The applications of standards of radioactivity

International Journal of Applied Radiation and Isotopes, 1956, Vol, 1, pp. 215--232. Pergamon Press Ltd., London

The Applications of Standards of Radioactivity H. H. SELIGER National Bureau of Standards, Washington

(ReceivedaTune6

1956)

The application of standards of radioactivity for (a) determination of absolute efficiencies of detectors, (b) determination of long half-lives, and (c) determination of mean energies are reviewed. Most of the available isotopes are listed and methods of routine assay are suggested. The various types of detectors used for routine radioactivity assay in the laboratory are discussed and their efllciencies are evaluated. Techniques are suggested for the preparation of "laboratory standards" from the standards issued by the National Bureau of Standards. The use of simulated standards in both beta- and gamma-ray measurements is described. Les applications des 6talons de radioactivit6 O n passe en revue les applications des 6talons de radioactivit6 (a) /t la d6termlnation de l'efficacit6 absolue des d6tecteurs, (b) /t la mesure des longues p6riodes, (c) /~ la mesure des ~nergies moyermes. On donne la liste de la plupart des Lcotopes utilisables et on sugg6re des mdthodes de mesure courante. Les diff6rents types de d~tecteurs utilis6s pour les mesures courantes de radioactivit6 en laboratoire sont discut6s et leurs constantes 6valu6es. On sugg6re des techniques de preparation d'"~talons de laboratoire" /t partir des ~talons du National Bureau of Standards. O n d6crit l'utilisation d'6talons simul~s pour les mesures de rayonnement fl et ~,. IIpHMeHeHHe CTali~apTHIdX 3Ta~'IOHOB pa~HoaHTHBHOCTH.

~[aH 0680p c~y~aeB HpHMeHeHH~ CTaH~apTH~X 9Ta//OHOB pa~Hoalrl"HBHOCTli ~JIH: a) onpe~e~eHH~ a6COJIIOTHI~IX9~eHTHBHOOTe~[ ~eTewropoB, 6) ~:I~ onpe~eJieHH~ 6o~,nmx noJIynepHO~OB H B) ;g.Ti~onpe~e~eHna cpe~HHX 9neprn~. HpitBe~eH cIIncoH 6OHI,IIIHHCTBa ;gocTyIInIa-X H3OTOHOB H npe~gy/araloTcH CTaH~IapTHLte MeTO~I aHaaHaa. O6cyh~aKrrca pas~Hqnl~e TH.U~ ~eTewropon, npHMeHfleg],IX B :la6opaTOpHOlt npawrHRe n ~aeTca cpanHHTem,Ha~t ollenRa Hx 9(~eKTHBHOCTH. IIpe~jio;ReHa MeTO~ja~a HSPOTOBHeHHH,, ~a6opaTOpHh'X CTalt~apTHhl'X 9Ta~/OHOB" Ha OCHOBe CTaHKapTOB, Bt,IIIyC~aeM~IX HaIIHOHa2II~HI,/M ]3Iopo CTan~apTOB. OHHCaHI~I ciioco6I~I IIpHMeHeHHH Mo~eJIbHI~IX 3Ta~IOHOB npH 6eTa n ra~Man3MepenHHx. Die Verwendung von radioaktiven Standards Es wird ein Ueberblick tiber die Verwendung von radioaktiven Standards gegeben, (a) zur Feststellung des absoluten Zahlenausbeute eines Detektors, (b) zur Bestimmung yon langen Halbwertszeiten und (c) zur Messung von mittleren Energien. I n einer Tabelle wird ein Grossteil der erh/ilflichen Isotope zusammen mit den jeweils in Frage kommenden Messmethoden angegeben. Verschiedene Detektortypen fiir routinm~tssige Aktivit~tsbestlmmungen in Laboratorien werden beziiglich ihrer Verwendbarkeit diskutiert. Methoden zur Herstellung yon "Laboratorium Standards" aus Standards die durch das National Bureau of Standards bezogen werden k6nnen, werden vorgeschlagen. Die Verwendung von angeggiichenen Standards bei der Messung von kurzlebigen t - oder y-Strahlern wird beschrieben. 215

216

H. H. Seliger

INTRODUCTION Determinations of the number of alpha based on (a) ; measurements of historical and particles emitted by radium per second per geologic dates, based on (b) ; and measuregram were first made in 1908 by RUTHER- ments of W (the energy expended per ion FORD and GEmER, m using their newly- pair), electron penetration, and depth-dosedeveloped spark counter, and by Sir JAMES rate for medical applications, based on (c). DEWAR, ~*) who measured the rate of pro- The use of the C 14 half-life for historical duction of helium from about 70 mg of dating has become an archaeological tool of pure radium chloride. In the same year, extremely high resolving power, the latest 1908, REGENER~3) attempted to detect alpha and most important contribution being the particles quantitatively by counting the accurate dating of the Dead Sea Scrolls. scintillations produced in ZnS. The deterO f a certain nonscientific but pecuniary mination of the efficiency of the ZnS interest is the use of standards of radioscintillation counter for alpha particles can activity to determine the cost of radioactive therefore be considered to be the first direct material to the consumer. application of a standard of radioactivity. The National Bureau of Standards is responAlso in 1908, SODDY{4) measured the sible for the development, maintenance, and production of helium in purified uranium distribution of radioactivity standards in the leading to a value for the half-life of the United States and standards described in this latter element. This standardization had a paper originate from that laboratory. direct application in answering one of the It is important to distinguish between the oldest questions of mankind, namely, how applications of radioactivity and the applicaold is the earth? Radioactivity and par- tions of standards of radioactivity. Any report ticularly uranium provided a process wherein encompassing the former would be monuthe rate of progress was independent of mental, since most measurements of radioexternal physical or chemical conditions and activity involve only precise relative measurein which the element was sufficiently long ments. However, even in these cases, for lived so as not to have decayed appreciably radiation safety, for planning experimental details, and for ordering and disposing of during the entire process. In 1912, MOSELY,~51 using R a B ÷ C , active material, a knowledge of the disdetermined that within his experimental integration rates of the material is essential. error there was only one electron emitted The experimenter must account for all of per beta disintegration. This result, while not the initial radioactivity in order to obtain a standardization per se, forms an essential a material balance in his experimental link in the interpretation of the subsequent results. Thus the total efficiencies under results of ELLIS and WOOSTER,~6) who differing experimental conditions of his determined the mean energy of R a E beta various radiation detectors must be known. disintegration. To do this they measured In this review a summary of the applications in a sensitive calorimeter the total energy of of standards of radioactivity will be presented. A large section will be devoted to suggested a previously standardized sample of RaE. From the three primary applications of methods for the use of radioactivity standards standards of radioactivity referred to above, for calibration purposes, and the various namely (a) determination of absolute effi- methods of measurement of radioactivity ciencies of detectors, (b) determination of will be discussed. The remainder of the long half-lives, and (c) determination of paper will be concerned with long-half-life mean energies, there have been derived measurements, historical and geologic dating, today a number of related applications. and measurements based on the mean There are measurements of absolute cross- energy of beta disintegration. The literasections, activation analyses, and activity ture references are by no means exhaustive measurements for medical, industrial, and but are representative of the applications of military uses and uranium prospecting, all standards of radioactivity.

The application of standards of radioactivity

217

T H E U S E OF R A D I O A C T I V I T Y S T A N D A R D S I N T H E L A B O R A T O R Y

T h e total efficiency e o f a detector for a source p r e p a r e d from a radioactivity s t a n d a r d xs defined as: counts per second observed for source e = disintegrations per second in source

(1)

T h e total efficiency is c o m p o u n d e d o f the geometry (i.e. the fractional solid angle s u b t e n d e d b y the detector) and the detector efficiency (i.e. the p r o b a b i l i t y o f a measurable response from a radiation striking the detector). Beta-ray solution standards distributed by the National Bureau of Standards are contained in flame-sealed glass ampoules and consist of approximately 3 ml of an aqueous carrier solution of the active nuclide. A certificate specifies the disintegration rate per second per milliliter as of the zero date. Ampoules intended for use as gamma-ray standards contain an accurately known volume of

solution (5 milliliters), and in these cases the total disintegration rate and the exact volume of solution at a given temperature are also specified. RaD + E reference sources, RaF alpha-ray standards, and U30 s alpha-ray standards consist of the active material already deposited on source-mounts. A complete list of standards of radioactivity presently available from the National Bureau of Standards has been given by W. B. MAnN(T) in a previous issue of this journal. In Table 1 are listed most of the important radioisotopes, their decay properties, and the preferred methods to be used in the laboratory for the routine assay of samples. Isotopes now available or shortly to be available as standards from the National Bureau of Standards are printed in italic type. The beta-ray energies available cover the range from the 18.9-keV spectrum of I-In to the 3.58-MeV spectrum of K 42. Gamma-ray energies cover the range from the 87-keV line of Cd 1°9 to the 2.758 MeV line of Na ~4, as well as the very low-energy X rays from the electron capturers such as Fens and Zn es.

M E T H O D S OF M E A S U R E M E N T

T a b l e 2 lists the most general methods available for the m e a s u r e m e n t o f alpha, beta, a n d g a m m a emitters, together with b r i e f description o f the counting conditions and the r a n g e o f applicability o f each method. I n this connection the t e r m BEA (Background E q u i v a l e n t Activity) has been found to be a convenient m e t h o d o f expressing the intrinsic sensitivity o f a detector. (a) I n all measurements o f radioactivity there is a r a n d o m b a c k g r o u n d effect which must be subtracted from the observed reading taken with the source present. T h e m a g n i t u d e o f this background, or m o r e specifically the ratio o f the source effect to b a c k g r o u n d effect, m a y place practical time limitations u p o n the c o n d u c t o f an experiment. It is for this reason that the t e r m Background E q u i v a l e n t Activity (BEA) was devised, this being the disintegration rate of the p a r t i c u l a r isotope required to just e q u a l the b a c k g r o u n d reading. T h e 2~rfl windowless gas-flow proportional c o u n t e r is considered to be the most sensitive and versatile a p p a r a t u s for the routine c o u n t i n g o f all types o f beta sources, followed closely b y the 2~tfl windowless

electroscope. T h e well-type N a I scintillation c o u n t e r has the highest sensitivity for g a m m a rays o f a n y detector now available.(9, z0) With these instruments one can c a r r y out almost a n y type o f m e a s u r e m e n t involving radioactive materials. W h e r e cost and trouble-free m a i n t e n a n c e are overriding factors, the electroscope and the end-window G - M c o u n t e r are efficient substitutes for the windowless flow counter, and a cylindrical a r r a n g e m e n t o f several g a m m a - r a y G - M counters surrounding the source can substitute for the well-type scintillation counter. F o r these substitute methods the BEA is, o f course, increased, especially so for the g a m m a - r a y a r r a n g e m e n t . T h e liquid scintillation c o u n t e r has recently come into limited use for the routine m e a s u r e m e n t o f low-energy beta emitters. At the present time, u n d e r p r o p e r experimental conditions, it is possible to p r e p a r e a liquid scintillator which is m o r e efficient t h a n anthracene. (z~) T h e most efficient liquid scintillator c o m b i n a t i o n reported to date is a p p r o x i m a t e l y 4 g/1,2,5diphenyloxazole ( D P O ) with 0.1 gfl. 1,4di- (2 (5-phenyloxazole)) :benzene ( P O P O P )

H . H . Seliger

2 18

TABLE 1. Preferred methods of measurement for selected radioisotopes

Nuclide

g/~mtx (MeV)

H s

0.0180

C14

0.155

Nags

0.542

NaS4

1.39

J~8

1.71

S*s

0,167

(11s6 AsT

0.713 EC

K 4o

1.33 EC 2.04 3.58

Kas

Principal E~, (MeV)

m

TI/g

Suggested methods for routine assay*

12.3 y

Liquid scintillation counter; internal gas counter or ion chamber 2*rfl windowless counter; liquid scintillation counter; internal gas counter

6000 y 0.511 1.28

2.758 1.380

m

2.6 y

2rrfl windowless counter; well-type scintillation counter

15.0 h

2 rrfl windowless counter; well-type scintillation counte~

14.3 d

21rfl windowless counter; well-type scintillation counter (Bremmsstrahlung) 27rfl windowless counter; liquid scintillation counter

87.1 d

C1K~

4 × 10ny 35.0 d

1.45, AK~

1.3 × 10~y

2rrfl windowless counter; well-type scintillation counter

12.44 h

2zrfl windowless counter; well-type scintillation counter

1.51

1.12, 0.89, 0.89, 0.32 VK~

85 d

2,r/~ windowless counter; well-type scintillation counter

Cr51

0.254 0.36 1.2 0.5% EC

27.8 d

X-ray counter; well,type scintillation counter; liquid scintillation counter

M n s,

EC

0.84 CrK~

291 d

.~e55

EC

MnK~

~59

0.260 0.460 0.310

Ca45 Sc 4S

CoaO NiS~ N i e*

152 d

2rrfl windowless counter; liquid scintillation counter Internal gas counter

{I:? I.17 1.33

EC 0.067

CoK~

EC 0.325 2.5%

Gae~

EC

As 7,

fl- 1.36, 0.69 fl+1.53, 0.92 0.465

0.596 0.547, 0.787 Complex

0,716 1,822 0,130

1.081

65

Br sl

R b s6

R b s7

0.034

X-ray counter; well-type scintillation counter; liquid scintillation counter X-ray counter; well-type scintillation counter; liquid scintillation counter

45.1 d

2*rfl windowless counter; well-type scintillation counter

5.2 y

2rtfl windowless counter; well-type scintillation counter

8 × 104y

85 y 1.11, CuK0t 0.511 Complex ZnKo¢

~n

2.94 y

2rrfl windowless counter; liquid scintillation counter

250 d

X-ray counter; well-type scintillation counter 27rfl windowless counter; liquid scintillation counter X-ray counter; well-type scintillation counter

77.9 h

X-ray counter; well-type scintillation counter

17.5 d 35.9 h

2 rrfl windowless counter; weU-type scintillation counter 2rrfl windowless counter; well-type scintillation counter

19.5 d

2rrfl windowless counter; well-type scintillation counter 2rtfl windowless counter; well-type scintillation counter

6 × 10 l ° y

The application of standards of radioactivity

219

TtmLZ 1 (continued)

Nuclide

EPmax (MeV)

Principal E~ (MeV)

S r 85

EG

Sr"

1.46

65 d 53 d

SrgÜ

0.61

19.9 y

Imo

2.180

61 h

2,rfl windowless counter; well-type scintiUation counter (Bremsstrahlung)

65 d

2*rfl windowless counter; well-type scintillation counter

35 d

2~rfl windowless counter; well-type scintillation counter

2 × 105y

2*rfl windowless counter; well-type scintillation counter

0.371 0.84 isomer

ZrgS

~95

0.160

Tc09

0.290

Kut0S

0.217 0.698

Rut°6 I AgnO

I

Cd1°9 I SntXa [ IatS

0.745

0.498

2~rfl windowless counter; well-type scintillation counter 39.8 d ly 270 d

2~'fl windowless counter; liquid scintillation counter 2¢rfl windowless counter; well-type scintillation counter

EC EC

0.087, AgKcc InKg

470 d l12d

X-ray counter; well-type scintillation counter X-ray counter; well-type scintillation counter

0.850 1.27

0.382

CSI $4

0.090 0.648 0.510

~$I$7

1.20

Bat*~ Ba t "

EC

Ce m Col4,

0.442 0.581 0.300, 0.170

T m 1~°

0.721

0.52 0.62 Complex

0.255 0.600

EuX54

X-ray counter; well-type scintillation counter 2~rfl windowless counter; well-type scintillation counter (Bremsstrahlung) 2~rfl windowless counter; well-type scintillation counter (Bremsstrahlung)

0.039 Complex

pat

pr144

0.51

Suggested methods for routine assay*

3.0

13d

2~rfl windowless counter; well-type scintillation counter

0.080 0.364 0.638

8.08 d

2~'fl windowless counter; well-type scintillation counter

0.602 0.794

2.3 y

2*r~ windowless counter; well-type scintillation counter

30 y

21rfl windowless counter; well-type scintillation counter

2.6 m lOy

21rfl windowless counter; well-type scintillation counter X-ray counter; well-type scintillation counter

Complex

33.1 d 282 d

2~'fl windowless counter; well-type scintillation counter 27rfl windowless counter; well-type scintillation counter

1%

17m

2*rfl windowless counter; well-type scintillation counter

0.661 0.085, CaK~ 0.141

EC

0,067, NdKot

200 d

0.300 0.700 1.900

0,778 1.2 0,336

16y

2~rfl windowless counter; well-type scintillation counter

0.866 0.970

--

129 d

2*rfl windowless counter; well-type scintillation counter

X-ray counter; well-type scintillation counter

220

H. H. Seliger TABLE 1 (continued)

ECmax (MeV)

Principal E r (MeV)

0.250 0.530

Complex

Ir T M

0.670

Complex

A u TM

0.970

0.411

H g 2°3

0.208

0.279

Tl 2°~

0.783

--

P b 21°

0.026

0.007

B i 21°

1.17

--

Nuclide

Ta

TM

Po ~1°

~5.3

10-3% 0.773

R a 22~

~4.8

Complex

Tll2

115 d 74.4 d 2.69 d 47.9 d 4.0 2.7 Y/ Y/ 22 y 5.0 d

138.3 d 1620 y

Suggested methods for routine assay*

21rfl windowless counter; well-type scintillation counter 2~rfi windowless counter; well-type scintillation counter 2~vt3 windowless counter; well-type scintillation counter 2~rfl windowless counter; well-type scintillation counter 2~fl windowless counter 2~rfl windowless counter; liquid scintillation counter 2~fl windowless counter ; well-type scintillation counter (Bremsstrahlung)

2~v/3 windowless counter;

ZnS ~ scintillation counter

47r7 ionization c h a m b e r ; well-type scintillation counter

* T h e m e t h o d s suggested are the most efficient. This does not preclude the use of the quartz fiber electroscope or the s t a n d a r d G - M counter for routine measurements.

in toluene. Under carefully controlled conditions of solution preparation and measurement it has recently been possible to intercompare tritiated-water standards received from AECL, Chalk River, Canada, and LASL, Los Alamos, New Mexico, with the NBS tritiated water standard with a precision of a few tenths of 1 per cent. In this case a water : alcohol : xylene ratio of 1 : 50 : 250 was used in the system with 4 g]l. D P O as the scintillating solute. For certain other tracer applications involving lower activities of material and therefore requiring the measurement of larger volumes of water, the use of dioxane as a solvent will permit as much as 25 per cent water to be incorporated into the measuring cell.

Considerations in beta-ray calibrations To use beta-ray solution standards it is necessary to open the standard ampoule and to prepare a standard source for comparison, or for calibration of a counting system. Assuming that the electronic equip-

ment is suitable and the detector adequately stable, the following are the most important considerations (a) Source self-absorption. Most beta-ray solution standards distributed by the National Bureau of Standards have a minimum of total solids present, consistent with chemical stability and freedom from adsorption effects. In some instances, however, where the original material may be of low specific activity, as, for example, in the carbon-14 solution standards, the absorption of beta rays from a dried source may be quite appreciable on account of the amount of solids present. Because of the low energy in the case of the carbon-14 beta spectrum (Em~ = 155keV), the observed counting rate will be a strong function of the amount of solids present as well as the method of source preparation. Even in the case of practically weightless deposits of cobalt-60 (Emax = 310 keV), the author has reported ~8) as much as a 10% difference in observed counting rate in a 47rfl counter, depending on the method

The application of standards of radioactivity

221

TABLE 2. General methods of measurement for alpha, beta, and gamma emitters Detector

Auxiliary equipment required*

Types of sources

Range of applicability and BEAt

27$l air-ionization chamber (recommended for secondary standardization at a large installation)

High-voltage supply, Vibrating-reed electrometer, or Lindemann-Ryerson electrometer Projector Calibrated condensers Voltmeter Stopwatch

Dried on planchet ; liquid in glass or metal cup forming part of periphery of chamber

Sensitive mainly to alpha and beta rays. Higher intensity gamma-ray sources can also be measured. Small selfabsorption effects for low-energy beta emitters. BEA for Ps* dried sources0.001 ,uc. BEA for Par liquid sources0.01 PC.

4x7 air-ionization chamber (recommended for secondary standardization at a large installation)

High-voltage supply, Vibrating-reed electrometer, or Lindemann-Ryerson electrometer Projector Calibrated condensers Voltmeter Stopwatch

Liquid in ampoule; test-tube inserted into re-entrant cylinder

Beta rays are completely absorbed in walls. Ionization is due to bremsstrahhmg and to gamma rays. Big advantage is nondependence of reading on position or volume of source within the chamber. Can be used to compare high-energy beta emitters from bremsstrahhmg measurements. BEA for I’s’ in solution-l.4 PC, BEA for Pas in solution-20 pc. Highpressure chambers are more sensitive.

2a/l windowless proportional flow counter (recommended for all beta-ray measurements on a routine basis)

High-voltage supply Nonoverloading amplifier Discriminator Scaler 90% argon, 10% methane or helium isobutane, or 100% methane

Dried on planchet; liquid, in formamide or other lowvapor-pressure mlvent, delivered to stainless-steel cup

Most sensitive method for beta-rays. Higher intensity gamma-ray sources can also be measured. The efficiency for beta rays is practically 100% over the central region, becoming 90% at periphery. Simple to operate and decontaminate. Alpha particles can be counted in presence of betas at lower voltages in the proportional region. BEA for Par dried sourcefor P”* as thick 0.0001 PC. BE4 sourcti.001 PC. BEA for Cl4 as liquid-O.001 PC.

Well type NaI scintillation counter (recommended for all gamma-ray measurements on a routine basis)

Precision high-voltage supply Nonoverloading linear amplifier Discriminator Multiplier phototube Scaler Single-channel pulseheight analyser

Liquid in ampoule; test-tube inserted into well. Source could also be cxternal in conditions of “good” geometry

Beta rays are completely absorbed in walls of crystal holder. Light output to phototube is due to bremsstrahlung and to gamma rays. Most sensitive method for gamma rays. Tremendous increase in sensitivity over air-ionization chamber. Light output is directly proportional to energy absorbedpermits identification of isotopes by gamma photopeaks. Window pulseheight and analyser permits counting one photopeak to exclusionof others in mixtures of isotopes. Relatively independent of source position or size inside well. BEA for 1181 in 5-ml ampoule-0.0003 ,uc. BEA for Crsl in 5-ml ampoule-0.003 ,uc. BEA for Par in 5-ml ampoulti.02 PC.

* All equipment listed is available commercially in U.S.A. t BEA: Background Equivalent Activity is a measure of the intrinsic sensitivity of a detector. It is the number of disintegrations per second of the particular nuclide that will produce an effect equal to the background effect.

222

H. H. TABLE 2

Seliger (continued)

Detector

Auxiliary equipment required*

Types of sources

Range of applicability and b.e.a.t

5. Liquid scintillation counter (recommended for double betalabeling experiments. Useful but not essential for low-energy beta emitters and electron capturers)

Precision high-voltage supply Nonoverloading linear amplifier Discriminator Multiplier phototube Scaler Single channel pulse height analyser

Liquid incorporated into solution of scintillator as water-alcoholtoluene, waterdioxane, or an organic tagged molecule such as C 14benzoic acid

Useful for measurement of low-energy beta-emitters such as H 3, Ni s3 and C 14 and electron-capturing nuclides such as Cr ~1, Fe 55, NiSL-eliminates selfabsorption effects. Useful for doubleand triple-labeling experiments because of linear response to energy absorbed. Can discriminate between 0o's and fl's. BEA for H~O in 5-ml cell--0.01 #c. BEA for C 14 benzoic acid in 5-ml cell--0.004 #c.

6. Mica end-window G - M counter (lower-prlced substitute for 2~r~ windowless counter)

High-voltage supply Scaler

Dried on planchet; liquid in glass or metal cup directly below window

Almost as sensitive to betas as 2~r//flow counter. 100% sensitive to electrons entering sensitive volume. Air and window absorption lower efficiency for low-energy beta emitters. Does not discriminate between ~'s and fl's as in 2~r flow counter. Efficiency for g a m m a rays same as 21rfl flow counter (1-2%). Halogen self-quenching tube gives indefinite, trouble-free service. Standard shelf arrangement gives reproducible geometry. BEA for p82 dried source in top shelf position--0.0001 #c.

7. Thin window argon, krypton, or xenon X-ray counter (specifically for use with electron-capturing nuclides)

High-voltage supply Linear amplifier Discriminator Scaler Single-channel pulse height analyser

Dried on planchet; liquid in glass or metal cup directly below window

Makes use of critical or near-critical absorption of gas for X-rays near critical absorption limit. Useful for pure electron-capturing nuclides. Window-absorption reduces efficiency for low-energy X-rays. Linear response makes identification and differential counting of mixtures of gamma-emitting isotopes possible. BEA for Fe 5s dried source---0.0003/~c.

8. Quartz-fiber electroscope (lowerpriced substitute for 27r/3 windowless counter)

70-volt battery for charging. 1.5-volt battery for pilot light Stopwatch

Dried on planchet, on slide mounted external or internal to electroscope. Internal liquid sources not recomm e n d e d due to difficulty in decontamination

Very-low-capacity system slightly more sensitive to internal beta sources than 2~rfl air-ionization chamber. Low sensitivity to g a m m a rays. Displacement of fiber is slightly nonlinear. Excellent for relative measurements over same region of scale. Simple, trouble-free operation with care. Can be used for low-energy beta emitters with same precautions as 21rfl airionization chamber and for higher intensity external g a m m a sources. BEA for psa dried source--0.001 pc.

9. Internal gas G-M counter

High-voltage supply Scaler Chemical conversion system

Isotope converted to inert or counting gas; small amount present in vapor phase so as not to interfere with operation

Useful in some cases for routine analyses of tritium or carbon-14 where material can be converted to gaseous phase. Can also be used for counting small amounts of T H O as vapor. 100% efficient for beta particles. Small end and wall effects. BEA for C 14 or Ct40~--0.00003 pc.

The application of standards of radioactivity Table 2 Detector

Auxiliary equipment required*

223

(continued)

Types of sources

10. Internal gas ionization chamber

High-voltage supply Vibrating-reed electrometer Chemical conversion system

Isotope converted to

I 1. ZnS alpha scintillation counter (used w h e r e / / o r ~, counting rates interfere with ~ detection)

Precision high-voltage supply Linear amplifier Discriminator Multiplier phototube

Dried on planchet

gaseous phase

Range of applicability and BEAt

Less sensitive than internal gas counter. However, the technique is less critical as to the type of gas present. Good G-M characteristics are not required so that larger amounts of the isotope as a gas may be counted. Sensitivity similar to 2~r~ air-ionization chamber. BEA for C 1' as Ca*Oz--0.001 #c. Detects &s in presence of high intensity fl's and ~,'s. EiBciency is practically 100% for ~ particles striking ZnS film.

Scaler

of preparation of the source. This was later confirmed independently by D. B. S~ITH and A. M. WILDBLOOD.~Is) It is therefore suggested in the cases of lowenergy beta emitters that comparison sources be prepared from carrier solutions of nearly identical solid concentrations when low to medium solid sources are desired. Otherwise "infinitely" thick liquid or solid sources should be prepared. The 21rfl gas-flow proportional counter has almost 100% efficiency for a beta particle entering the sensitive volume. A method of counting, using this type of counter for infinitely thick liquid samples is described by A. SCHWEB~L et al. ~14, 15). The radioactive material is dissolved in formamide, an organic solvent having a very low vapor pressure compared with water. A diluting solution offormamide containing 1% carrier is prepared. The solution to be standardized is then diluted by a factor of 100 with the formamide carrier solution to reduce the water concentration to 1%. At this low concentration water vapor does not appear to change the counting characteristics of the 2~r flow counter. Usually 1 ml of the 1% solution is pipetted into a stainless-steel cell. The counting rate is proportional only to the solution activity and to the area of the cell. I f the volume and hence the depth of solution is always the same, any effects due to g a m m a rays will be constant. Thus the formamide method of

liquid counting can also be used for beta emitters that have accompanying g a m m a rays. "Infinitely" thick solid samples can be prepared uniformly from slurries of active material containing carrier. Care must be taken in the use of slurries or partial digestions or wet ashings because of slight differences in self-absorption and scattering due to adsorption on glassware, changes in deposition of colloidal particles and possible settling. It may be necessary to stir the sample continuously or to use rotating planchets to average differences in concentration over the source. Even in the cases of the infinitely thick solid and liquid sources it must be remembered that the effective atomic number of the material will to some extent determine the amount of selfscattering and therefore the self-absorpfion.~t6,1~) In order to obtain reproducible measurements the chemical compositions of the samples being compared must be as alike as possible. For example, if identical areas of infinitely thick sources of sodium carbonate C 14 and barium carbonate C 14 are compared, the barium-carbonate C 14 will give the higher reading even though the sources have the same specific activity. For beta emitters of Emax greater than 300 keV the problem of source self-absorption is not so important, and samples can be intercompared by drying aliquots on planchets and inserting these in

224

H. H. Seliger

position in the 2~r flow counter. However, there is yet another important consideration. (b) Baekscattering. For the routine analysis of dried samples of material various workers have used planchets or cups made of aluminum, glass, copper, stainless steel, palladium, or platinum, on which aliquots of solution were dried. The conditions of the experiment and the economic limitations of the cost of sample holders will determine the type of backings used. In any case, one must carry out all the measurements using the same type of backing, since the backscattering of beta rays is dependent on the atomic number of the backing material. This Z-dependence has been investigated for both electrons and positrons .cls) U n d e r certain conditions the use of a nonconductor such as glass as a source backing can induce electric charges inside a counter. We dry the sources on palladiumfaced silver discs. Palladium is less expensive than either gold or platinum, is close to silver in atomic number and therefore in backscattering properties and, unlike aluminum, copper, and steel, is inert to most acids and bases. (c) Low solids source preparation. For the delivery of a small-volume sources there is available a delivery-type mercury-displacement ultra-microburet ~19) that can deliver quickly and precisely volumes of solution as small as 0.01 ml with an accuracy of 0.05%. No particular problems are encountered in the preparation of most medium and highenergy beta sources. The technique of air-drying at room temperature has been quite satisfactory. For low-energy beta emitters and even in those cases of the higherenergy beta-emitters such as Na~2C1 and Srg°CI~, which form crystalline salts, it is necessary to prepare sources carefully to avoid self-absorption effects. Methods that are employed to obtain minimum selfabsorption include freeze drying, precipitation by addition of specific salts (such as AgNO3 added to NaI T M solutions to precipitate AgI), and slow evaporation in a saturated ammonia atmosphere, cs~ in which ammonia induces precipitation in the form of an ammonia complex.

Considerations in gamma-ray calibrations Except under special conditions, it is not intended that a gamma-ray solution standard be opened or destroyed. The following procedure is suggested for the application of a 5-ml gamma-ray standard. A stock solution of the same isotope is diluted so that 5 ml of the final dilution contains approximately the same activity as the standard ampoule. A "laboratory standard" ampoule of the same dimensions as the standard ampoule is prepared containing an accurately known volume close to 5 ml. The laboratory standard is then compared directly with the primary standard under identical conditions of geometry, e.g. in a well-type scintillation counter, thus standardizing the entire solution. In this way the radioactivity standard can be retained as a check on further solutions, or in case of any doubts as to chemical stability or losses from the solutions used for the actual experiments. The detector efficiencies can be determined again as in equation (1) from the newly-prepared "laboratory standard" or else other types of special sources can be prepared from it. Some electron-capturing nuclides such as M n 54, Zn 65 and Cd 1°9 have nuclear gamma rays associated with their decay. These therefore can be used for standardization in the same manner as other gamma-ray emitters. Other electron capturers, such as Fe 55, and Ni 59, have only low-energy daughter-atom X rays associated with their decay. In these latter cases the method of measurement mav involve the opening of the ampoule for the preparation of dried sources on planchets for X-ray counting and 2rrfl counting, or for the preparation of liquid sources for liquid scintillation counting. The same precautions should be observed here as for the low-energy beta emitters. Simulated standards It is sometimes advantageous in the cases of the short-lived radioactive nuclides to have available a simulated standard with which intermediate checks can be made of the detectors and of the total efficiency during the course of an extended experiment. A R a D + E reference source (T1/2 ----22 years)

The application of standards of radioactivity

225

serves this purpose adequately for beta- R a D + E Reference Number emitters in the medium- and high-energy -----Activity of Standard Source range. (C/S)extrapolatea (RaD + E) It can further be used as a means of × (C/S)extrapol,tea (Std. Source) (3) standardization which is accurate to + 5 - 1 0 % for isotopes whose beta spectra are not too dissimilar to the R a E beta spectrum. These reference numbers may be slightly The procedure briefly is as follows: An different for each isotope. Once more it unknown source is prepared from a stock should be emphasized that the very important solution on a palladium-faced silver disc considerations of stability of electronic equipsimilar to the standard disc, and an absorp- ment, technique of source measurement, and tion curve in aluminum is obtained for each statistical design are always present, although source with, say, an end-window beta-ray they are not discussed in this paper. In some cases there exist long-lived beta G-M counter in a fixed geometry. Extrapolation of these curves to zero total absorber emitters whose beta spectra are nearly will correct to a first approximation for the identical to those of the short-lived isotopes. differences in the beta energies a n d for Sources prepared from these long-lived beta slight differences in geometry from one emitters can be assigned reference numbers calibration to the next. Since the back- as in equatio n (3), with the simplification scattering in the energy range 300 keV to that the absorption curves in aluminum are 1.TMeV is independent of the energy parallel, i.e. the ratio at only one thickness distribution, one may assume that the frac- of absorber is necessary. A. H. AT~N, JR./~m tional backscattering for the unknown source using end-window G-M counters, has found and for the R a E beta rays is the same. The that the absorption of Na z~ positrons is the disintegration rate of the unknown source is same as that o f I TM electrons up to 90 mg/cm ~ of aluminum absorber, and that T m 17° given by electrons are absorbed in the same manner as A -- c/s zero total absorber (unknown) Au 198 electrons between 2 0 m g / c m 2 and c/s zero total absorber (RaD + E) 100 mg/cm z of aluminum absorber. At the present time there is a sufficient range of × RaD -t- E disintegration rate (2) energies of beta-ray standards available for The R a D + E disintegration rate is given on use as simulated standards, so that practically the certificate accompanying the standard any beta emitter can be standardized to and represents the total number of RaD ~- 10% routinely in the laboratory. Mixtures disintegrations per second contained in the of fission-product nuclides can be assayed, source. Thirty days after preparation of the using T F °~ as a simulated standard.~*l, 22~ There are several simulated gamma-ray standard the R a E will be in equilibrium with the RaD. Therefore this number is also the standards available, the most notable of disintegration rate of the RaE. The use of a which is the "Mock 1181'' standard developed minimum of 6 mg/cm * of total absorber by M. BRUC~R.~3) In this case an empirically between the source and the counter will determined mixture of Ba 138 and CslST-Ba 137 insure that there is no contribution from produces, with proper filtration, a gammaeither low-energy RaD beta rays or from ray spectrum almost identical with that of isotopic I TM. Two sets of such "standards" the R a F alphas which are also present. The method can be made exact by have been "standardized" in a 4try ionizaperforming the above measurements using tion chamber by direct comparison with as an "unknown," a source prepared from a I~1 previously standardized in the 4~-fl standard solution of the isotope. In this case a counter. The long half-lives of the Ba la3 reference number for the isotope can be and the Cs 137 will permit the simulated assigned to the RaD + E source, as in standards to be used for approximately ten years before there is much change in the equation (2)

226

H. H. Seliger

spectrum due to the differences in decay. Not only is a long-lived simulated standard advantageous as a check on calibrations during the course of an experiment, but the material can be incorporated into actual phantoms and into various organ shapes in order to serve as a reference in, say, iodineuptake studies in h u m a n beings. At the present time such simulated standards of pat are being used throughout the United States in intercomparisons of I T M uptake measurements, in an effort to make diagnoses more precise and to standardize the method of measurement. Another example of the use of a simulated standard for gamma rays is that of Co 6°, whose gamma rays are suitable for the standardization of Fe 59, Na 22, and Zn 65. Since the response of a NaI(T1) crystal is dependent upon the energy of the gamma rays being detected, it is possible, using previously standardized nuclides having widely separated gamma-ray lines associated with their decay, to determine empirically a smooth curve of response per unit activity versus energy for a given NaI(T1) scintillation counter. C24~ With this smooth curve it will then be possible to standardize by interpolation or extrapolation gamma emitters of practically any energy. This will be of especial interest to nuclear spectroscopists and those interested in determining absolute gamma-ray yields from nuclear reactions. Complex standardization procedures There arise m a n y cases in which standardizations are to be made of mixtures of radioisotopes. Fission products, for which T12°4 serves as a simulated standard, ~*L~2~ are an example of a mixture being standardized by means of a single average standard. In other cases, mainly in medical and biological research, the simultaneous use of two or more different tracers is of great assistance to the researcher. In this way the effects to be observed separately with each tracer isotope have had the same host under exactly the same experimental conditions. Double-labeling experiments involving H a and C 14 make use of the difference in the beta spectra of these two nuclides. The method of measure-

ment involves (a) incorporating the samples in a liquid-scintillator solution and (b) a twochannel pulse-height analysis of the observed pulses. The relative positions and widths of the two windows of the two-channel pulse height analyser are adjusted, using standard solutions of the isotopes, so that most of the H 3 counts occur in the lower window and most of the C 14 counts occur in the upper window. With the H 3 and C 14 standard solutions the channels and the response can therefore be calibrated and the overlap reduced to a minimum. A description of the simultaneous use of Na 24 and K 42 as tracers is given b y J . F. TAIT and E. S. WILLIAMS.C2al One method of separating the components of a mixture of beta emitters is the graphical analysis of beta-absorption curves. ~z6~ In reference 26 a table of the semi-logarithmic slopes of a large number of beta emitters relative to p32 and Ca 45 is given. A number of isotopes emit several gamma rays with different energies. In these cases the various gamma rays can be separated, using the well-type scintillation counter with pulse height analysis. Similarly, mixtures of gamma-emitting isotopes can be separated by counting under the photopeaks corresponding to the most suitable g a m m a rays. By means of an efficiency curve obtained with gamma-ray standards the components of the mixture can be analyzed quantitatively. The simultaneous use of Cr51-Fe s9 and Na24-K 42 samples is described by HINE et al. ~27~ A similar application of the energy dependence of the light output of the NaI(T1) crystal was made by UPSON et al. ~28~ in counting Pu *a9 gamma rays in the presence of fission-product gamma rays. Bremsstrahlung In addition to its versatility and high efficiency for nuclear gamma rays, the welltype scintillation counter can be used to compare beta sources by measurement of the internal and external bremsstrahlung. R. LO~.WNGER and S. FEn'~.LB~RG~29~ have recently described a method of counting pa~ routinely by means of the bremsstrahlung from solutions of the isotope in a well-type NaI(T1) crystal. The efficiency is given as

The application of standards of radioactivity 0.8 count per 100 disintegrations. For a beta emitter an approximate expression for the energy radiated as bremsstrahlung, (3°) measured in M e V per beta ray, is B = 1.23 × 10-4(Z + 3)E~,

(4)

when/~ is the effective atomic number of the material in which the beta particles are brought to rest. As a rule of thumb, 85% of the total intensity of bremsstrahlung from a beta emitter in t h e range 0-0.3Emax, in the ratio 45 : 25 : 15 for 0-0.1Emax, 0.1-0.2Eraax, and 0.2-0.3Emax, respectively. From equa-

227

tion (4) the energy lost per disintegration from sulfur-35 (Emax = 166 keV) would be onehundredth that of phosphorus-32. In addition, 45% of the bremsstrahlung would have energies less than 16 keV, and would therefore be highly absorbed in the housing of the scintillation crystal. The technique should therefore be used for the higher-energy beta emitters. The only disadvantage of the method of measurement is its extreme sensitivity to gamma-emitting impurities, although this can be turned into an asset in the determination of radiopurity.

D E T E R M I N A T I O N OF A B S O L U T E EFFICIENCIES OF D E T E C T O R S

A number of examples of the use of betaand gamma-ray standards for the determination of the absolute efficiencies of detectors have been reported. S~TX-I, SELIOER, and STFYN(81) have used standardized solutions of ps2, CoSO, Srg0__ygo, I131, and TI 2°4 to determine the absolute efficiency of the 4rr crystal scintillation counting technique for beta rays of varying end-point energies. Gamma-ray standards have been applied to the absolute calibration of experimental equipment used to investigate Coulomb excitation of nuclei. TEMMER and HEYDENBERG(32,33) and FAGG eta/. (34) have made use of the gamma-rays from standards of sodium-22, iodine-131, cerium-141, and gold-198 in order to calibrate their equipment and so determine absolute gamma-ray yields. In the Coulomb excitation experiment a very thin layer of the target material is laid on top of a sodium-iodide (TI) crystal

and is bombarded with alpha particles or protons. The gamma rays emitted from any low-lying excited levels are then detected by means of a scintillation spectrometer. To determine the absolute efficiency, a standardized gamma-ray source of the same size as the beam cross-section is delivered to the target material. The ratio of source strength to area under the photopeak then yields one point of an absolute photopeak efficiency curve. A similar application has been made by C. P. SWANN and F. R. M~TZGER (35) for the absolute calibration of their gamma-ray detectors. These were used in investigating the cross-sections for the production of isomeric states excited in nuclei b y inelastic scattering of monoenergetic neutrons. R. E. HEFT and W. F. LmsY (3e) have measured absolute cross-sections for deuterons on beryllium from absolute measurements of the disintegration rate of the tritium produced in the reaction.

D E T E R M I N A T I O N OF LONG HALF-LIVES In determining the half-life of a radioactive nuclide by counting measurements for several half-lives, the familiar relationship

N(t) = Noe -a'

(5)

is reduced to In ~N = In ~g0 -- At (6) and the disintegration constant ,~ is found from the slope of a linear plot, whence the half-life if' is given by

f

= In 2 / x

(7)

When it is impracticable to measure for a sufficiently long period, the disintegration constant can be obtained by using the more fundamental decay law:

- v/dt

=

(8)

I f the half-life is so long that the disintegration rate dN/dt of a sample does not change

228

H. H. Seliger

appreciably during a measurement, ;t can be deduced from a knowledge of N, the number of nuclei present at the beginning (or end) of the experiment. For isotopically pure elements such as radium the determination of N can be made by gravimetric means or in other isotopes by quantitative conversion to the gas phase and a subsequent measurement of pressure under defined volume and temperature conditions. In most cases, however, isotopic analysis by means of the gas-density balance or the mass spectrometer as necessary, in addition to quantitative gas conversion, due to the presence of isotopes. The determination of the half-life of carbon-14 is an example of these latter procedures. The carbon-14 half-life is of particular importance in archaeology for the accurate dating of organic relics. In principle the procedure for the determination of the half-life is as follows: A sample of, say, C14-tagged carbonate is converted quantitatively to carbon dioxide. A small but accurately-known fraction of this gas volume is mixed thoroughly with an appropriate counter gas at a definite pressure, and the sample is counted with 100% efficiency. Appropriate end and wall corrections are made as discussed previously. The quantitative combustion and the final pressure and volume measurements give the total number of molecules of carbon dioxide within the sensitive volume of the counter but do not differentiate among C TM, C 13, and C 14. The isotopic abundance ratio of the carbon-dioxide gas must then be determined HISTORICAL

in the mass spectrometer or in the gas density balance. Since the quantitative count in the gas counters is due only to C 14 and the isotopic abundance ratio yields

C14/(C12 + C 13 "~--C 14) we have ~ ( C 14) and N ( C 14) as required by equation (5). At the present time the reported values of the carbon-14 halfqife range from 5360 years(37) to 6360 years.(3s) Fresh measurements are proposed, using various counting methods as described by W. B. MANN(7) and using 40% abundant carbon-14 (instead of the 4% abundant material used previously) to reduce errors caused by isotope fractionation in the mass spectrometer. In an analogous manner to the carbon-14 half-life determination, JONES~39) has used a disintegration-rate standard of tritium to determine the half-life of tritium. In this case the isotopic abundance ratio measurement can be quite accurate because of the large relative differences in the atomic weights of the hydrogen isotopes. A determination of the half-life of potassium-40 has recently been made by S. D. SUTTLI~ and W. F. LIBBY.~4°) In this case the g a m m a rays from a standardized sample of cobalt-60 were used to determine the absolute g a m m a efficiency of the experimental arrangement, a known amount of cobalt being homogeneously incorporated into the sodium-carbonate sample. A small correction was necessary for the differences in gamma-ray efficiencies of their counters between the 1.17- and 1.33-MeV gamma rays of cobalt-60 and the 1.46-MeV gamma-ray from argon-40.

AND GEOLOGICAL DATING

The constancy of the disintegration constant of any group of nuclei, independent of external physical or chemical environment, forms the basic assumption of all age determinations based on radioactivity decay. From investigations of pleochroic halves in mica it has been shown that the ranges of alpha particles from a number of natural radioactive species have always remained constant. Since it has also been shown, both empirically m) and theoretically(42,~) that

FROM HALF-LIVES

the decay-constant is uniquely related to the energy and therefore to the range of the alpha particles (log ~. = a + b E ) , it follows that the decay-constant has not changed. In particular, it is 1.54 × 10 -1° year -1 for U 238, corresponding to a half-life of 4.49 × 109 years. With this constancy definitely established, the half-life becomes a unit of time measurement. For example, in the naturally occurring uranium there are eight successive

The applicationof standards of radioactivity

229

alpha transformations from uranium-238 to lead-206. Each alpha particle at the end of its range becomes a helium atom, and since 1 cm 3 of helium contains 2.687 × 1019 atoms, it can readily be shown that 1 gram of uranium-238 in secular equilibrium with all daughter products produces 1.16 × 10 .7 cm 3 He per year. Assuming no loss of daughter products, the age "of the rock is given by

dioxide with the atmosphere which is in equilibrium with the C1402 produced in the atmosphere by cosmic rays. When the organism dies, the C140, already present is trapped, and loss of C 14 by decay is no longer compensated for by exchange of carbon dioxide. The ratio of the amount of C 14 per gram present in any dead organic matter to the amount of C a4 per gram present today in living organic matter, together with the knowledge of the half-life Volume of He in cm 3 × 8.6 million years. of carbon-14, permits the determination of Mass of U *3s in grams the "age" of the specimen. The sensitivity Alternatively, the age can be determined of the carbon-14 method has been extended from the Pb2°e/U 238ratio, the pb2°7/U~5 ratio, to 40,000 years by means-trf-targe=volume the pb2°S/Th 232ratio, or the Pb2°n/pb 2°4 ratio. liquid-scintillator counting/47~ where the In each of these cases no loss of daughter pro- carbon as incorporated into the liquid ducts must have occurred between the time of scintillator, and by high-pressure gas-phase formation of the rock and the time of counting ~48) of acetylene. The limitations of available isotopes with measurement. A review of this field and a fairly complete reference list can be found appropriate half-lives and in proper chemical composition leaves a wide gap in the dating in references 44 and 45. Near the other end of the dating scale is scale between 40 thousand and 40 million the carbon- 14 method developed by LIBBY.(46) years ago. In this intermediate range, In this case the time unit is 6,000 years ages have been determined from both instead of 4.49 × 109 years. There is an ionium-230 (T = 8 3 , 0 0 0 years) measureimportant further qualification to the funda- ments and chlorine-36 (T = 4.4 × 105 years) mental assumptions of constancy of decay, measurements, the former for deep-sea however. In the naturally radioactive series sediments and the latter for relatively young the material is incorporated into the rock surface rocks less than 1 million years old. and the quantitative amount of decay is The ionium-230 method makes use of the measured. Carbon-14 on the other hand thorium-uranium ratio, assuming that the is a relatively short-lived isotope produced rate of adsorption of uranium by the sediby cosmic-ray bombardment in the reaction ment has been constant. The chlorine-36 N14(n,p) C 14. Only under the further assump- in surface rocks is built up by the C135(n),)CP 6 tion that the intensity of cosmic rays has reaction due to slow neutrons from cosmicbeen constant throughout the dating period ray stars. Both of these methods suffer is the carbon-14 dating method applicable. from difficulties due to leaching and Living organic matter exchanges carbon erosion. DETERMINATION

OF M E A N E N E R G I E S

When the beta disintegration-rate standardization is combined with a measurement of the total energy-emission rate, one obtains the average energy E defined by

6A

where N(E) dE is the number of beta particles having energies between E and E + dE. The only direct experimental method for the determination of the quantity

m' (E)E dE

= r~mjo.~N(E)E dE

m~N(E)E dE

AND ENERGY PER ION PAIR

(9)

is by calorimetry, as originally used by ELLIS and WOOSTER.~"~ The quantity

230

H. H. Seliger

d e t e r m i n e d using e q u a t i o n (10). T h e r a t e o f e n e r g y emission of a n a p p r o x i m a t e l y 400-millicurie source was m e a s u r e d in a the source. Peltier m i c r o c a l o r i m e t e r (49) a n d the total Generally it is easier to measure the disintegration ionization c u r r e n t f r o m specially p r e p a r e d rate of a source than to measure directly the total sources was m e a s u r e d in a large cylindrical energy-emission rate. Thus for biological applications ionization c h a m b e r . (5°) T h e v a l u e o f W ~ = where one is interested in energy delivered to a 33.71 ~ 0.24 electron volts p e r ion p a i r particular volume, the quantity N/~ is of interest, o b t a i n e d agrees quite well with the v a l u e /~ having been determined from equation (9). In o f 33.8 electron volts p e r ion p a i r r e p o r t e d actual practice, calorimetric methods for determining the rate of energy absorption are quite b y GROSS a n d FAILLA,(51) based on m e a s u r e difficult, and use is made therefore of the ionization m e n t s m a d e in a n e x t r a p o l a t i o n ionization c h a m b e r . C52~ A review of the entire field effects produced by the radiation. o f the m e a s u r e m e n t of W for o t h e r gases as T o relate the ionization m e a s u r e m e n t well as for air a n d also for a l p h a particles in air to the rate of e n e r g y a b s o r p t i o n in air, is given b y W. BINKS.~53) one uses the ratio Wair, the m e a n e n e r g y in Conversely, f r o m the k n o w l e d g e of the electron volts e x p e n d e d in the p r o d u c t i o n m e a n e n e r g y it is sometimes possible to o f a n ion p a i r in air. Since the r o e n t g e n d e t e r m i n e the disintegration rate. T r i t i a t e d o f X - a n d g a m m a - r a y dosage is defined in w a t e r solution standards, for e x a m p l e , were terms of ionization in air, a n d since most p r e p a r e d f r o m a high specific activity source dosimeters, w i t h the exception of the chemi- of H T O whose rate of e n e r g y emission was cal dosimeters, m a k e use ,of this ionizing m e a s u r e d in the Peltier m i c r o c a l o r i m e t e r p r o p e r t y , it has b e e n o f considerable interest (loc. cit.). D i v i d i n g this r a t e b y the m e a n to o b t a i n a c c u r a t e values o f Wait for elec- energy of d e c a y o b t a i n e d b y JENKS et al., (~4) trons. E x p e r i m e n t a l l y one obtains the disintegration rate of the source was found. EmaxN(E)Ed E (10) S t a n d a r d s of r a d i o a c t i v i t y f o r m an essential link b e t w e e n the m e a s u r e m e n t a n d the W a i r ~--- ' Yair i n t e r p r e t a t i o n of physical a n d c h e m i c a l w h e r e Jair is the total ionization c u r r e n t p r o - p h e n o m e n a in b o t h the biological a n d the Emsx physical sciences. T h e i r i m p o r t a n c e will d u c e d b y the source a n d N(E)EdE u n d o u b t e d l y g r o w as the increasing availaJ0 is d e t e r m i n e d with a c a l o r i m e t e r . Wair bility o f r a d i o a c t i v e materials m a k e s their for sulfur-35 b e t a rays has recently b e e n use m o r e w i d e s p r e a d . EmaXN(E) d E is the disintegration r a t e o f

~

REFERENCES 1. RUTHERFORDE. and GEIOER H. An electrical method of counting the number of c¢ particles from radioactive substances. Proc. Roy. Soc. A81, 141 (1908). 2. DEWAR J. The rate of production of helium from radium. Proc. Roy. Soc. A81, 280 (1908). DEWAR J. Long-period determination of the rate of production of helium from radium. Proc. Roy. Soc. ALL3,404 (1910). 3. REGENER W. Uber ZRhlung der a-Teilchen durch die Szintillation und die Gr6sse des electrischen Elementarquantums. Verh. d.D. Phys. Ges. 10, 78, 351 (1908). 4. SODDY W. Attempts to detect the production

5. 6. 7. 8.

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