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71

CALIBRATION OF FIVE

GAMMA-EMITTING NUCLIDES

FOR EMISSION RATE

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS

technical ^lote

71

AUGUST 1960

CALIBRATION OF FIVE

GAMMA-EMITTING NUCLIDES

FOR EMISSION RATE

J. M. R. Hutchinson

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Price 75 cents
ABSTRACT

Meroury-203 and niobium-95 were calibrated by a ^-nS-y coincidence method for y-emission rate, zinc-65 by comparison v/ith the 1.12

Mev peak of scandium-A-6, sodium-22 by a Y-annihilation-cuanta coincidence method and by a triple coincidence method, and strontium-85 by

x-y coincidence counting. The accuracy of the calibration in all cases

was -2 i°. The half-life of the isomeric state of rubidium-85 was

measured and found to be .98 microseconds.
Calibration of Five y -Emitting Nuclides for Emission Rate

J. M. R. Hutchinson

Introduction

In order to supply a need for point source 7-ray standards for

calibration purposes in nuclear physics, five 7 -emitting nuclides have

been calibrated by the coincidence method for 7-emission rate. The

nuclides are listed below -with some of their properties.

Table I

Nuclide Emitted rays 7 Energy (mev) Half -life

Mercury-203 7,$~ 0.2V? k'Jd.

Strontium-85. y, x (k capture) 0.513 6Ud

Niobium-95 y, P~ O.765 35d

Zinc -65 7, p+

, x 1.119 245d

Sodium-22 y, p+ 1.28 2.8y

The nuclides will be provided in solid, point source (diameter of

deposit < 1 cm) standard samples of 7-emission rate. Ideally one

desires nuclides emitting a single 7-ray, whose energies are well known

and evenly spaced with respect to energy from 0.1 Mev to 2.0 Mev and whose

half lives are long enough to make them useful as accurate standards.

for a period of years. In the kit initially produced the range of
energies used is indicated in Table I. The activity of each standard

h i is approximately 5 x 10 7's/sec. The estimated accuracy of the

standardization for 7-emission rate is ± 2$.

This article will be divided into three parts devoted to

(i) methods of source preparation, (ii) a description of the calibration of the standards, (iii) various checks on the 7-emission rate of

the standards.

SOURCE PREPARATION

Mounting

It is desirable that in a small solid-angle, at least, the

7 radiation emitted by the source should be unscattered. Also the

mounting has to be firm in order to allow a fixed geometry intercomparison with other sources of the same nuclide. Toughness and

resistance to temperature changes are also desirable qualities. All

these requirements were satisfied by covering an annulus of aluminum

2" o.d. 1 l/2" i.d. with thin adhesive polyester tape under tension.

The source was deposited on the adhesive side of the tape, approximately in the center and covered with another layer of similar

polyester tape.

Source material was received in highly concentrated form and

diluted to a strength of approximately 5 x 10 7's/sec per 50 mg. of

water solution. 5 ml* ampoules containing accurately weighed" amounts

of the diluted solution were prepared and sealed. The ampoules were
then measured in the N.B.S. ionization chamber for ion current per

gram of solution relative to a fixed radium source (See Emission Rate

Checks). The ampoules were then broken open and a number of sources

of approximately 50 mg were weighed onto the source mounts using a

picnometer and a balance accurate to 0.1 mg. The latter operation

was performed quickly to prevent much evaporation during the weighing.

The accuracy of these weighings in the worst case as checked by

relative counting rates in a fixed geometry was found to be ± 0.6$*

Since each source was checked for counting rate relative to a fixed

standard, the error in intercomparing standard samples is less than

± 0.4$. The sources" were allowed to dry, after which tape was placed

over them securing them in a sandwich.

CALIBRATION

Mercury-203 (4jtB-7 coincidence counting)

The energy of the conversion electron near the high energy limit

of the continuous 8 spectrum caused difficulty in monitoring 3's in

the continuous spectrum without counting the conversion electrons.

Moreover the total conversion is too large (22.6$) to be corrected

for accurately in any easy manner in low efficiency counters. The

obvious solution is to use a 4jt3 counter which has high enough

efficiency so that most of the time a beta is counted in, say a

coincidence apparatus, a conversion electron is also counted, the

two being recorded as one pulse in the B channel. For 100$
P efficiency the counts in the p channel equal the p emission rate

alone without any contribution from the accompanying conversion

electrons.

In view of these considerations a kitfi-y coincidence measurement

203

")f Eg was made. Since the coincidence analyser could not accept

counting rates of 10 /sec an accurate further dilution of the. source

material was made and deposited by picnometer onto thin collodion

source holders. Several sources were counted for B's, y

l s and

coincidences. Approximate disintegration rates and efficiencies were

determined after correcting for background in all channels. The

observed {3 efficiency was approximately 9h^o. A final disintegration

rate was determined after correcting for (i) accidentals (ii) dead

time losses (iii) internal-' conversion. The accidental rate was given

by N_ NT(2-€ -€ ). The dead time loss rate for both channels was

small.

A multiplicative factor correcting for internal conversion is

1 + —P . ce w = ft channel rate

€p(l-KX) P

N = 7 channel rate

Thus the final disintegra- ^ tion rate is given by: € = p efficiency

(N

g

-B

p

)(N

y

-B

7

) \

(N -B -A) a(l-Oe e = 7 efficiency % c c , p ce 7

N

1 +

p a = total conversion coefficient

6 = conversion electron efficiency

ce T = resolving time ( l.V? x 10" sec)
and inserting some typical numbers

(all in per second)

N = coincidence rate

c

N = disintegration rate

(356. 87-0. 90)(H- 879-^109) 1 „ a v * N

o

= S (70227-0.1160-0.00639) ^0113 B

P

= p backeround

B =7 background

€ was taken as unity _ . , _ . ce * B = coincidence background

A = accidental rate

95 A method similar to the above was used to calibrate Wb and to

22 check the disintegration rate of Na described in later sections of

this article. A total conversion coefficient of 22.57$ based on the

2/ numbers of Nijgh and Wapstra-' was used.

Strontium-85 (x-7 coincidence count)

The long half -life (10~ sec) of the isomeric state of Kb ^

required special consideration when making an x-7 coincidence count

85

on the decay of Sr . In order that all the coincidences be recorded,

a resolving time long compared to the half life of the isomeric state

was necessary. It was also important to make the resolving time small

enough to minimize the error due to variation in accidental rate. The

half-life was determined and coincidence counts were made at two

resolving times.

85 Half-life determination of the 0.513 Mev state of Kb ^

The schematic experimental arrangement used in the half -life

determination is shown in Figure IV. A 1-mm thick Nal crystal with
an electron-multiplier phototube was used to detect the x-rays and a

2" x 2" Nal crystal with phototube was used to detect the 0.513-Mev

7 radiation. The y ray was received by the 7 channel some time after

the x-ray had been detected in the x-ray channel. To measure the

half -life of the isomeric state a plot on semi -log paper was made of

the number of real coincidences divided by the product of the two

singles rates versus delay in the x channel for a resolving time short

compared to the half life. A straight line resulted and the half -life

was read from the graph. The resolving time was 0.2 ^s. The

accidentals were found by increasing the delay to 20 ^s. The variable

delay was calibrated with a crystal oscillator. Six values of the

85

half life of the isomeric state of Kb were obtained - O.96, O.96,

0.97> 1.00, O.98, 1.00 giving an average corrected value of

T

xy2

= 0.98 ± %

The major source of error in this measurement is due to inaccurate

calibration of the time delay.

85 Calibration of disintegration rate of Sr '

The disintegration rate of a Sr standard sample was measured

by coincidence counting with the equipment of Figure IV using resolving

times of 2.57 us and 8.5 us in the coincidence analyser. Knowing the

half life of the isomeric state of Kb and the resolving time of the

coincidence circuit, the fraction of the total real coincidences which

were lost due to the delay of the 7 in decaying from the isomeric state
were calculated and a correction in the disintegration rate was made

for them. To obtain the 8.5 ^s resolving time a gate 7*3 \xs long was

placed in the 7 channel and a pulse of 1.2 jis shaped by a shorted delay

line was introduced into the x channel. The time relationship between

the two channels was found by placing an annihilating positron source

between the detectors and adjusting the delay in the x channel to where

coincidences just disappeared, i.e., where the total delay in the

x channel was 7.3 |j.s longer than the delay in the 7 channel. The

channels were then adjusted to overlap by approximately 0.6 [is (i.e.,

the delay in the x channel was reduced by 0.6 pis) to remove any loss

of coincidences due to differing rise times of the pulses in various

parts of the circuit. Under these circumstances 7 rays which were

emitted within 7.3 + 1.2 - 0.6 = 7.9 us after the associated x-rays

were emitted could produce a coincidence. The coincidence equation

for this experimental arrangement is then: . 1 v N N

N =(l - 7-9 ) -I21 ° V

2

O^B ' Nc

where N = singles counts in the x channel, N = singles counts in the x 7

7' channel, N = coincidences.

c 85 A coincidence count of Sr was also made with resolving times

. , , , L , , , -,. „ 2.57 us in both channels. The pulses shaped by a shorted delay line of ^ *

were made to overlap by 1.034 p.s. The disintegration rate for this

setup was then calculated with the following formula

N N

Nf =(l[1 - 4.11 )) -%?!- where 4.11 = 2.57 - 1.034 + 2.57*

V n o^S /

N

c
85 The disintegration rate of the Sr master solution obtained

using these two resolving times agreed with each other within 1$ and

"both fell within 1$ of the predicted N /gm of the master solution

using the ionization chamber curve.

Zinc -65 (comparison with the 1.12 mev scandium-46 peak)

Zinc was calibrated for 7 emission rate by comparing the 1.112 Mev

65

" V

peak of Zn as seen in a 2" x 2" crystal with the upper 1.119 Mev peak

46 46

of a calibrated Sc source. The disintegration scheme of Sc is

similar to that of Co

46 A Sc source was calibrated by y-y and $-y coincidence counting.

46

The 7-7 coincidence method used in calibrating Sc followed that of

Hayward, Hoppes and Mann in their calibration of Co .» Briefly,

their method was to set their crystals at different angles with respect

to the source and monitor both channel's singles rates and the coincidence rate. Corrections for the angular correlations of the cascade

7*s and the finite size of the crystals can be computed using theoretical values given by Rose.-' The fact that the crystals cannot be made

to monitor the 7-'s separately can be taken into account by adjusting

the discriminator settings on the amplifier outputs to give equal

efficiencies and by introducing a factor. of l/2 into the coincidence

equations.-^ For two, two incL crystals each at a distance of four

N N jt

* inches from the source, the theoretical^ ratio 1 2 at it and — for a N

c 4-2-0 transition is 1.155. The experimental ratio was 1.155 ± 0.010.

6
A P-7 measurement on the same source was also made and the value

for the disintegration rate fell within 0.5$ of that obtained by y-y

coincidence counting. The estimated accuracy of the measurement of the

kG

Sc source was ± 0.5$.

A comparison of the Sc and Zn sources was made by comparing

kG the counting rate in the 1.12 Sc peak as "seen" through a 5v window

65 with the counting rate of the Zn y peak as seen through the same

window. Measurements were made at different settings to check the

effect on the ratio of y emission rates of the slightly differing

kG energies of the two y

r s. Comparison in this manner of a Sc source

and a Zn source provided the same ratio of Sc to Zn for differential discriminator settings varying over a range of 0.03 Mev - i.e.,

a range known to" be greater than the difference in energy between the

upper Sc peak and the Zn peak.

Sodium-22 (triple coincidence and 7 -annihilation quanta coincidence)

Sodium-22 decays 90$ by positron emission and 10$ by electron

22 capture to the 1.28 Mev excited state of Ne . This state then decays

promptly to the ground state. The coincidence experiment was as follows.

A source was encased in a lead sheath to produce complete annihilation of the positrons within a small volume. The annihilation quanta

were received in coincidence in a and a which were l80° apart with

respect to the source (see Figure VHI^nd fed through amplifiers into

coincidence analyser C . The coincidence pulses out of C represented

a a

a sampling of positrons. These pulses were fed into coincidence
analyser CQ . Pulses into the 7 channel were amplified (A ) and

P7 7

discriminated against annihilation quanta (d) and also fed into C

P7

Thus at C . one had the arrangement for an oridnary $-y coincidence

experiment. f3, y, and coincidence rates were taken. Accidental and

"background rates were also taken. Turning to the p channel again,

part of the p counts resulted from coincidences between the 1.28 Mev y

and annihilation photons. This so-called y background in the "pn

channel and other effects such as accidentals in the "p" channel etc.

were corrected for by subtracting the counts in SQ when a was rotated

P 1

away from the l80° position with respect to the source from the "p"

singles rate. The advantage of this method lies in the fact that the

"7 background in the p channel" can be subtracted directly without

making an independent estimate of the 1.28 Mev 7 efficiency in the

0.511 Mev region.

7 - Annihilation 7 Coincidence Count

A coincidence count was then taken between the 1.28 Mev 7 and the

annihilation 7's. The experimental arrangement consisted of an

ordinary two channel coincidence analyser with two, 2" x 2" crystals

facing each other but which had the freedom to rotate with respect to

the line connecti-ng the source and other crystal. One channel was fed

through a window which contained the annihilation peak. The positrons

were again completely annihilated close to the source by a lea*d sandwich. The coincidence equations are in general:

10
y n ' = sum coincidence in 7 channel

N = N € + n

7 7

7 07 (37 Nomenclature.

, R n

R

= sum coincide1106 i n P channel

NQ = N

€

Q + n + n^ P7

P op 7 P7

t

, n = 1.28 Mev comptons in p channel

c o p 7 P7 P

£ = probability that a p will be

detected through p window if

the p strikes the crystal

Hence if the two crystals oppose each other with the source in the

center we get:

N N 3

Q

(N-€ + n + n£ )(Ne + n£ ) 7 P =

v oy 7 P7'

v oP frr

N„

" Ne Q<£ + n /d c o p 7 P7 p

If the crystals are not aligned with the source n' € ' =0

P7 p

since the annihilation quanta are emitted in opposite directions. From

22 •

' a detailed analysis of the Na spectrum £ was estimated at 50$.

n was measured by counting the Compton recoils from a calibrated Co

f>C> source through the p window and by assuming that the two Co 7's with

an average energy of 1.25 Mev would produce twice as many Compton

22 recoils per disintegration in the region of O.5H Mev as one Na 7

with an energy of 1.28 Mev.

n„ € ( 0.5 n„ ) was estimated from the difference between

p7 p

^ ' p7

'

N_N

p y

r.

' when the crystals and the source were and were not aligned.

c NQN

8 y Some typical numbers will help illustrate the calculation. il ' c

11
at 180 ° was found to be 20$ smaller than at 90°. Thus N ' £ „ -wao

approximately 20$ of the true coincidence rate. N was about 1

count/sec, hence n '€ (0-5 n_ ) = (20$) 1 count/sec = 0.2 counts/sec.

Hence n„ • = O.k counts/sec. Now N was about 70 counts/sec hence the

$7

' 7

' summing correction in the 7 singles channel was approximately

P

/ yj v = 0.6$. With regard to the "£" channel, it was evident that

the summing should be less than in the 7 channel because the £ channel

was "seen" through a window and the 7 channel was not. Analysis of the

spectrum showed this estimate to be correct —in fact the summing

in the B channel was calculated to be approximately 0.1$ of the

annihilation 7 counting rate.

N -was counted at an angle of l80°. After the above corrections

o were made and the background and accidental coincidences were sub22

tracted, a disintegration rate of the Na agreeing within 1$ of the

triple coincidence measurement was counted.

22

Two Other Checks on the Calibration of Na

Calibrated sources of Sr and Co were used in the right

22 proportions to reproduce the spectrum of Na in the region of the

annihilation peak, the Co reproducing the Compton-recoil background.

Counting through a 6v window (this just took in the annihilation peak)

an intercheck on the calibration of three sources was made since the

Co gave degraded quanta which were equivalent to the 1.28 Mev Na

85

quanta in that energy region and the Sr produced /'s which were of

the same energy (within 0.5$) as the annihilation quanta. The following

12
formula was used to intercompare the sources;

N N N

jS-- 2(0.898).^ + f-, os or oc where N , N , N are the counting rates of Na , Sr and Co sources

s r c respectively and N , N , N are the corresponding disintegration

rates

In this equation the fact that two annihilation quanta appear for

22

every positron in a Na disintegration and the fact that for each

60

decay of Co there are two /'s emitted were taken into account.

0.898-^ is the positron branching fraction from a decaying Na nucleus,

Using an average value of disintegration rate of both double and triple

22 coincidence counting for Na , the left hand side of the equation

agreed with the right hand side to within 1.5$.

A second intercomparison was provided by the fact that the peak

60 22 efficiencies of the upper peak of Co , the 1.28 Mev peak of Na and

the peak of Zn fell on a straight line on a log-log-graph in agreement with the curves of Lazar, Davis and Bell. sJ

EMISSION RATE CHECKS

In addition to the source calibration two checks on the accuracy

of calibration of the disintegration rate of the standard samples' were

made, these were namely (i) the final weighed sources were intercompare

for counting rate thus checking the weighed amount of the master

solution, and (ii) a comparison with a group of nuclides calibrated by

13
hx$-y coincidence counting which included among others Na , Nb , Hg but not Sr , Zn was made using the ion chamber as a means

of comparison. A smooth curve was drawn through the points given by

these 4jrfJ-7 calibrated nuclides on a graph of > versus energy.

t j -it j> ion current , , . , _ _ 85 , „ 65 Interpolated values of 7 were obtained for Sr and Zn , gm 7/ sec ' and a comparison of these nuclides was made. It should be emphasized

85 65 that in the cases of Sr ' and Zn y the quoted values for the 7 emission

rates of the samples were those obtained directly "by other methods, the

ion chamber measurements served only to check the limits to within 2 or

yjo. As it turned out, calibration of these two nuclides agreed to

within ± 1$ of the calibration of the same two nuclides using inter-

, , n ., ion current _ ... polated values on the -> versus energy curve. The dismtegra22 tion rate of the Na master solution obtained in' other ways agreed

with that obtained by 4jt(3~7 coincidence measurement.

In conclusion the author gratefully acknowledges continued

interest in the work by R.W. Hayward, numerous helpful discussions

with S. Garfinkel and D. D. -Hoppes, and aid with the dilutions by

R. Medlock.

14
BIBLIOGRAPHY

1. Campion, P.J., J. A. R. I. k, 232 (1959).

2. Nijgh, G.J., Wapstra, A.H. Ornstein, L. Th.K. , Salomonsgrobben, II. and Huizenga, J. R. Nuclear Physics £, 528

(1958-1959)

3. Singh, P.P., Dosso, H.W. and Griffiths, G.M. U. B. C. Can.

J. Physics H, 1055-8 (1959).

W. Strorainger, D. , Hollander, J.M. and Seaborg, G. T. R. M. P. 10,

636 (1958).

5. Hayward, R. W. , Hoppes, D.D. and Mann, W. B. Journal of Research

of the National Bureau of Standards ^f, No. 1 (1955).

6. Rose, M.E. P. R. £1, 610 (1953).

7. Putman, J. L. Measurement of disintegration rate, Beta- and

Gamma-Ray Spectroscopy (Editor: K. Siegbahn; North Holland

Publishing Co.) Chapter 26, p. 8*H (1955).

8. Ramaswamy, M.K. (Johns Hoskins University, Baltimore) India

J. Phys. 11, 285-294 (1959).

9. Lazar, N.H. , Davis, R. C. and Bell, P.R. Nucleonics 14, 52 (1956).

10. Strominger, Hollander, and Seaborg, Op_. Cit .

, p. 609.

11. Ibid. , p. 659.

12. Ibid. , p. 667.

13. Slack, L„. and Wey, K. , Radiations from Radioactive Atoms in

Frequent Use, pr. 27 (February 1959)

14. Strominger, Hollander, and Seaborg, 0p_. Cit . , p. 609.

15
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CD

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DELAY TIME IN MICROSECONDS

4.0 4.5 5.0

Figure V. Decay curve of the isomeric state of 3ubid.iu.-n-8 5
rov

SO

^Jo

CO

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DETECTORS AND

PRE-AMPLIFIERS

AMPLIFIERS

DISCRIMINATOR TO ACCEPT

PHOTONS ABOVE .5 MEV

COINCIDENCE

ANALYZERS

SCALERS

Figure VIII. Schematic diagram of the triple coincidence apparatus for

the determination of the absolute disintegration rate of oodium-22

sources.

DEPARTMENT Oh COMMERCE

Frederick H. Mueller, Secn-lurv

, \TIONAI. BURKAU OF STANDARDS

A. V. \siin. Director THE NATIONAL BUREAU OE STANDARDS

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Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Research. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Propagation Obstacles Engineering. Radio-Meteorology, l^ower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High Frequency Impedance Standards. Electronic Calibration Center. Microwave Physics. Microwave Circuit Standards.

Radio Communication and Systems. Low Frequency and Very Low Frequency Research. High Fre- quency and Very High Frequency Research. Ultra High Frequency andSuper High Frequency Research. Modulation Rf search. Antenna Research. Navigation Systems. Systems Analysis. Field Operations.
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