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55
PROTON REACTION ANALYSIS FOR LITHIUM AND FLUORINE I N GRAPHITE,
USING A SLIT SCANNING
R . L. Macklin, J. H. G i b b o n s , and T. H. Handley
NOTICE This document contains information of a preliminary nature and was prepared primarily for internal use a t the Oak Ridge National
Laboratory. I t i s subject to revision or correction and therefore does not represent a final report.

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L E G A L NOTICE -
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T h i s report was prepored as on account of Government sponsored work. the Commission, nor ony person acting on behalf of the Commission:
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ORNL-'IN-2238
C o n t r a c t N o . W-7405-eng-26
Physics D i v i s i o n
PROTON REACTION ANALYSIS FOR LITHIUM
FLOURINE I N GRAPHITE,
U S I N G A SLIT SCANNING TECHNIQUE
R . L. M a c k l i n , J. H. G i b b o n s , and T. N. H a n d l e y
JULY 1968
OAK R I D G E NATIONAL M O R A T O R Y
Oak R i d g e , T e n n e s s e e
O p e r a t e d by
UNION CARBIDE CORPORATION f o r t h e
U . S . ATOMIC ENERGY COMMISSION
U
*

.
, iii
CONTENTS
Page
Abstract
. . . . . . . . . . . . . . . . . . . . . e .
.
1
Introduction
. . . . . . . . . . . . . . . . . . . . .
.
1
Method and Apparatus
. . . . . . . . . . . . . . . . .
.
2
Standards
. . . . . . . . . . . . . . . . .
O I .
.
.
3
Sample
. . . . . . . . . . . . . . . . . . . . . . . .
* 3
Results and Discussion
. . . . . . . . . . . . . . . .
3
Microscopic Examination of Sample
. . . . . . . . . .
. 4
References
. . . . . . . . . . . .

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PROTON REACTION ANALYSIS FOR LITHIUM AND FLUORINE IN GRAPHITE,
USING A SLIT SCANNING TECHNIQUE
R. L. Macklin, J. H. Gibbons, and T. H. Handley
ABSTRACT
Protons from the ORNL 3
Van de Graaff accelerator were brought to a line focus and collimated thru slits of either 0.025 or 0.0075 cm. Cross sectional cuts of graphite samples were moved across the beam to study the distribution of Li and F with depth by measuring the yields of neutrons and of g a m a rays from the reactions TLi(p,n) and lgF(p,q).
A graphite sample exposed to molten fluorides in a loop experiment at the Oak Ridge Research Reactor showed about 20 and 100 ppm (by weight) Li and F respectively, 2 m exposed surface. The observed ratio of Li to F was close to that characteristic of the molten salt down to
3 mm below the exposed surface.
INTRODUCTION
The proton induced reactions 7Li(p,n) and ''F(p,q) have been
1 used to measure the concentration of the target nuclides in graphite.
This can be done in the presence of considerable radioactivity from fission products, making the method attractive for studies of penetra- tion in graphite moderated molten salt reactors.2
1
In the previous work
, the exposed surface of a sample was ground away a layer at a time and the concentrations near each freshly exposed surface measured with a
0.32 cm diameter, 0.1-1 special handling for the radioactive material ground off and introduced considerable delay in the measurements.

- 2 -
METHOD
APPARATUS
The ORNL 3 MV Van de Graaff accelerator proton beam is normally focussed to a cy
1 diameter spot at the target with a magnetic quadrupole strong focus lens. By deliberately detuning this lens the beam can be brought to a horizontal (or vertical) line focus about 1.0 x 0.08 cm. By using several microamperes of current and a metal slit, sufficient beam could be passed through to a sample to perform the analysis over an area 0.0075 cm x 0.60 cm, or with better sensitivity
(compared with background from F impurities in the slit collimator material) 0.025 cm x 0.60 em. With this arrangement, a sample exposed in a reactor can be sectioned, cutting perpendicular to an exposed face and the distribution of 7 L i and with depth studied by simply moving the sample across the proton beam. The apparatus we used for this is
indicated schematically in Fig. 1.
The graphite sample was mounted on a micrometer head and moved in to intercept the beam. The proton beam passing the sample induced a blue fluorescence in the quartz viewing plate at the left so the point at which the edge of the sample just intercepted all of the protons could be easily noted. The profile of the beam passing through the slit was measured with a 0.005 cm A1 foil
glued to the end of the graphite blank. Figures 2
and 3 show the
27Al(p,y) yield from this foil using 0.025 ern and 0.0075 cm gold slits respectively. Slits of tantalum showed too much contamination with fluorine. The slight asymmetry of the scans is probably due to a few protons striking the exposed face of the A1 foil rather than the edge as the blank sample was pulled up out of the beam.
I
.
2.
I

- 3 -
Gamma r a y s were d e t e c t e d by a 12.5 cm d i a . x 17.5 cm l o n g N a I ( T 1 ) c r y s t a l through a 1 . 2 5 ern lead f i l t e r a t t h e s i d e of t h e sample housing.
Neutrons were d e t e c t e d i n t h e s t r a i g h t ahead p o s i t i o n as i n r e f e r e n c e (1).
A s t h e g r a p h i t e b l a n k sample could r e a d i l y be brought i n t o t h e beam t o check t h e background w i t h o u t dismounting t h e sample, w e u s e d 2.06 MeV p r o t o n s ( j u s t below t h e ’Be(p,n) t h r e s h o l d ) throughout, r a t h e r t h a n dropping t h e energy below t h e “ L i ( p , n ) t h r e s h o l d (1.88 MeV) t o check background.
STANDARDS
P r e s s e d samples of g r a p h i t e powder c o n t a i n i n g weighed q u a n t i t i e s of
7
LiF were u s e d as s t a n d a r d s .
The sample s t u d i e d w a s t a k e n from a molten s a l t convection l o o p exposed i n t h e Oak Ridge Research Reactor
year.’ The p i e c e a v a i l a b l e for s t u d y i s shown i n F i g . 4 (ORNL S l i d e No. 74245). The s t r a i g h t e s t s i d e shown i s t h e w a l l of a molten s a l t e x i t channel s e c t i o n e d a x i a l l y a f t e r exposure. The proton b e a m w a s c e n t e r e d a l o n g t h i s s u r f a c e , as n e a r l y p a r a l l e l t o it as i t s i r r e g u l a r i t i e s would p e r m i t .
19 f a s t n e u t r o n s p e r s q u a r e c e n t i m e t e r (79% of i t w i t h 235U b e a r i n g molten s a l t ) and a cracked o u t l e t p i p e w i t h consequent a i r contamination.
RESULTS
The homogeneity o f t h e s t a n d a r d s w a s d i s a p p o i n t i n g . Scans of o r i g i n a l and f r e s h l y c u t s u r f a c e s showed c o n s i d e r a b l e v a r i a b i l i t y . The s t a n d a r d d e v i a t i o n a t a s i n g l e p r o t o n beam p o s i t i o n appeared t o b e about

- 4 -
3546. An uncertainty of that size is indicated for the absolute scales of concentration. The standardization was, however, based on the grand average of all our measurements on the 200 ppm (
7
LiF by weight) stan- dard. Until more uniform standards are available or the cause of the observed nonuniformity is better understood, we hesitate to claim greater accuracy for the concentration scale.
The fluorine and lithium concentrations in the sample as a
function of distance from the flow channel surface are shown in Figs.
5
and 6. The rapid decrease in concentration in the first ten mils or so (0.025 cm) is expected. The persistence of moderate concentrations to much greater depths is puzzling. Figure 7 shows the ratio of F to
Li concentration. The ratio is persistently near the ratio characteristic of the molten salt mixture rather than that for the LiF molecule or the progressively lower values expected for free ionic diffusion. It has
4 been suggested that the bulk of the material seen at depth may repre- sent liquid phase intrusion via a slant crack. It should be noted that the promotion of graphite wetting by the air contamination experienced in the ORR Loop should make smaller cracks than usual effective in this regard.
MICROSCOPIC M A M I N A T I O N OF SAMPLE
The graphite sample was viewed (x 10 and x 2 0 magnification) with a binocular microscope. The surface is relatively quite rough, with saw markings a few thousandths of an inch deep and pock-marked with many voids several thousandths of an inch in diameter. There is evidence of at least one long groove or crack in the sample at about 20' inclination to the exposed surface. The surface is also irregularly
W t

- 5 - discolored, reminiscent of differential heating effects. When wetted with acetone numerous bubbles r o s e to the surface, clearly implying penetration of the liquid into sub-surface voids. In short, this sample shows gross irregularities and imperfections compared to the sample
1 taken from the MSRE that we studied earlier.

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REFERENCES
1. R . L. Macklin, J . H. Gibbons, E. R i c c i , T. Handley, and D. Cuneo, t o be p u b l i s h e d i n Nuclear A p p l i c a t i o n s .
2 . H. G. MacPherson, Power Englneering, January, 1967, p. 2-8.
3 . Molten-Salt Reactor Program Semiannual P r o g r e s s Report f o r P e r i o d
Ending August 31, 1967. ORNL-4191, P a r t 1 5 .
4. E . G. Bohlmann, p r i v a t e communication. v
.
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A

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FIGURE CAPTIONS
Fig.
1
-
Schematic side view of the scanning system. The graphite sample
(from a specimen exposed to molten salts containing 7Li and
19F) is cut so that the exposed surface is uppermost in the
7 figure. The Li(p,n) and 19F(p,q) yields indicate concentration as a function of depth as the sample is moved up across the collimated proton beam.
Fig. 2
-
Proton beam profile using the .O25 ern slit-collimator as measured by the 27Al(p,y) reaction yield from a .005 em A1 foil seen edge-on. The small random deviation of the points from the curve is largely an indication of mechanical precision and reproducibility in the experiment (about 0.0003 em).
Fig. 3
-
Proton beam profile for the .0075 em slit-collimator measured using the .OO5 em A l foil. At the left the beam is hitting clean graphite, whereas at the right it can graze tangentially the flat face of the A1 foil (see Fig. 1) producing a slight tail in the composite resolution function shown.
F i g . 4 -
L o c a t i o n a n d o r i e n t a t i o n of t h e sample c u t from t h e
1967 ORR
Loop Specimen. The molten salt flow was largely upwards through the drilled holes and channels shown in the photograph.
Fig.
5
-
Fluorine concentration as a function of depth in the ORR Loop graphite sample. The data taken at two slit widths are self- consistent. The results cannot be accounted for by simple ionic diffusion and may reflect non-homogeneous structure of the graphite.
Fig. 6
-
Lithium concentration as a function of depth in the ORR Loop graphite sample. The data obtained with the two slit-collimators

- a - are self-consistent. The lithium concentration beyond 100 mils depth decreases more rapidly than expected.
Fig. 7
-
Mass concentration ratio, F/Li, versus depth. The closeness of the ratio observed to that typical of the bulk salt suggests that most of the material found (at depths down to 30 mils) may represent a liquid intrusion.

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MICROMETER
DRIVE MOUNT
ORNL-DWG 68-655f
CLEAN GRAPHITE FOR BACKGROUND CHECK
0 . 0 0 5 c m AI FOIL FOR SCANNING B E A M PROFILE
GAMMA RAYS
A-
NEUTRONS
MICROMETER
DRIVE MOUNT
0.0075 x 0 . 6 0 c m COLLIMATING S L I T
SAMPLE TO BE SCANNED
CONVERGING
PROTON B E A M
(2.06 MeV)
Fig. 1

-
10
-
6000
5000
W
2
4000 k z
3
8
W
$
-J
W
U
2000 io00
ORNL- DWG 68-6552 t
40 42 44 46 48 50 52
DISTANCE (mils)
Fig. 2
54 56 58 60

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11
-
ORNL- DWG 68- 6553
0,000
8000
6000
4000
2000
0
44 16
20 22
DISTANCE
24
Fig.
26 28

-
12
-
ORNL- DWG 68-6736
SURFACES O F
MOLTEN SALT FLOW CHANNELS
CROSS SECTIONAL
SURFACE CUT
A F T E R EXPOSURE
- - - ---
-------a
PROTON B E A M POSIT10
AT VARIOUS DISTANCE
FROM EXPOSED SURFACE
/
/ COLLIMATED
PROTON B E A M
Fig.
L

-
1 3
-
.
ORNL-DWG 68-6556
40,000
5000
2000
1000
E
2
500
Y
In m
+ I c
LL
200
100
50
20
2 5 10 20 50 100
DEPTH BELOW SURFACE (mils)
200 500
Fig. 5

- 14 -
ORNL-DWG 68-6554 io00
500
200
-
400
E
CL
6
In rr)
+I
.-
-I b
20
(0
2
'
4 i 2
'
!
I
,
1 , l . l
i o 20
I
50
1 1 1 1 '
I
400 200 500
DEPTH BELOW SURFACE (mils)
Fig. 6

-
15
-
ORNL-DWG 68-6555
I000
500
200
0
F 50 a tY
.- r-
-I
\ 2 0
5
2 i 2 5 io
20 50 100
DEPTH BELOW SURFACE (mils)
200 500 1000
Fig. 7

- 17 -
ORNL-TM-2238
1. A. M. Weinberg
2. H. G. MacPherson
3. E. P. Wigner
4. G. E . Boyd
5.
L. Fowler
6. E. L. Compere
7. F. C u l l e r
8. L. Dresner
9. R . B. Evans, I11
1 0 . M. S. Wechsler
11. D. K. Holmes
12. S. S. K i r s l i s l 3 * D. R. Cuneo
14. F. F. Blankenship
15. W. H. Cook
16. W. S. Lyon
17. M. T. Kelley
18. E. R i c c i
19. T. H. Handley
20. E . S. B e t t i s
21. E. G. Bohlmann
22. R . B. Briggs
23. S. J . D i t t o
24. W. P. E a t h e r l y
25. D . E . Ferguson
26. W. R . Grimes
27. A. G. G r i n d e l l
28. P. N. Haubenreich
29. P. R . Kasten
30. R . E. MacPherson
31. H. F. McDuffie
32. H. E. M ~ C O Y
33. R . L. Moore
34. E. L. Nicholson
35. L. C . Oakes
36. A. M. P e r r y
37-39. M. W. Rosenthal
40. Dunlap S c o t t
41. M. J. Skinner
42. R . E. Thoma
43. J. R. Weir
44. M. E. Whatley
45. J. C. White
46. D. S. B i l l i n g t o n
DISTRIBUTION
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49. Document Reference Section
50-52. Laboratory Records Department
53. Laboratory Records, ORNL R . C .
54. ORHL P a t e n t Office
55-69. DTIE
70. Lalrsratory
Division, OR0
7 1 - 9 - R. L. Macklin