Design and testing of ruggedized, thin window proportional counter tube for detecting X rays in light element

Schnopper, H. W.

Shields, R. A.

English

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N A T I O N A L  A E R O N A U T I C S  A N D  S P A C E  A D M I N I S T R A T I O N  
Te chnica 1 Memorandum No. 33- 22 7 
A Ruggedized Thin- Window Proportional-Counter Tube 
C O I N  V E S T I G A  T O R S  
Herbert W. Schnopper Rex A. Shields 
Cornell University Jet  Propulsion Laboratory 
R. H. McFee, Manager 
Lunar and Planetary Sciences Section 
J E T  P R O P U L S I O N  L A B O R A T O R Y  
C A L I F O R N I A  I N S T I T U T E  O F  TECHNOLOGY 
P A S A D E N A ,  C A L I F O R N I A  
September 15, 1965 
https://ntrs.nasa.gov/search.jsp?R=19680001899 2020-03-12T11:55:47+00:00Z
Copyright 0 1965 
Jet Propulsion laboratory 
California Institute of Technology 
Prepared Under Contract No. NAS 7-1 00 
National Aeronautics & Space Administration 
JPL TECHNICAL MEMORANDUM NO . 33-221 
CONTENTS 
1 . TubeDesignandAssernbly . . . . . . . . . . . . . . .  1 
II . Windowlnstallation . . . . . . . . . . . . . . . . . .  3 
111 . Testprocedures . . . . . . . . . . . . . . . . . . . .  3 
A . EnvironmentalTests . . . . . . . . . . . . . . . . .  3 
I3 . Pulse-HeightDistributions . . . . . . . . . . . . . . .  3 
IV . CounterAppCications . . . . . . . . . . . . . . . . .  5 
References . . . . . . . . . . . . . . . . . . . . . . .  5 
TABLE 
1 . Resolution of aluminized-Mylar-windowcounters . . . . . . . . .  3 
FIGURES 
1 . Thin-window counter before andafter assembly . . . . . . . . .  2 
2 . Pulse-height distributions of test counters . . . . . . . . . . . .  4 
111 
FOREWORD 
The work described in this Memorandum was performed by the 
authors as a cooperative effort during a period in which Dr. Schnopper 
was stationed at JPL under special assignment by the U. S. Army. 
Dr. Schnopper is now associated with the Physics Department and the 
Laboratory of Atomic and Solid State Physics at Cornell University. 
IV 
JPL TECHNICAL MEMORANDUM NO. 33-221 
ABSTRACT 
The design and testing of a thin-window proportional-counter tube 
are described. The objective of this investigation was the development 
of a component suitable for use in space experiments involving the 
detection of X-rays from light elements. The stability, efficiency, and 
resolution of the tube described here are consistent with space- 
application requirements. In a series of tests, it was determined that 
0.25-mil aluminum-coated Mylar, supported by a nickel screen, pro- 
vided maximum strength with minimum absorption. Two methods of 
sealing the window are described: (1) an O-ring and (2)  a potting 
compound. Sodium Ka pulse-height distributions from the thin-window 
tubes are compared with those from a standard commercial tube hav- 
ing a mica window. 
1. TUBE DESIGN AND ASSEMBLY 
For space applications, a proportional-counter tube 
must exhibit good stability under changing intensity and 
other environmental conditions, and als2 good efficiency 
and resolution in the region of 7 to 12 A. Although tube 
design is already a well developed art (Ref. l), a need 
has existed for improvement in window construction and 
installation techniques. The window described here pro- 
duces results consistent with the objective of detecting 
X-rays from light elements in space experiments. 
The tubes used in the present investigation were made 
from a 3-in. length of %-in. square brass tubing with a 
0.038-in. wall thickness (Fig. 1). Convenient connections 
for filling and sealing were' fabricated from ?&in. copper 
tubing, silver-soldered to the tube. End caps with glass- 
to-metal feedthroughs were soft-soldered to each end of 
the tube as mounts for the anode wire. 
The recessed-window body was soft-soldered to the 
tube body at the midpoint on the length of the tube. A 
groove for a Viton 2-14 O-ring was machined on the 
floor of the %-in. recess, the depth of the groove allow- 
ing approximately 10% compression of the O-ring. A 
%-inl window opening was machined through the tube 
wall. After each assembly step, the tube was liq(id- 
honed inside and out to maintain a clean surface, 1 
The last assembly step prior to the window installa- 
tion was the installation of the tungsten anode wire, 
which was 0.002 in. in diameter, and 1.7 in. long. This 
wire was welded at each end to a length of 0.050-in. 
stainless steel wire, which was then soft-soldered into the 
small tubing in the glass-to-metal feedthroughs. 
The window support screen was of etched nickel? 
0.001 in. thick, having 40 wires per inch and a transmission 
1Screen supplied by Buckbee-Mears Company, St. Paul, Minnesota. 
1 
JPL TECHNICAL MEMORANDUM NO. 33-221 
Fig. 1. Thin-window counter before and after assembly 
of 70%. The wires were found to have rough edges 
capable of puncturing the window material. This rough- 
ness was eliminated by lapping the screen with a fine 
diamond dust between two microscope slides. The screen 
was cemented to the compression surface of the window 
cap by applying a small amount of Selectron.’ While the 
Selectron was curing, the assembly was held in place by 
a lightly greased, weighted microscope slide. After cur- 
ing, the window-cap assembly was placed in a lathe, and 
the Selectron that had flowed through the screen was 
machined smooth and flat to within approximately 
0.002 in. of the screen. The entire tube assembly was 
then cleaned and dried before installation of the window. 
2Commercial potting compound supplied by Pittsburgh Plate Glass 
ComDanv, Torrance, California. 
2 
JPL TECHNICAL MEMORANDUM NO. 33-221 
111. WINDOW INSTALLATION 
Several pieces of 0.25-mil Mylar were vacuum-coated, 
each with a different thickness of aluminum. Absorption 
tests were performed to select the thickness of coating 
that would render the Mylar leak-free and still give 
reasonable transmission for Na K a  X-rays. 
One piece of aluminized Mylar was selected, after 
inspection to ensure complete aluminum coating. A thin 
film of vacuum grease was applied to the compression 
surface of the window cap, over which was placed the 
aluminized Mylar with the aluminum surface toward 
the inside of the tube. The O-ring was lightly greased 
before it was placed in its groove. With the aluminized 
Mylar held in place by the vacuum grease, the window 
cap was tightened against the window body. 
Because the use of an O-ring was a new approach in 
window sealing, another tube was prepared for compari- 
son. The second tube was identical to the first except that 
it contained no O-ring groove. In place of the O-ring, a 
small amount of Selectron was painted on the floor of 
the window body to seal the window assembly. 
Both tubes were filled and sealed with P10 gas at a 
pressure slightly above atmospheric, thus reducing win- 
dow flexing by holding the window against the screen. 
After a short test period in atmosphere, both counters 
were placed in a vacuum chamber with an FeS5 source 
over each window, and were connected so that they 
could be checked while in vacuum. At the end of a 
month's testing, during which both counters operated 
satisfactorily, they were removed from the vacuum. 
Because the window sealed by the O-ring was more 
easily installed and replaced, this design, designated as 
S-3/8, was chosen as the test counter. 
111. TEST PROCEDURES 
A. Environmental Tests 
Counter S-3/8 was subjected to a standard series of 
environmental tests, including: an acceleration test of 14 g 
for 5 min; a shake test of 100 g, repeated five times in 
three planes; and a vacuum temperature test at -50 to 
70°C. There was no apparent change in the counter 
operation after the tests. 
B. Pulse-Height Distributions 
As a test of the performance of the thin-window 
counter after the environmental tests, a series of pulse- 
height distributions was obtained from X-ray sources, 
including NaF and NaCl. Three counters filled with P10 
gas were used for this test: (1) an Amperex counter hav- 
ing a mica window 0.25 in. in diameter and 1.5 mg/cm2 
thick; (2) a modified Amperex counter incorporating a 
0.25-in.-diameter 0.25-mil aluminized-Mylar window; and 
(3) counter S-3/8. The samples were excited by an elec- 
tron gun operzted at 12.5 kv and 5p amp. The counters 
were masked by metal disks with small holes to cut down 
intensity. The only variation in the operating conditions 
was the adjustment of the counter voltages to control 
peak positioning in a pulse-height analyzer. Figure 2 
shows the NaF and NaCl distributions for the three 
counters. The X-ray peak of sodium is clearly seen in the 
record for the two aluminized-Mylar counters, but is 
indistinguishable above background in the data for the 
mica-window counter. Table 1 lists the NaF and Fez5 
peak resolutions of the two thin-window counters. These 
resolutions are of a quality sufficient to permit use of 
these counters for nondispersive X-ray analysis. 
Table 1. Resolution of aluminized- 
Mylar-window counters 
3 
0. 33- 
Q C L  
PEREX THIN- COUNTER (PI0 
QF 
T TER S-318 (P I0  
O F  CL 
ero 
8 
JPL TECHNICAL MEMORANDUM NO. 33-221 
Transmission values were obtained by absorption At this writing, twelve counters have been constructed: 
six having 0.25-mil aluminized-Mylar windows; four of 
the S-3/8 design (0.71-cm2 window area); and two of the 
modified Amperex type (0.31-cm2 window area). All are 
measurements at several wavelengths. The transmission 
of the aluminized Mylar used in counter S-3/8 is 7% 
for Na Ka X-rays, 30% for Mg Ka X-rays, and 52% for 
Al Ka X-rays. operating satisfactorily. 
IV. COUNTER APPLICATIONS 
A rugged proportional counter Providing adequate from the lunar surface will be induced by solar radia- 
transmission in the range of 7 to 12 A can be utilized for tion; thus, the detector could also be used to measure 
several types of space experiments. The counter de- this emission from a spacecraft orbiting the Moon 
scribed here was developed for the detection of Na, Mg, (Ref. 3). A third application would be for the measure- 
Al, and Si in lunar material by  an X-ray spectrograph ment of interstellar X-ray fluxes from sounding rockets 
placed upon the surface (Ref. 2). Fluorescent emission or satellites. 
REFERENCES 
1. Curran, S. C., "The Proportional Counter as Detector and Spectrometer," Handbuch 
der Physik, Vol. 45, p. 174, Springer-Verlag, Berlin, 1958. 
2. Metzger, A. E., An X-Ray Spectrograph for lunar Surface Analysis, Technical Report 
No. 32-669, Jet Propulsion Laboratory, Pasadena, California, October 16, 1964. 
3. Wittry, D. W., Arrhenius, G., and Cohen, L. H., "Determination of the Surface Com- 
position of the Moon ,from a Lunar Satellite," Geofisica Infernacional, Vol. 1, p. 95, 
1961. 
5 


A ruggedized thin-window proportional-counter tube

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