Chemical lnsfrumenfafion
.-a-
feature
S. 2. LEWIN, N e w York University, Washington Square, N e w York 3, N. Y.
T h i s aeries of articles prerents a suruey of the basic principles, characteristics, and limilations of those instruments whichflnd important nppliratwns i n chemical work. The emphosis is on commercially available epipment, a n d approzimate prices are quoted to show the order of magnitude o j cost of the vnrious types of design a n d construction.
12. Nuclear Radiation Detectors The detection and cstimstion of n r d e a r radiations, by whirh will be meant in the following discussions those radiations which are encountered in the laboratory when working wit,h radioactivie dements, pmes several unique instrumental p r o b loms. These stem from ( 1 ) t,he great vxricty in properties of these radiations, and (2) the different kinds of inform.zt,ion it, is necessary to obtain nhout thrm. Nuelens radiation includes: (1) heavy prrlirle rays, such as fission product nnrlri, alpha particles, neutrons, and heavy mesons; (2) light particle rays, such as negative and positive electrons and light mrsons; and (3) electros?aanetic rays, such as gamma and x-radiation. The encrgy per particlc or photon may range from fractions of a n rlrctron volt (CV) up to several millions of electron volts (Mev). The flux of radiation encountered may rangc from less t,han one pcr hour t o more than 10"per rm' per nccond. The kinds of information a nuclear radiation detector may be aalled upon t o provide include: (1) number of disintegrations per second occurring in t,he sample, (2) t,ype(s) of nuclear particles or photons emitted, (3) quantum energy and energy distrihrdion of the radiations, (4) angular distribution of intensity, or angular spread between individual particle directions, (5) simultaneity in t,ime of rertain elementary events, ( 6 ) rate of, or t,otal integrated, production of electric charge hy the- radioactive sonroe in a unit volume of a standard substance (i.e., dosc rate and total dose). T o cope a i t h this great variety of conditions and requirements, i t has been nreessary t o develop a versatile arsenal of detector instnunentation. These inchldc: (1) electroscopes, (2) ionization chamhers, (3) proportional counters, (4) Geiger-Mueller counters, (5) scintillation detect,ors, (6) eryst,al and semiconductor co~mtrrs,(7) cloud and bubble chambers, (8) calorimeters, (9) sensitive photographic emulsions. These detectors have, in many cases, heen developed t o a very advanced stage, and take a numher of different physical forms. Also, they are commonly coupled t o sophisticated electronic circuitry for seding down, selecting, or int,egrating the rate of informat,ion
resd-out, and for automating the bask of recording t,he dcsircd information.
Electroscopes The earliest and simplest of the t,mnnducers for ionizing radiations is t,hc elmtroscopo, which is hased upon the mnnifcstation of the forces of attraction or repulsion due t o electric chargcs. If a lightwright gold leaf is nttachrd hy one m d to a rigid metallic suppart, and a charge is placed on t,he metal, the fwc end of the gold leaf will stand away from its support t o such a distance that the restoring force due t o the action of gravity an tho mass of the leaf just balanecs the rcpdsive force of the likc charges on the leaf and its support. Any ionization prodmpd in the air surrounding the leaf and support can result in ncntraliaation of the charge on these metals, causing the free end of the l e d to i d 1 hack t,oward the rigid support. Thun, the rate of movement of the leaf is a meamre of the rate of dischergc of the e l ~ c t r o s e o p and, ~, under sufficirntlv cont,rolled eondit,ions, of t,he intensity of ionization of the gas within the active volume of the instroment. Although gold leaf was the usual moving member in early electroscopes hecause i t could he fabricated in very thin, light strips, i t has been replaced in modern rlectroscopes by quarts fibers, which can be made very light, can he coated with a surface film of metal if condnctivit,y is desired, and are extremdy rugged and constant in properties. Figure 1 shows the ronstruction of the Lauritsen quartz fiber electroscope, which has been widely used.
Figure 1.
The Louritsen quartz flber electro-
scope. The quartz fiber and it,s support are charged hy making contact with the pasitive side of the battery. While this contact is made, the image of the fiber on a. microscope eyepiece reticle is set t o a
eonvenicnt reference point, and the contact is hroken. The isolated fihpr is then ohserved as function of timr. In t,he ahsrnce of a. ssmplr, the discharge rate is dne in part t o the leakage of chaw? through tho insolat,ors, and in part tu the inevituhlr ambient radiation I ~ v P I . The activity of B sample is drterrninrtl from t,he increasr in discharge mtt. ovcr the 1,srkgroond rate when the sarnplr in present nrar t h charged ~ rlertrodr. Thc advantages of the dertrosoopr an. bhnt i t is simple, ruggcd, in~spensivc~, s a d portable. Only n simple hatter? is required for the power soppl?, and the wad-out ia direct and visual. Its disarlvantugcs are its don.ness and limited smsitivit,~. Most of the purposes for which clrrtrosroprs were used in the past are now better served I,>- other inxtrnments, such RS ion chambers, (kigcrMwller tuhm, and scintillation detertors. The principal applirationa for rlrrtrosropes a t prrsent are in radio-assays under firld conditions, pnrticularl?- for rtlpha rmitt,ers, and in p ~ r s a o n r l dosimrtry, for whirh they are admirably suited.
Design of o pocket electrorcopr personnelexposure monitoring. Figure 2.
for
Figure 2 shows thr design of s typiral pocket rlectroscop~. These arc the sisr of R fountain pen, and ran br ronstruct~d t o show intcgrsted radiation dose from as low as milliroentgens t o ss high a kilorocntgens ( I roentgen = srnount of radioactivity capal~le of producing 1 esu of charge pt-r ml of air). An examplc of a quartz fiher elratroscope is the Modcl L i 5 l ) Elcrtrometer ($100) mmufnctured by the Landsvrrk Rlert,romet~rCo., availshlc in the U.S. through W. D. Johnson and Assor., Boonton, Pjew Jprse?-, U.S. Kwlear Carp., Bnrhsnk, California, and several other stqqdirrs of mtclenr instrumentation. The cylindrical ehamher consists of a n nlominum tuhe with an inside volume of 200 ce. The axially-mounted quartz fihcr is charged hy means of a. transistor oscillator power supply based on a 1.3-v mercury battery and producing a n nubput, voltage of about 100 v. The zero adjustment of the fiber is itecomplished by generating a voltage I,y means of R. hand-operated friction charger. This auxiliary rlectrostntie rhargr is applied t o the fiher in whatever amount is necessary to correct its position. The Imkage rate is about 5Y0 per hour, and a i t h samples inserted direet,ly in the ionization chamber, a sensitivity of dct,ection of 3 X lo-' micr* curies of a. soft beta emitter is attainable. An alpha source (dry, 5 hfev part,icles)
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A135
can be detected a t s, concentration corresponding t o 1 disintegration per minute. Qusrtz-fiber packet electroscopes for personnel dosimebry are available from this manufacturer, as well as from Bendix Aviation Corp., Cincinnati, Ohio; Vietoreen Instrument Co., Cleveland 3, Ohio; Keleket division of Tracerleh, Waltham 54, hlassarhusetts; Landis and Gyr S.A., N e a York 19, New York; and Fricseke and Hoepfner, available through Rahl Scientific Corp., El Csjon, California. A unique electroscope intended to give warning of major exposures t o radiation is the Radnd dosimet~r (Model 520, 52.95) made by Eleetromation Co., Burbank, California, and shown in Figure 3.
Figure 3. The "Radod" personnel dosimeter, bosed upon the mutual electrortotic repulsions of chorged polystyrene bolls.
I t consists of a tube containing R number of small polystyrene balls. If i t is shaken in the direction of its long axis 20 or 30 times, the balls acquire a, static charge due t o ruhhing friction, and will ding t,o the inside walls of the glass tube. Under normal conditions, w r y few of the bends will drop down in a sir-hour period. Ii the dosimeter ha8 been exposed t o 6 or more roentgens, most of the heads will be found t o have disappeared below the edge of the metal ferrule.
Ion chambers I n a n ionization chamber, the charge produced by the nuclear radiation and collected by a charged electrode is measured in an external circuit, rather than by the motion of the collecting electrode itself, as in the electroscope. The collected charge mav be measured as s. flov of current, as a voltage drop across a large resistance, or as a rhange in the state of charge of a capacitor. The constn~ction of an ionization chamber is simple, consisting essentially of a, pair of electrodes which are well insulated from each other. Since the magnitude of the current t o be measured is very small, the quality of the insulation (Continued on page A138)
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Journal of Chemical Education
is of fundamental importance. For example, s. particle such as an alpha or a beta ray expends an average of about 32 ev of its energy for each ion pair i t produces hy collision with molecules in air. Thus, a 1 M w alpha, creates about 3 X lo4 ion pairs in its total path. Since the chargr on a n electron is l . G X canlomhs, the maximum n e g a t i v ~charge that could be collected, if no recomhinatkm of ions occurred, would be 5 X lo-'" co~domb. If $1 t,his chargp were colleetrd in I second, the r o r r m t Honing during t h a t time wauld be 5 X amperrs. By mcnns of present-day high sensitivity elcct,romet~r-amplifiereircuitty, i t is possil& t o make reliablc current measurrments as low as 10-l6 amperes. However, for these small currmts to he interpreted in tprms of radioactivity, i t is necessary that the leakage rnrrents across the insulation he made as small as possible. The leakage current in an ionization chamber is due t o ( I ) surface rondurtivity, through any moisture and dirt on the surfare of the insulator, (2) volume rondurtivity, through t,he- bulk of the insulator, and (3) ntrrss current, d w t,o the relaxation of strain in the insulator resulting from mechaniral or elcrt~rieal stressrs. To minimiap surface rond~trtivity, the insulator should he h y d m phobic, and t,hesurface should be polished smooth, so t,hat no scrstahes or porcs are nvailablr for capillary condensation of moisture. T o minimizc volulnc conductivity, t,he resistivity of the insulator should he ss great as possihlr. The volume wsistiviti~s,in ohm-cm, of rommonly used insolators are: polystyrene, >los; fused quartz, >5 X Teflon, 10'l-lO'a; Ceresin wax, > 5 X 10IB; polyethylene. 10"; amber, 5 X 10". Stress currents tend t o be- gwater for soft insulators bhan for hard ones, but the rate of relaxation of t,he strain is fastpr for thr softer materinls. Ion chamhers for us? with solid samples a t at,maspheric pressure are not subjected t o much stress, and polystyrene insulators can he used with advantage, However, if radioactive gases are t o be memored, pressure differentials on tho insulators during the filling and emptying manipulations can cause disturhing stress currents, and hard insulators such as fused quartz or alumina arc t o he preferred. $0- GW . sR-
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