N O T E S FROM THE NATIONAL BUREAU O F STANDARDS *

ELECTRONIC INSTRUMENT T E S T S G E I G E R COUNTERS

An e l e c t r o n i c g a t i n g i n s t r u m e n t h a s b e e n d e s i g n e d a n d c o n s t r u c t e da t the N a t i o n a l B u r e a u of S t a n d a r d s for the a c c u r a t e d e t e r m i n a t i o n ofthe d e a d t i m e a n d r e c o v e r y c h a r a c t e r i s t i c s of G e i g e r - M ~ i l l e r c o u n t e r s .The d e a d t i m e a n d r e c o v e r y c h a r a c t e r i s t i c s are i m p o r t a n t n o t onlyb e c a u s e the e r r o r s of c o u n t i n g d e p e n d on t h e m , b u t also b e c a u s e theyare i n t i m a t e l y a s s o c i a t e d with the e l e c t r i c a l d i s c h a r g e p r o c e s s .

W h e n i n c o m i n g r a d i a t i o n c a u s e s a n i o n i z i n g e v e n t in a G e i g e rc o u n t e r , n e g a t i v e e l e c t r o n s are a t t r a c t e d t o the p o s i t i v e w i r e , the e l e c -t ron s t r e a m p r o d u c i n g a n e g a t i v e v o l t a g e p u l s e a c r o s s t h e t e r m i n a l s ofof the c o u n t e r . T h e h i g h l y m o b i l e e l e c t r o n s r e a c h t h e p o s i t i v e w i r e ofa G e i g e r c o u n t e r w i t h i n a f r a c t i o n of a m i c r o s e c o n d a f t e r i o n i z a t i o n h a so c c u r r e d . A t the s a m e t i m e , t h e h e a v y p o s i t i v e ions m o v i n g t o w a r dthe n e g a t i v e o u t e r c y l i n d e r r e m a i n in the v i c i n i t y of the w i r e longa f t e r the e l e c t r o n s h a v e r e a c h e d i t . If a p a r t i c l e e n t e r s the c o u n t e rw h i l e the ions a r e s t i l l c l o s e t o the c o l l e c t i n g w i r e , G e i g e r a c t i o n can-not o c c u r .

T h e d e a d t i m e is d e f i n e d as the time i n t e r v a l a f t e r a p u l s e haso c c u r r e d d u r i n g w h i c h the c o u n t e r is i n s e n s i t i v e t o f u r t h e r i o n i z i n ge v e n t s . P h y s i c a l l y i t is the time r e q u i r e d for the p o s i t i v e ions t o t r a v e lfar e n o u g h from the w i r e t o let G e i g e r a c t i o n r e s u m e . S i m i l a r l y , ther e c o v e r y t i m e is d e f i n e d a s the time i n t e r v a l t h a t m u s t e l a p s e a f t e r ap u l s e h a s o c c u r r e d b e f o r e a full-sized p u l s e c a n a g a i n o c c u r ; p h y s i c a l l yit is the time r e q u i r e d for the p o s i t i v e ions t o r e a c h the c o u n t e r w a l l .Both the d e a d t i m e a n d the recovery, c h a r a c t e r i s t i c s of a G e i g e r c o u n t e r ,a s well a s the i n p u t s e n s i t i v i t y of the a s s o c i a t e d c i r c u i t s , are i n v o l v e din the " r e s o l v i n g t i m e , " d e f i n e d as the m i n i m u m time i n t e r v a l byw h i c h two p u l s e s m u s t be s e p a r a t e d t o be d e t e c t e d as s e p a r a t e p u l s e sby the c o u n t e r a n d i t s a c c e s s o r i e s . S i n c e the c o u n t e r is d e a d for as h o r t p e r i o d a f t e r each p u l s e , s o m e d i s i n t e g r a t i o n s are n o t d e t e c t e d ,r e s u l t i n g in a d i s c r e p a n c y b e t w e e n the n u m b e r of p a r t i c l e s e n t e r i n g thec o u n t e r a n d the n u m b e r of c o u n t s r e c o r d e d .

Wi th the B u r e a u ' s e l e c t r o n i c g a t i n g i n s t r u m e n t it is p o s s i b l e t os t u d y the p u l s e s t h a t o c c u r in the time i n t e r v a l b e t w e e n the e n d ofthe d e a d t i m e of a G e i g e r c o u n t e r a n d the e n d of a p r e d e t e r m i n e d g a t i n gp e r i o d . W h e n the g a t i n g i n s t r u m e n t is in o p e r a t i o n , r a n d o m p u l s e sfrom a G e i g e r c o u n t e r are fed t h r o u g h the p u l s e h e i g h t s e l e c t o r a n dp u l s e s h a p i n g c i r c u i t s t o give s h a r p n e g a t i v e p u l s e s of u n i f o r m h e i g h t .

* Communicated by the Director.537

538 NATIONAL BUREAU OF STANDARDS NOTES 13. t:. I.

T h e s e n e g a t i v e p u l s e s follow two p a t h s : one t h r o u g h a n a m p l i f i e r t o ah e a v i l y b i a s e d s t a g e a n d the o t h e r t h r o u g h a d e l a y c i r c u i t t o a g a t i n gm u l t i v i b r a t o r . An i n i t i a t i n g p u l s e e n t e r i n g the s y s t e m f i n d s C h a n n e l1 b l o c k e d by the h e a v y b i a s , but by w a y of C h a n n e l 2 it t r i p s the m u l t i -v i b r a t o r a n d o p e n s C h a n n e l 1 for p u l s e s t o p a s s . B e c a u s e of the d e l a yin C h a n n e l 2, C h a n n e l 1 is not o p e n e d u n t i l the i n i t i a t i n g p u l s e h a s dieda w a y . T h e d e l a y u s e d is 15 m i c r o s e c o n d s . S u b s e q u e n t p u l s e s findC h a n n e l 1 o p e n , go t h r o u g h it, a n d are c o u n t e d . A f t e r a time d e t e r -m i n e d by the c i r c u i t c o n s t a n t s of the m u l t i v i b r a t o r , C h a n n e l 1 isb l o c k e d off a g a i n a n d the w h o l e p r o c e s s is r e p e a t e d . In effect , thes y s t e m s e l e c t s p a i r s of p u l s e s with a time s e p a r a t i o n less t h a n a s e l e c t e dv a l u e d e t e r m i n e d by the s e t t i n g of the c o n t r o l s on the m u l t i v i b r a t o r ,a n d one c o u n t is r e c o r d e d for e a c h s u c h pair l e c e i v e d .

EFFECT OF BORON ON THE STRUCTURE AND PROPERTIES OF CAST IRON

W a r - i n d u c e d s c a r c i t i e s of e l e m e n t s s u c h a s c h r o m i u m a n d m a n g a n e s ec a l l e d a t t e n t i o n t o the p o s s i b l e u s e of b o r o n t o i n c r e a s e the h a r d n e s s a n dw e a r r e s i s t a n c e of c a s t i r o n . H o w e v e r , specific d a t a r e g a r d i n g thee f f e c t of b o r o n on the s t r u c t u r e a n d p r o p e r t i e s of c a s t i r o n s h a v e b e e ns o m e w h a t l i m i t e d . A t the N a t i o n a l B u r e a u of S t a n d a r d s ' e x p e r i m e n t a lf o u n d r y , a n e x t e n s i v e s t u d y was r e c e n t l y made of the e f f e c t of b o r o nin p l a i n g r a y c a s t i r o n s . U s e f u l i n f o r m a t i o n on solidification c h a r a c -t e r i s t i c s , s t r e n g t h , h a r d n e s s , m i c r o s t r u c t u r e , a n d c h i l l i n g p r o p e r t i e swas o b t a i n e d . T h e s e d a t a s h o u l d be of v a l u e t o t h o s e i n d u s t r i e s w h i c hp r o d u c e c a s t iron for s p e c i a l i z e d u s e s r e q u i r i n g definite characteristics.

C a s t iron c o n s i s t s e s s e n t i a l l y of g r a p h i t e p a r t i c l e s d i s p e r s e d t h r o u g h -ou t a s t e e l m a t r i x . O r d i n a r i l y the f r a c t u r e d s u r f a c e of the m e t a l isg r a y , b u t w h e n c a s t iron is c o o l e d r a p i d l y from the m o l t e n s t a t e , itc h i l l s , f o r m i n g w h i t e i r o n . T h e core of s u c h c a s t i n g s c o n s i s t s of softg r a y iron w h i l e the o u t s i d e p o r t i o n is the m o r e d u r a b l e w h i t e i r o n .B e c a u s e of i t s h a r d n e s s a n d r e s i s t a n c e t o w e a r , w h i t e c a s t iron is u s e din s u c h a r t i c l e s a s s t r e e t - c a r w h e e l s a n d r o l l s for r o l l i n g m i l l s . T h eb r i t t l e n e s s of the iron chip a n d the l u b r i c a t i n g p r o p e r t i e s of the g r a p h i t em a k e g r a y iron e a s y t o m a c h i n e . T h e g r a p h i t e p a r t i c l e s also a c t asc u s h i o n s a g a i n s t t h e r m a l s h o c k a n d t h u s p r e v e n t c r a z i n g . F o r thisr e a s o n , g r a y iron is the only m a t e r i a l t h a t has b e e n s u c c e s s f u l l y u s e d ona l a r g e s c a l e for b r a k e s h o e s or b r a k e drum l i n i n g s , w h i c h m u s t a b s o r bt r e m e n d o u s e n e r g y , c o n v e r t it t o h e a t , and s t i l l not s e i z e the w h e e lor a x l e .

In t h e B u r e a u ' s i n v e s t i g a t i o n , p l a i n g r a y c a s t i r o n s of nine d i f f e r e n ts t o c k s w e r e s t u d i e d . T h e i r o n s w e r e m e l t e d in h i g h - f r e q u e n c y i n d u c -t ion f u r n a c e s , a n d b o r o n in the form of f e r r o - b o r o n was a d d e d t o them o l t e n m e t a l . W h e n e x a m i n e d u n d e r a m i c r o s c o p e , the s t r u c t u r e ofi r o n s with a r e l a t i v e l y l a r g e b o r o n c o n t e n t was f o u n d t o c o n s i s t e s s e n -

Dec., I949.] )X'ATIONAL BUREAU OF STANDAR1)S .~N'()TES 539

t ially of a matrix of cementite containing islands of pearlite as well asother structureless islands.

Transverse strength was determined by subjecting bars to a breakingtest. A disk was cut from each broken transverse test bar and used forBrinell hardness determinations and for metallographic examination.Fractures could be classified as gray or white-network. For bars withgray fractures, an increase in boron content was accompanied by asmall increase in the transverse strength, Brinell hardness, and relativemodulus of elasticity of the sample. Howevm, the appearance of awhite network or white specks on the fractured surfaces of bars con-taining larger amounts of boron usually was accompanied by a drop inthe transverse strength and deflection, and an increase in the Brinellhardness and relative modulus of elasticity.

An important proper ty of cast iron is its abil i ty to form white iron,when chilled. Depth of chill may be controlled by varying the coolingrate or by adjusting the composition of the iron. In the present studythe whiteness of the fractured surface of p la te and wedge specimenswas taken as a criterion of the iron's susceptibility to chill. For allcastings there was a pronounced increase in depth of chill with an in-crease in the boron content . This was found to be accompanied bv agreater development of the white network in the gray portions of thecastings.

In general, the results of the investigation show that boron increasesthe depth of chill of cast iron as well as its hardness. Boron was alsofound to decrease the size and quant i ty of graphite particles, to increasethe free carbide content , and to produce an undercooling effect whicht)ecomes more pronounced with greater percentages of boron.

NBS LIQUID OXYGEN CONVERTER

A fully automatic liquid oxygen converter having significant ad-vantages for use in high alti tude flight has been developed at theNational Bureau of Standards. Its features include compactness,greatly reduced weight, sturdiness, simplicity of design and operation,rapid attainment of operating pressure, economy of oxygen, and deliveryof warm gas.

The NBS Liquid Oxygen Converter consists of a standard 25 literm e t a l Dewar flask modified by the addition of a bot tom drain, togetherwith two coils, one for build-up of pressure and one for warming the gasas it is delivered. Four valves, two on the flask, one in the build-uptube, and the o t h e r in the delivery line, prevent excessive rise in pressure.

When oxygen gas pressure is to be built up, liquid is directed throughthe bot tom drain and a liquid trap into the build-up coil, in which it isevaporated and w a r m e d by the atmosphere. The warm gas risesthrough the coil by thermal convection and is then returned to the top

54 ° NATIONAL BUREAU OF STANDARDS NOTES [J. F. I.

of the flask, where it mixes with the gas already present above theliquid. Gas pressure rises quickly as this cycle continues. Afterpressure has been established, oxygen may be withdrawn by forcingliquid through the bot tom drain into the withdrawal coil, where it isevaporated and w a r m e d by the atmosphere as it passes to the outletmechanism.

The total converter unit is 25 in. high and 18 in. in diameter. Itweighs 60 lb. and holds 62 lb. of oxygen, enough for ten men for 10 hr.To supply this quant i ty of oxygen from gas cylinders at 1800 psi. wouldrequire 21 t a n k s of 514 cu. in. each, which weigh a total of 350 lb. whenempty . For flows up to 150 liters per minute (5 cu. ft. per minute), gasis delivered within 5° C. of atmospheric temperatures. Greater flow"rates may be obtained, but the delivered gas is colder. Pressure maybe built up, when the container is full, from 0 to 65 psi. in 10 sec.

The converter has successfully withstood tests u n d e r conditions ofsevere vibration (3000 cycles per minute), rapid acceleration (nine timesthe acceleration of gravity), extreme cold ( - 4 0 ° C.), and severe heat(75 ° C.). Performance at tilts up to 45 deg. was not materially dif-ferent from that observed in the erect position. Converters built onthe NBS design by commercial manufacturers have operated satis-factorily in flight tests at 40,000 ft.

In reliability and safety the new converter compares favorably withhigh-pressure gaseous oxygen systems. Wide variation is possible incontainer capacity, and in delivery flow, temperature, and pressurespecifications. Thus, although designed specifically for aviation use,the device may be adapted with m i n o r modifications to industrial andmedical applications.

ELECTRON-OPTICAL SHADOW METHOD

As the result of a series of electron-microscope experiments in theElectron Physics Laboratory of the National Bureau of Standards, anelectron-optical shadow technique has been developed which providesa valuable tool for the quantitative study of electrostatic and magneticfields of extremely small dimensions. The new development shouldalso provide a powerful means for broadening present knowledge con-cerning space-charge fields, waveguide problems, and the basic magneticproperties of metals. Though similar in some respects to the electron-optical Schlieren method previously developed at the Bureau, theshadow m e t h o d is much better adapted to precise determinations offield intensity.

In practice, the object to be studied, a recording wire for example,is placed between an electron source and a sys tem of electron lenses.The lens sys tem focuses the electron beam to form an image of the wireon a fluorescent screen. By placing a wire mesh of known gage just

D~.c., ~!~40.~ N A T I O N A L ]~UREAI7 (tl: ~TANDARI)S -"XTOTES 5 4 1

beyond the back focus of the lens system, a shadow image of the meshis superimposed on the image of the wire. The portions of the shadownetwork adjacent on the screen to magnetized regions of the recordingwire are then found to show considerable distortion.

A complete theoretical analysis of this effect at the National Bureauof Standards has shown that the distortion of the shadow image is dueto the deflection of the electron beam by the field of the recording wireat each magnetized region. The displacement and change in size ofthe shadow image depend on the strength of the field of the magnetizedwire. Although it is possible to compute field intensity from the in-tensity distribution of the pattern obtained by the Schlieren method,the shadow method is of far greater utility for quantitative work sincethe image displacement and magnification can be measured much moreaccurately than can the intensity distribution across the Schlieren pat-tern on a photographic plate.

Perhaps the greatest value of the electron-optical shadow methodlies in its utility for exploring complex electric and magnetic fields ofextremely small dimensions or in which a probe of greater size than theelectron would disturb the field under study. The shadow techniquenow provides data for accurate calculation of the absolute value of theintensity in the neighborhood of a specimen of any size or shape withoutaltering or disturbing the field. The method is thus well adapted toinvestigation of the fundamental nature of ferromagnetism.

The Bureau is also applying the principle of the shadow techniqueto the study of spherical aberration in electron lenses. When a finewire mesh is placed in the focal region of a lens having spherical aberra-tion, the shadow image of the network is enlarged. The resultantpattern may be interpreted to give information of value in correctingthe lens.

NBS M E T H O D F O R M I C R O S E C T I O N I N G

An improved and relatively rapid method for cutting extremely thinsections of biological tissue for study with either the light or electronmicroscope has been developed by the National Bureau of Standards.The application of the electron microscope to many biological problemshas been seriously hampered by the lack of such a method, because ofthe very slight penetrating power of the electron beam in commercialinstruments and the relatively great depth of field involved. Yet thetechniques that have been available for preparing thin sections arequite elaborate and difficult, requiring expensive equipment and pro-ducing few usable sections.

An important feature of the Bureau's method is an easily made, in-expensive device which holds the embedded specimen and at the sametime advances it gradually and uniformly toward the knife of a micro-

542 NATIONAL BUREAU OF STANDARDS NOTES [J. E. L

tome. The specimen is embedded in polybutyl methacrylate to producean optically clear medium having highly desirable cutting properties.The material is fed to the microtome in extremely small incrementsby m e a n s of the thermal expansion of a metal specimen holder. Thespecimen holder is cooled by carbon dioxide gas and thus contracts.As the holder begins to warm and expand, it moves toward the cuttingedge of the microtome. The length of time between cuts and the con-trol of the carbon dioxide determine the thickness of the microsection.Thus, sections of tissue having undistorted structure and uniformthickness of fractions of a micron may be cut, one at a time.

Although the Bureau's new method for obtaining very thin sectionshas given excellent results, it possesses certain limitations. The greatestchance for failure appears to lie in the polymerization of the embeddingmass. However, use of low-temperature catalysts will minimize dif-ficulties at this stage. Efforts are now being made at the Bureau toapply the new sectioning method to the study of the structure ofnatural and artificial fibers found in leather, p a p e r and textiles.

EFFECT OF SIMULATED SERVICE CONDITIONS ON PLASTICS

The use of laminated plastics in aircraft has increased during thepast few years, especially in accessories and semi-structural parts, suchas partitions, linings, propellers, and wing flaps. There has accordinglydeveloped a need for more information regarding the action of theweather, temperature, and humidity in order to evaluate these plasticsfor aircraft applications and to prepare specifications.

To provide the necessary d a t a , the National Bureau of Standardsu n d e r the sponsorship of the National Advisory Committee for Aero-nautics undertook an investigation to determine the effects of simulatedservice conditions on the weight, dimensions, and flexural propertiesof phenolic and unsaturated-polyester plastics. The test materialswere commercial products and included nine laminated plastics whichare the types commonly employed in aircraft.

For accelerated weather testing the specimens were subjected tocycles of ultraviolet l igh t and fog while the accelerated service testsconsisted of exposure to various temperatures and relative humidities.Three sets of specimens were exposed for one year to outdoor weathering.Changes in weight, dimensions, and flexural properties were the cri-t e r i a used in analyzing the data gained. In most of the tests the changesin weight and dimensions were negative, any positive changes beingin thickness. In several cases increases in flexural strength were notedand attributed to further cure of the resins.