A slow positron beam generator using well-annealed polycrystalline tungsten moderators and a Na-22 positron source was developed. A 250 micro c source, deposited on a 2.54 micron thick aluminized mylar, is sandwiched between two (2.54 cm x 2.54 cm x 0.0127 cm) tungsten pieces. Two (2.54 cm x 2.54 cm x t cm) test polymer films insulate the two tungsten moderator pieces from the aluminized mylar source holder (t=0.00127 to 0.0127). A potential difference of 10 to 100 volts--depending on the test polymer film thickness (t)--is applied between the tungsten pieces and the source foil. Thermalized positrons diffusing out of the moderator pieces are attracted to the source foil held at an appropriate potential below the moderator pieces. These positrons have to pass through the test polymer films before they can reach the source foil. The potential difference between the moderator pieces and the aluminized mylar is so adjusted as to force the positrons to stop in the test polymer films. Thus the new generator becomes an effective source of positrons for assaying thin polymer films for their molecular morphology

Eftekhari, Abe

Singh, Jag J.

St.clair, Terry L.

English

NASA Technical Reports Server

. 
NASA Technical Memorandum 101590 
A SLOW POSITRON BEAM GENERATOR FOR LIFETIME 
STUD I ES 
ti89-25433 
Unclas 
63/35 Oil2659 
JAG J, SINGH, ABE EFTEKHARI, AND TERRY L, ST, CLAIR 
APRIL 1989 
National Aeronautics and 
Space Administration 
Langley Research Center 
Hampton, Virginia 23665 
https://ntrs.nasa.gov/search.jsp?R=19890016062 2020-03-20T01:45:22+00:00Z
A SLOW POSITRON BEAM GENERATOR FOR LIFETIME STUDIES 
Jag J. Singh, Abe Eftekhari, and Terry L. St. Clair 
We have developed a slow positron beam generator using well-annealed 
polycrystalline tungsten moderators and a Na22 positron source. 
250 pc source, deposited on a 2.54 pm thick aluminized mylar, is sand- 
wiched between two 2.54 cm x 2.54 cm x 0.0127 cm tungsten pieces. Two 
2.54 cm x 2.54 cm x t cm test polymer films insulate the two tungsten 
moderator pieces from the aluminized mylar source holder (t may range 
from 0.00127 to 0.0127 cm). A potential difference of 10 to 
100 volts--depending on the test polymer film thickness (t)--is applied 
between the tungsten pieces and the source foil. Thermalized positrons 
diffusing out of the moderator pieces are attracted to the source foil 
held at an appropriate potential below the moderator pieces. 
positrons have to pass through the test polymer films before they can 
reach the source foil. The potential difference between the moderator 
pieces and the aluminized mylar is so adjusted as to force the posi- 
trons to stop in the test polymer films. Thus the new generator 
becomes an effective source of positrons for assaying thin polymer 
films for their molecular morphology. 
A 
These 
I. INTRODUCTION 
Polymers are finding increasing applications in aerospace industry. 
They are strong, lightweight, and can be developed to have desirable 
mechanical, electrical and optical properties. Some of the more chal- 
lenging applications often call for polymers in the form of thin films. 
The properties of these films are strongly dependent on their molecular 
morphology. 
have different physical properties depending on their processing 
history. It is therefore necessary to develop a technique that can 
give information about the internal structure of the "finished" thin 
polymer films. 
Films with the same chemical composition and density can 
We had previously used Positron Annihilation Spectroscopy (PAS) for 
measuring free volume in polymer discs(ls2). 
to adapt conventional PAS to the study of thin films. The first 
attempt involved sandwiching the test films between suitable aluminum 
-degraders such that positrons of progressively higher energies were 
forced to stop in the test films. Figure 1 shows the source-target 
assembly. In order to eliminate the effects of positrons annihilating 
in the aluminium energy degraders, lifetime spectra were taken with 
and without the test films in the target assembly. The difference 
between these two spectra represented the test film lifetime data. 
However, only a small fraction of the incident positron beam stopped in 
the test films due to the original Fermi energy(3) distribution of the 
Na22 positrons. Consequently, it required 24 hours or more to accumu- 
late data with adequate statistics, even with a 25 pc Na22 source. 
It was therefore decided 
2 
In order to expedite the data accumulation with better statistics, the 
following slow positron beam generation scheme was devised and imple- 
mented: Isotropic positron beams were allowed to penetrate a 0.0127 cm 
thick annealed, high purity (99.95%) tungsten foil. After quick ther- 
malization, the incident positron beams suffered multiple elastic scat- 
tering from tungsten atoms, forcing some of the positrons to diffuse 
out of the same side whence they entered the foil. Since tungsten has 
a negative work function for positrons(4) , those positrons that survive 
annihilation and trapping would eventually diffuse out of it. 
positrons can be accelerated to preselected energies by simply applying 
an appropriate electrical potential difference between the tungsten 
moderator and a metal electrode positioned in front of it. The exper- 
imental details and some applications of this concept are described in 
the following sections. 
These 
11. EXPERIMENTAL DESIGN 
High purity (99.95%) polycrystalline tungsten foils in the form of 
2.54 cm x 5.08 cm x 
vacuum by repeatedly heating them to red heat and allowing them to cool 
down while still in vacuum. The annealed strip was cut into two 
2.54 cm x 2 . 5 4  cm x 0.0127 cm pieces which served as positron beam 
moderators. 
ized mylar was inserted between two moderator pieces. 
films, introduced between the source foil and the moderator steps, 
helped electrically insulate the source holder from either tungsten 
strip. The source-moderator assembly is shown in figure 2. 
0.0127 cm strips were annealed in 6 x pascal 
A 250 pc Na22 source deposited on a 2.54 pm thick alumin- 
The test polymer 
After injection into tungsten moderator the positrons are quickly 
thermalized. Subsequently, they suffer multiple elastic scattering 
over distances of the order of 1000 A' before annihilating with the 
electrons(5). Some of these positrons diffuse back to the entrance 
surface whence they are emitted with an energy spread of the order of 
0.2 ev. These positrons can be accelerated to a desired energy(6) by 
applying appropriate potential difference between the moderator and the 
source foil. It should be noted that the positrons strike the modera- 
tor surface isotropically, thereby enhancing their probability of even- 
tual escape from the entrance surface. Also, the back diffusing posi- 
trons come mainly from the near surface regions because of limited 
acceptance time selected for the positron lifetime measurement system. 
111. EXPERIMENTAL PROCEDURE 
As shown in figure 2, the test polymer films are inserted between the 
source foil and the moderators. Consequently, all re-emitted positrons 
attracted towards the source foil must pass through the test films. By 
adjusting the voltage between the source foil and the moderator strips, 
all positrons emitted from the moderator surfaces can be made to stop 
in the test polymer films(6). Thus, thin polymer films which could not 
be conveniently studied for their free volume characteristics using the 
3 
multi-degrader assembly of figure 1 can be studied readily by appl-ying 
appropriate voltage on the source foil. 
In order to separate the moderator annihilation spectrum from the test 
film spectrum, it is necessary to make two separate lifetime 
measurements. The first measurement is made with the source foil held 
at -V volts. The second measurement is made with the source held at a 
much lower potential--such as -10 volts. The difference between these 
two spectra then gives the effects of annihilation in the interior of 
the test polymer film. By measuring the total number o f  counts in the 
lifetime spectra with +V volts at the source foil, the efficiency (E) 
of the slow beam generator can be calculated as follows: 
I(-V) - I ( + V )  
I(-V) + I(+V) € = 2  
The value of E: for the present generator is of the order of 6 %  
IV. TEST OF THE SLOW POSITRON BEAM SYSTEM 
In order to test the validity of the assumption that the thermalized 
positron beams diffusing back from the moderator surfaces do not suffer 
inordinate delays with respect to the 1.28 MeV gamma ray time marker, 
we compared the lifetimes measured in a 0.0089 cm thick teflon film 
using the present system with the corresponding values in a 0 . 2 5 4  cm 
thick disc obtained by using the conventional thick disc procedure. 
The results are summarized in Table I. 
It is apparent that the agreement is reasonably good, particularly in 
the case of ( ~ 3 / 1 3 )  values which are the basis for free volume measure- 
ments in polymers. 
V. APPLICATIONS 
We have applied the procedure described earlier for the measurement of 
free volume fraction in two polyimide films. The physical properties 
of these films are summarized in Table 11. The polyimide, PMDA/ODA, is 
a NASA-Langley pre ared polymer which has the same chemical structure 
as that of Kapton8 (Dupont's commercially available polyimide) . The 
polyimide, ODPA/p-PDA, is an isomer (same chemical formula with differ- 
ent arrangement of chemical units) of Kaptonm. 
chemical structure of the two isomers. 
totally amorphous polymer while the ODPA/p-PDA has a significant level 
of  crystallinity. This crystallinity results in a high modulus in the 
latter polymer, 9 7 5 , 0 0 0  psi, as compared to 380,000 psi for PMDA/ODA. 
Even though the two systems are isomers of each other, they obviously 
have considerable differences in their morphologies. The free volume 
fractions in these films were measured using the relationst~ip(~) 
Figure 3 shows the 
The PMDA/ODA is essentially a 
4 
between the ortho-positronium lifetime (7') and the average microvoid 
size (R): 
where Ro = Radius of the spherical potential well 
(The potential is infinite for r > Ro and constant 
for r < Ro) 
R = R  - A R  
0 
( A R  is the thickness ok the electron layer") inside the 
potential well and h a s  been assumed t o  be 0.1656 nm) 
Equation 1 is based on the assumption that the orthopositronium (0-Ps) 
lifetime in the electron layer is cv 500 picoseconds. 
Positron lifetimes in the polyimide films were measured using standard 
fast-fast coincidence lifetime measurement technique based on the slow 
positron beam generator. Figure 4 shows typical lifetime spectra in 
ODPA-p-PDA films. 
the average free volume cell volumes, <Vf>, in the two films are summa- 
rized in Table 111. Clearly, ODPA-p-PDA films have large-sized micro- 
voids, but comparatively fewer of them. 
The 0-Ps lifetimes and the corresponding values of 
I VI. CONCLUDING REMARKS 
I A slow positron beam generator suitable for Positron Annihilation Spec- 
troscopic measurements in thin ('< 0 . 0 2 5 4  cm) polymer films has been 
developed. Its efficiency, determined by the ratio of the total counts 
in the "Difference" spectrum and the total counts in the moderator 
life-time spectrum, is of the order of 6 % .  Furthermore, the present 
scheme also successfully resolves the difficult timing problem that can 
be encountered with the positron moderator foils. I 
I 
5 
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2 .  
3 .  
4 .  
5 .  
6 .  
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Moisture Dependence of Positron Annihilation Spectra in Nylon-6, 
Nucl. Instr. &Methods, Vol. 221, 4 2 7 ,  1 9 8 4 .  
Singh, J. J . :  Applications of Positron Annihilation Spectroscopy 
in Materials Research. Proceedings of 33& International Symposium 
on Materials--Pathway to the Future, held at Anaheim, California, 
March 7 - 1 0 ,  1 9 8 8 .  Edited by G .  Carillo, E. D. Newell, W. D. Brown 
and P. Phelan (published by SAMPE, Covina, 1 9 8 8 )  pp. 4 0 7 - 4 2 1 .  
For example, see Nuclei and Particles (Second Edition) by E. Segre, 
Chapter 9--Beta Decay, pp. 3 9 1 - 4 4 5  (published by W. A .  Benjamin 
Publishing Company, Reading, 1 9 7 7 ) .  
Vehanen, A., Lynn, K. G., Schultz, P. J. and Eldrup, M.: Improved 
Slow Positron Yield Using a Single Crystal Tungsten Moderator, 
Appl. Phys.; m, 163, 1 9 8 3 .  
Vehanen, A.: A High Intensity C 0 5 8  Slow-Positron Beam and Applica- 
tions to Probe Near-Surface Disorder. Appl. Phys.; m, 2 6 9 ,  1 9 8 7 .  
Berger, M. J. and Seltzer, S. M.: Tables of Energy Losses and 
Ranges of Electrons and Positrons. NASA SP-3012 ( 1 9 6 4 ) .  
7(a). Nakanishi, H., Wang, S .  J. and Jean, Y. C.: Microscopic Surface 
Proceedings of Inter- Tension Studied by Positron Annihilation. 
national Symposium on Positron Annihilation Studies of Fluids, 
held at Arlington, Texas, June 8 - 1 2 ,  1 9 8 7 .  Edited by S .  C. 
Sharma (published by World Scientific Publishing Company, 
Singapore, 1 9 8 8 ) ,  pp. 2 9 2 - 2 9 8 .  
7 ( b ) .  Eldrup, M. Lightbody, D., and Sherwood, J. N.: The Temperature 
Dependence of Positron Lifetimes in Solid Pivalic Acid, Chem. 
Phys.; Vol. 63, 5 1 ,  1 9 8 1 .  
8 .  Tao, S. J . :  Positronium Annihilation in Molecular Substances. J. 
Chem. Phys.; 56(No. ll), 5 4 9 9 ,  1 9 7 2 .  
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TABLE 111. Summary of Free Volume Results in the Test Polyimide Films 
ODPA-p-PDA 
No. 1 Film Composition 
I 
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Positron Lifetime Values 
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Report Documentation Page 
1. Report No. 2. Government Accession No. 
NASA TPl- 10 1590 I 
17. Key Words (Suggested by AuthorW) 
Variable Energy Positron Beams. Tungsten 
?(oderator, Negative Work Function, Thin 
Polymer Films, Orthopositronium Lifetime, 
Free Volume 
I 
4. Title and Subtitle 
A Slow Positron Beam Generator for Lifetime 
Studies 
18. Distribution Statement 
UNCLASSIFIED - UNLINITED 
Subject Category 35 
7.  Author(s1 
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages 
UNCLASSIFIED UNCLASSIFIED 14 
Jag J. Singh, Abe Eftekhari, and Terry L. St. Clair 
22. Price 
A0 3 
9. Performing Organization Name and Address 
NASA Langley Research Center 
Hampton, VA 23665 
12. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
Washington, DC 20546 
3. Recipient’s Catalog No. 
5. Report Date 
April 1989 
6. Performing Organization Code 
~~ 
8. Performing Organization Report No. 
10. Work Unit No. 
506-43-2 1-05 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
15. Supplementary Notes 
Jag J. Singh and Terry L. St. Clair: Langley Research Center, Hampton, Virginia. 
Abe Eftekhari: Analytical Services & Materials, Inc., Hampton, Virginia. 
16. Abstract 
We have developed a slow positron beam enerator using well-annealed 
polycrystalline tungsten moderators and a Na2’ positron source. A 250 pc source, 
deposited on a 2.54 pm thick aluminized mylar, is sandwiched between two 
2.54 cm x 2.54 cm x 0.0127 cm tungsten pieces. Two 2.54 cm x 2.54 cm x t cm test 
polymer films insulate the two tungsten moderator pieces from the aluminized 
mylar source holder (t may range from 0.00127 + 0.0127). A potential difference 
of 10 to 100 volts--depending on the test polymer film thickness (t)--is applied 
between the tungsten pieces and the source foil. 
out of the moderator pieces are attracted to the source foil held at an appropriate 
potential below the moderator pieces. 
test polymer films before they can reach the source foil. The potential difference 
between the moderator pieces and the aluminized mylar is so adjusted as to force 
the positrons to stop in the test polymer films. Thus the new generator becomes 
an effective source of positrons for assaying thin polymer films for their 
molecular morphology. 
Thermalized positrons diffusing 
These positrons have to pass through the 


A slow positron beam generator for lifetime studies

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