Chemical Instrumentation 5. Z. LEWIN, N e w York University, Washington Square, N e w York 3, N. Y.
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feature
T h i s series ojarticles presents a survey of lice basic principles, charncleristirs, and limitations o j those insfrumenfs which And i m p o ~ t a n tapalications in chemical work. The emuhasis i s on commerriallu available ~ of magnitnde o j equipment and appfozimnte prices are quoted to show t h order cost of the various types of design and construction
13. Nuclear Radiation Electronic Gear Few ficlds of instrumentation sre as well endowed with commereid firms producing a aide variety of models for all types of u s r as is the nuclear field. This is in l a r g ~part due t o the subsidization of cnginerring dov~lopmentsin this field hjbho federal government heci~use of its intsrcst in fissionable mnberials. The inst,rumont user is the fortunate beneficiary of this situation, for he no~q-h a available a a i d e range of instrumma from which to choose the specific ones that are best suited to his needs and hudgot. The following paragraphs summarize the t,ypes of instrumentation currently xvailable from the major U.S. manufaet,lrrers. Since in many cases equivalent models of instruments are produced hy several mnnufnrturers, not all the models of enah supplirr are mentioned in detail. Also, t,he design of nuclear detrct,ion inntrument,ztion has devdapetl rapidly in the p a ~ decade, t and model chengcs havc taken place with grcntcr frcqueney in this firld than in most other areas of instm~nentation. This summary emphasizes the most recent models of t,he variom mznufactnrcrs.
Lionel/Anton
Electronic
Laboratories
One of thc principal suppliers oi nuclear detector t u l q and a manufacturer of n line of radiological survey and counting equipment, ia Lionel Electronio Lahoratories, Brooklyn 3,NOWYOTIC,formwly known as Anton Eloetronic 1,ahoratories. This company's dct,crtor t h e s have been mentioned rarlirr in this ~ e r i e s( c j . No. 12, "Nurlesr Radiz~tion I)eteetors"), and inrludp a wkle range of siees and shapes, ranging from a. 0.1-inch diameter thinwall hetn-gamma dipping or prohe tuhe t o cosmic ray and neutron detectors several feet long and sovcral inohes in diameter. A ttseful and nniquc feature of this line is x nnmher of "pancnke"-shaped detectors that are sensitive to alphas and hrtas, hut relatively insensitivp to gammas, which mostly pass through the thin, flat detector without producing ionizing eveot,s. This manufacturer sp2cil~lizes in the production of halogenquenched Geiger-Muollcr detectors, which have the advantage of very long life (not limited by total numher of count3 recorded, as is the case with organic vaporquenrhed OM-tubes), and the ahility to withstmd high overvoltagen.
1.5-volt flashlight hhtteries which are contained in a removal,lc conlpartment a t the bottom of the instrument, as shown in Figure 28. The high voltage (930 =t 20 v) for the G.M-tube is obtained hy means of n transistor oscillatar, and is regulat,ed by a corona discharge voltsgeregulator tuhe. The read-out is aural via the hesd-set,, or visual via a conntrate meter. The ranges of this instntment are calihmted for 0-0.5, 0-5, and 0-50 millirocntgcns per hour, xhieh are cquivnlcnt (far n radium sourre) to (t300, 0-3000, and 0-30,lllNl counts per minute. A ealihmtcd Ra I ) and E source is imh d d e d in thr wnll of the instrument, cam, and t h r mctrr s r ~ l r scan he standardized simply hy plaring the prohe nest to this r:lrrr.k aourrr and nmhing apprapriate adjmtment of :i vnrinhle resistor. A morc prt,eise measuring instrument is tho Morlcl 45 Radioactivity Scaler-Ratemotcr ($495), thc electronic design of which in illustrated in Figure 20. An amplifi~r-discriminator rircuit shapes the drtretor pulses, and passes only those the amplitudrs of ~\-hirharc ahavc a minimum value, which ma)- he chosen by means of an intwnal control. The pulscs may he fed rithrr t o :I rst,e-met~r oirruit that converts them into a metpr rrading whosc deflection is proportional to tho counbing rate, or to SCBIPL. circuit that registers the total count detected in a givm timr interval. .In audio circuit is provided far producing a. laud click for e:~oh ptilsp. The pea-cr snpply rireoitry providrv 110th regulated and onregnlntcd supplv v o l t a ~ r sfor the o t h ~ r circuits. Rrcnuw of the ver~atilityof t h ~ rrad-out,, this instrument is useful for I ~ c t o r ed~monstr,ztion pilrpoi.cs, as d l
iContinued
A port,able, t,ransistarisrd survey meter recently introduced by this mnnufaaturcr is shown in F i p r r 2 i . This instrument (hladrl 701, $130.50 with l,rt,a-gamma
Figure 27. Model 701 Alpho-beto-gomma survey meter of Lionel/Anton Electronic Lobs. The beto-gommo p m b e is rhown plugged in to the measuring circuitry. It consists of o thin-wall halogen-quenched Geiger-Mueller tube. The olpha-beta detector rhown beride the instrument has o mica end-window.
probe only; $225 wit,h additional mica end-nindow probr) is powred by five
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SPRINT
8-,
Figure 28.
K N U l i l O THUMB SCREW
P U C E SPRING
CON~~CT
ovra ernrra crcl
Battery section of Lionel/Anton portable survey meter.
Volume 38, Number 6 ,June 196 1
/
A389
Chemical Instruntentation as for survey and roubine radiological work. T h r rntemrter arenraey is 1 5 % of full senlc; t h r scaling section has a resolving timr of 2 X lo-' sccunds
L * I Figure 29. Biock dingram of eledronic~ of M o d e l 45 Radioactivity Scaler-Rotemeter of Lionel/Anton Electronic Loborotorier.
Tracerlab
One of the most cxtensivr lines of detoctors and counting systems in the nuclear field is available from Tracerlnh, Ine., Waltham 54, Massachusetts. This campans, mhich rvas among thc first to produce commercial radiologird instrumentation during and aftcr World \Var 11, offcrs not only inst,roments, but also I ~ h d e deon~poimds, rsdionctive sources, dcrontamination srrvices, and other supplies and services. An example of this mamdactur~r's survey instruments is the Model SU-I? Alpha Beth Cnmma Survey Metw ($260), the circuitry of which is sho~t-nin Figure 30. The lowrr portion of the diagram shows the porrcr supply section, which is hased upon four 1.6-vdt fiashlight hatteries, and three 45-volt portable-radio type batterips. .4 rrlaxation o d l a t o r , consisting of V-1, R-I, and C-l feeds pulses of a sawtooth waveform through a pulse sharpening network, R-8 and C-8, to the grid of VS. The suhseqnent pulscs of platc current through T-l produce large ar voltage pulses which are rectified I,y V-4 and filtered hy the If-C filtor. A de voltage a t 7 - 5 causes a. corona discharge t o occur, and this current Rows through R-1.9. Any smnll e h a n g ~ sin this corona regulation current through R-1.3 enusrd h,y b a t t ~ r ydeca.v of B-1 and B--2 will affect the hias voltage on the grid of V-2. The plntc voltage of V--2 will i n c r c ~ s r if its grid voltngr drcreases. The Iias voltage a n 17-3 is thpreiare inr r e a d as the regulator cnrrmt throilgh drops. T i t h an increase in hias, TI-5 will draw a large p u l ~ ecurrent through T-1, thereby increasing the valt,nge lewl at the corona regulntor t ~ h r . The upper portion of Figitre 30 s h o w tho ratrmeter circnit. Tube T'-7 is the normally-on tuhe of n mlivihmtor circuit. Since- its plate voltagp and grid-bias are drrived from n regulntcd supply voltage, the ratemeter srnsitivity is ronstant, independent of hatter? age or condition over a considerahlc range. The m n g s of this instrument art. 04.25, 0-2.5, and 0-25 mr/hr. The neewary is +15t70 of tho mr/hr scales for 0 - 6 0 radiation. Trawrlab produers a numher of athcr (Cotalinued on page A3.92)
A390 / lournol of Chemical Educofion
Chemical l N S t I ' ~ n t f ? N t f l f i ~ N
Figure
30. Schematic of
A392
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survey instrument,^, including pockrt dosim&crs, ionization rhamher survPy meters, and laboratory monit,ars. This company also manufactrms a varidy of
the electronic circuitry of the Tracerlab Model SU-14 Survey Meter.
journal o f Chemical Education
s r : h s and ratemcters, comprising cxamples of most of the types of read-out. These inelode binary scalers, d ~ r i m n l scalers, glow transfer t,uhr decade scalers. and automatic data print-orrt devircs. Somc idea of the rhnrart~risticsof the sevcral types of srnlers a v d a b l e can br gained from the data in Tablo I , 11. A 4 0 4 One of thr moro reecnt radiation drteation systems developed by Tr~eerlall is thc Omni/Gonrd Low Bnclrgronnd Beta Counting System (84390 t,o $i!lSO, depending on rtrreusorics). The principlrs of t,hr design of this system are illustrat,ed I,?- the- black diagram shown in Figure 31. The sample is positioned close (within 1 m m ) to the x!-indaw of an ult,ra-thin endwindow (Icss than 125 microgram/cm2) gas How eoont,rr. This detector yields p,ds:s due t o t,he sample, together with pulses due t o cosmic radiation, and surrounding tprrpstrial radiation. A domeshaped gas flow eormtrr surrounds t h r samplr counter, enclosing all hut t h ~ n-in do^\- of the latter. The pulses from the g m r d counter are compared with those from thr snmplr rounter by means of thr anti-roineidenru unit,. Cosmic radiation and rnergetir berrestrinl radiation will e a u s ~ .fiimultaneous ionizations t o occur in 110th the gnard and sample counters, and such roincidcnc~swill not he counted. Only pnlsm originating in t,he samplr! connt,w, and which do not roinride v i t h pnlsps occurring in the gum1 counter, are passrcl by thc anti-coincidener unit. Thus, bhe S ~ P can T he s r t t o count only t h l ~snmplr, f r w of most of the barkgro~ind
(Continued on poye . W I f l l
ing of lug
