Z
o
r_ o
I- o
o
a_ 102 - SHORT LIFETIME
o o o _ _-oo / COMPONENT
o_Oo_ _ _- +ZEROT,ME T,=(384_20_.,
I01 _ I 1 ' I I I 1 I I
0 40 60 80 I00 120 140 160 180 200 220 240
CHANNEL NUMBER (ONE CH.= 116.75ps)
FIGURE 6(b)- TYPICAL POSITRON LIFETIME SPECTRUM IN A LUCITE SPECIMEN
" USING FUll Noz2 SPECTRUM.
2 o
I0 -- 0
o o
(D o
I--- o o LONG LIFETIMEZ o
i,I COMPONENT
> o T2= (1812_183) psW o o
Z o
0 o
o o o o o
I 0
IO -- o
_1 o o o o
-r o o o o ooo o o
0 0 0 0 0 O0 0 0 0
Z 000 O0 0 0 0 0 0 0 0
Z O0 0 O0 0 0 000 O0
,_ o
0 0 0 0 0 0 0Z o
O o o
n-
I-- o
(.D
0 I0 0 _ o
n
ZERO TIME SHORT LIFETIME
COMPONENT
r I = :387 ps
I I I I I I I
40 60 80 I00 120 140
CHANNEL NUMBER (ONE CH.= 117.65 PICOSECONDS)
FIGURE 7(0)-POSITRON LIFETIME SPECTRUM OBTAINED WITH 250 KEV POSITRONS
INCIDENT ON A NYLON-6 TARGETASSEMBLY-2 (DRY/DRY).
i I
11 .i •
4
I0 --
I03 _
0
o
03 o
I-- 2
Z I0 --W o
ILl LONG LIFETIME
Z COMPONENT
0 oO T2= (1841 ± 153) ps
._1
I
_ 0
Z i01 o o o
•_ o °o
0 o
Z o o
0 0 0 o oo o o OO
O0 000 O0 0 0 0000 0 0 0 O0k-
_'_ CO 0 0 O0
0
o_ o o o
i00 _ o o
ZERO TIME
SHORT LIFETIME
:OMPONENT
_'1 = 387 ps
I I I I I I I
30 40 50 60 70 80 90 I00 I I0 120
CHANNEL NUMBER (ONE CH.= 116.67 PICOSECONDS)
FIGURE 7(b)- POSITRON LIFETIME SPECTRUM OBTAINEDWITH 250 KEV POSITRONS
INCIDENT ON A NYLON-6 TARGET ASSEMBLY-3 (DRY/WET).
6
I0 -
)
5
I0 --
o
4
t0 o
I--
Z
I.iJ
>bJ
°= 2 o
.J
_ "l-
Z o
Z
'_Z _ LONG LIFETIME
bX _COMPONENT
0Z 102 _ Tz ={1753_ 30)ps
-\rY o
or) o o o0
('i oo
o °o
oo o o o o
ooo o
I o °° oooo o ;HORT LIFETIME "_ °° o o _ o10 - o oo
COMPONENT _o oo oo oo o o
o Ti = 575ps o_.. o o o o
o (_ o oo oOo o o o oo o olo oo o
0 O0 0 _0 0 0 0003 0
ZERO _ o o o o o
oo TIME ocb coo o oooooo cD\ Oo
I I I IX,i0° I I I I I I I l I
0 20 40 60 80 I00 120 140 160 180 200 2ZO 240 260 2BO
CHANNEL NUMBER (ONE CH.-- 117.65 ps)
FIGURE 8(.0)- TYPICAL POSITRONLIFETIME SPECTRUM IN A DRY NYLON -6 SPECIMEN
USING FULL No22 SPECTRUM.
6
I0 --
Z
I,I
> 104ILl
Z ..
0 o
o
J
-1-
Z 103 _Z
<_Z LONG LIFETIME
COMPONENT
Z T 2 = (1945 + 24) ps0
n-" o
F-
Or)
0_0 Id -- o° 0°E'_O%O%OOOo^
o° SHORT LIFETIM
OO%ooOo o oOO^
o° COMPONENT o
OOooO oo O Ooo°>°°-°oOo o oo o ooo TI: 380 ps _ oo ooo o_ oo o o °°o° o°°° (3Oo
ZERO TIME
i01 I I I 1 I I "_ I I I I I
0 20 40 60 80 I00 120 140 160 180 200 220 240 260 280
CHANNEL NUMBER (ONE CH. = 117.65 PICOSECONDS)
FIGURE 8(b)-TYPICAL POSITRON LIFETIME SPECTRUM IN A WATER-SATURATED
NYLON-6 SPECIMEN USING FULL No22 sPEcTRUM.
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM-84535
4. Title and Subtitle 5. Report Date
Positron Annihilation Spectroscopy with Magnetically AuguSt 1982
Analyzed Beams 6. Performing Organization Code
505-33-23-05
7. Author(s) 8. Performing Organization Report No.
Jag J. Singh, William H. Holt, and Willis Mock, Jr.
10. Work Unit No.
9. Performing Organization Name and Address
Langley Research Center '11. Contract or Grant No.
Hampton, VA 23665
13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address Technical Memorandum
National Aeronautics and Space Administration 14. ArmyProjectNo.
Washington, DC 20546
5. Supplementary Notes
16. Abstract
Lifetime measurements with magnetically analyzed positron beams have
been made in condensed media with uniform and non-uniform properties. As
expected, the lifetime values with magnetically analyzed positron beams in
uniform targets are similar to those obtained with conventional positron
sources. However, the lifetime values with magnetically analyzed beams in
targets which have non-uniform properties vary with positron energy and are
different from the conventional positron source-derived lifetime values in
these targets.
17. Key Words (Suggested by Author(s)) 18. Distribution Statement
Positrons, Positron Lifetime, Moisture
in Polymers, Magnetic Analyzer, UNCLASSIFIED - UNLIMITED
Monoenergetic Positrons, Moisture
Distribution Subject Category 24
19. S_urity Classif. (of this re_rtl 20. Security Classif. (of this _ge) 21. No. of Pages 22. Price"
Unclassified Unclassified 19 A02
"For _le by the National Technical Information Se_ice, Springfield, Virginia 22161

iP


Positron annihilation spectroscopy with magnetically analyzed beams

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server