Accurate measurement of the intensity of 255.1 keV gamma ray in the decay of 113Sn(115.1d)

Accurate measurement of the intensity of 255.1 keV gamma ray in the decay of 113Sn(115.1d)

Appl. Radio?. ht. Vol. 44, No. 4, pp. 731-735, Printed in Great Britain. All rights reserved 1993 Copyright 0883~2889/93 S6.00 + 0.00 0 1993 Pergam...

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Appl. Radio?. ht.

Vol. 44, No. 4, pp. 731-735, Printed in Great Britain. All rights reserved

1993 Copyright

0883~2889/93 S6.00 + 0.00 0 1993 Pergamon Press Ltd

Accurate Measurement of the Intensity of 255.1 keV Gamma Ray in the Decay of 113Sn(115.1d) ANJALI

Saha Institute

MUKHERJEE, SUDEB BHATTACHARYA BINAY DASMAHAPATRA* of Nuclear

Physics,

l/AF

Bidhan

Nagar,

Calcutta

700064,

and

India

(Received 22 July 1992; in revised form 3 September 1992) Using large volume HPGe detectors the intensity of the 255.1 keV y-ray relative to the 391.7 keV y-ray in the decay of “?Sn(llS.ld) has been accurately measured. This value (3.37 _+0.05) is found to be significantly large (x 18%) compared to the one adopted in the literature. On the basis of the present results a revised decay scheme of “‘Sn has been constructed.

II. Experiment

I. Introduction

and Results

1. Source and detector

The decay scheme of “‘Sn(l15ld) is very simple (Lyttkens et al., 1981). The EC decay mainly populates two states of ‘131n: 3/2- 646.8 keV (~1.9%) and l/2- 391.7 keV (298%). These two states are deexcited by the 255.1 keV (646.8 + 391.7 keV) and 391.7 keV (391.7 keV +O) transitions, respectively. A survey of the literature shows that the measurement of the intensity of the 255.1 keV y-ray relative to the 391.7 keV y-ray (taken as 100) varies from 2.85 to 3.33 (Delucchi and Meyer, 1976; Inoue et al., 1973; Fogelberg and Backlin, 1968; Bosch et al., 1967). Although the latest measurement of Delucchi and Meyer (1976) with the lowest error (2.85 + 0.07) has been adopted in Nuclear Data Sheets (Lyttkens et al., 1981) and Table of Isotopes (Lederer and Shirley, 1978), considering the very simple decay scheme of ii3Sn, the large discrepancy (Z 17%) in the intensity value is difficult to understand, particularly in view of the fact that all these measurements were done with high resolution Ge(Li) detectors. To clarify this, we have tried in the present work to measure the intensity of this y-ray as accurately as possible considering all possible sources of uncertainties. On the basis of the results of the present work, the absolute intensity of the y-rays and the EC branching and log ft values for different states of ‘131n in this decay have been determined and a revised decay scheme has been constructed.

‘13Sn in the form of stannous chloride (SNCl,) in HCl solution was obtained from Bhabha Atomic Research Centre, Bombay. The solution was further diluted and several sources were prepared by drying small drops on thin mylar (~25 pm) and covering them with very thin (Z 3.6 pm) x-ray mylar film. Two sources, one being about 7 times stronger than the other, were used. Besides ‘13Sn, the activity contained small amounts of “9mSn(t,,z = 250 d) and ‘21mSn(t1,2 = 55 y) as impurities. However, these two impurities give rise to two low energy y-rays (23.87 and 37.14 keV) and Sn and Sb x-rays and thus their presence does not interfere with the y-rays of interest in the ‘13Sn decay. We used 3 large volume HPGe detectors. The particulars of these detectors are given in Table 1. The measurements were done at large source-todetector distances (10-30 cm) and the count rates were such that the ADC dead time was practically zero. 2. Area of the peaks Figure 1 shows a typical y-ray spectrum in the decay of i13Sn obtained with one of the HPGe detectors. The uncertainty in the peak area results mainly from the statistical error. However, since the estimation of the area is closely related to the choice of background under the peak and if fitted, on the approximation of the analytic function representing the region of fit, these two factors also influence the calculation of uncertainty in the peak area. As

*Author for correspondence. 731

132

ANJALIMUKHERJEE et al.

Table 1. Particulars of the HPGe detectors used in the present work Detector NO I

2 3

Active volume (cm?

Window material

FWHMI

Manufacturer

Diameter/ length (mm)

Princeton Gamma-Tech Princeton Gamma-Tech ORTEC

SO/49 51/42 5X8/42.5

90 80 z90

Be Al Be

1.71 2.06

CkeM

1.90

*At 1332.5 keV of “Co.

the peak-to-background ratio is large (> 10) for both the y-rays, the uncertainty resulting from the statistical error can be made very small (~0.5%). To illustrate the latter two we have shown the analyses of the 255.1 keV y-ray peak in Fig. 2. In this figure, (a) corresponds to the usual way of the area determination with a linear background interpolated from the average of the data on both sides of the 255.1 keV y-ray peak. Figures 2(b) and (c) show the fitted spectra taking the shape of this peak to be gaussian with associated background as linear and quadratic respectively. From the peak area evaluated in the three analyses (as shown in the figure) we find that maximum difference in their value is ~0.3% and it is of the similar magnitude as that of the error in the individual analysis. The uncertainty, arising out of the choice of the background line under the peak, is determined by the regions selected on the two sides of the peak as well as on the form. Since a number of allowable background lines can, in general, be drawn consistent with the data, a certain error must be considered for such arbitrariness in the choice of this background line. To estimate this uncertainty in

the case of 255.1 keV y-ray, we have drawn [Fig. 2(d)] two lines with one standard deviation of counts per channel (a) above and below the average background line of (a). It is evident from this figure that in this particular case any allowable background line is unlikely to be outside the region defined by these two lines. Taking Co (where sum is over the channels defining the peak region) as a measure of the uncertainty due to the shape and position of the background line, the uncertainty in the peak area is zO.7%. As 391.7 keV y-ray line is very intense (z 20 times the area of 255.1 keV y-ray) it has a pronounced low energy tail. The shape of the underlying background, therefore, has a step-like structure in the peak region. However, it is found that the maximum difference in the value of the area is ~0.4% even with a large difference in choice of the background line. It is also found that in the peak fitting procedure, taking the shape of the peak as an asymmetric gaussian, does not make much difference in the estimated area. From the examples shown above it appears that the area of either of the two y-ray peaks from any of

106

105

.

.

. .

ill z =j 104

.

6

103

102 ’ 1400

I

I

I

I

I

I

1500

1600

1700

1800

1900

2000

Channel

number

Fig. 1. Gamma

ray spectrum

in the decay

‘%n(l15.

I 2100

Id) obtained

I

I

I

2200

2300

2400

with a 90 cm3 HPGe

detector.

I 2500

733

Intensity of 255.1 keV gamma ray in “)Sn decay

@I

(a) (

255.1 ’ Area = 231712 f 862

255.1 ! Area = 232530 f 651

255.1 4rea = 232321 f 864

I

I

I

I

I

1500

1600

1640

1500

1600

Channel

I 1640

number

Fig. 2. Analyses of the 255.1 keV y-ray peak for the evaluation of its area. (a) By graphical method taking the peak region defined by the markers 3 and 4 and the background (dashed line) as a linear interpolation of the data, on both sides of the peak, defined by the markers 1, 2 and 5, 6 respectively. (b) By computer fitting method taking the peak as a gaussian and the associated background as a linear function (of channels). (c) The same analysis as (b) but taking the background as a quadratic function (of channels). (d) The same analysis as shown in (a) but with two lines above and below the background line representing the uncertainty (kg) of the background line. See text for details.

these analyses tainty

can be determined

within

an uncer-

< 1%.

3. Eficiency

calibration

of the detectors

For an accurate measurement of the relative intensity of the y-rays, besides the accurate ratio of the

area, one needs an accurate ratio of the efficiency of the detector for the two y-rays as well. The usual method for the efficiency calibration of a Ge detector is to use a number of calibrating sources. The uncertainty in the efficiency results from the uncertainty in the area of the y-rays, their relative intensities and the normalization of the data of one source to the other. Considering the nature of the efficiency curve, however, it is possible to minimize the error in the efficiency ratio for the two y-rays of interest in the present work. A plot of the efficiency of a Ge detector vs the energy of the y-rays in a log-log scale shows that the relation is linear from E, > 200 keV with upper limit (> 1 MeV) depending on the size of the detector (Hajnal and Klusek, 1974). In fact, with the available radioactive sources in our laboratory we have verified that for the three detectors used in the present work the upper limit of linearity is > 1.77 MeV. This straightline with minimum error can be constructed using an ‘08mAg(t,,,= 127 y) source (Dasmahapatra and Mukherjee, 1973). From the decay scheme of ioBmAg(Blachot, 1991), the three

E2 transitions (722.9, 614.4 and 433.9 keV) being in cascade are of equal intensity. Because of the very small internal conversion coefficients (< 1%) of these transitions, the y-ray intensities (100.7, 100.6 and 100) are equal within 1%. As the area of these strong y-rays can be measured with negligible error ( < 1%), the efficiency for a y-ray from the least square fitted straightline obtained with the above y-rays is expected to be accurate with an error z 1%. The reliability of this way of calibrating the detectors for the measurement of the intensity of the 255.1 keV y-ray relative to the 391.7 keV y-ray is checked with the results obtained with 3 pairs of y-rays of widely used calibrating sources (“*Eu, ‘33Ba and ‘Se) having energy close to the above pair. 4. Relative intensity of the y-rays Using the area of the two y-rays as obtained by one type of analyses [for example (a) in Fig. 21 with uncertainty as illustrated in Section II.2 and the efficiency of the detectors as obtained in the previous section (Section 11.3), the results of the determination of the intensity of the 255.1 keV y-ray relative to the 391.7 keV y-ray are summarized in Table 2. It is interesting to note that the ratio of the efficiency for the two y-rays as obtained from the 3 detectors are almost the same (x0.65). One should expect this because of the similar size and volume of the

734

ANJALIMUKHERJEZ et al.

Table 2. Measurement

of the intensity of the 255.1 keV y-ray relative to the 391.7 keV y-ray

Table 3. Comparison of the measured value of the intensity of the 255.1 keV y-ray (relative to 391.7 keV y-ray as 1001 with other works

Detector No 1

2

3

Source

A,,, ,/A,,, I

~3Pl7/%55 1 IL5IP~W 7

strong Weak (Average)$

0.0504 (4) 0.0499 (5) 0.0502 (3)

0.664 (9)

0.0333 (5)

strong Weak (Average).S

0.0539 (4) 0.0536 (5) 0.0538 (3)

0.633 (9)

0.0340 (5)

strong

0.0528 (I I)

0.642 (9)

0.0339 (8)

% Weighted average of the two values A Area. L Efficiency. I Intensity.

tors (Table 1). One should also note that although we can minimize the error in the ratio of the area of the two y-rays by more number of observations, different sources and detectors, the error in the efficiency ratio is difficult to minimize. Though this is small (Z 1.4%) the error in the average value of the intensity of the 255.1 keV y-ray, 3.37 + 0.05 is actually limited by this uncertainty. Nevertheless, as can be seen in Table 3 our measurement has the minimum uncertainty. The value obtained by us agrees very nicely with those of Inoue et al. (1973) and Fogelberg and Backlin (1968). However, it differs appreciably from the latest measurement by Delucchi and Meyer (1976) which has been adopted in Nuclear Data Sheets (Lyttkens

Present work

Delucchi and Meyer (1976)

Inoue er al. (1973)

Fogelberg and Backlin (1971)

Bosch et al. (1967)

3.37 (5)

2.85 (7)

3.33 (13)

3.22 (20)

2.9 (3)

et al., 1981) and Table of Isotopes (Lederer and Shirley, 1978). It may be mentioned that the latter authors used a very strong (30 mCi) source in their work since their primary aim was to observe the very weak 646.8 keV y-ray in this decay. Although it is difficult to figure out the actual reason for this large discrepancy, the use of such a strong source for precision y-ray spectroscopy is usually not recommended. 5. Absolute intensity of the y-rays and the decay scheme With curof 391.7 keV y-ray equal to the theoretical value 0.557 (Lyttkens et al., 1981), the normalization factor for converting the relative intensity of the y-rays to the absolute scale (per 100 disintegrations of the parent) is found to be 0.6423. If on the other hand we use the experimental value 0.540 + 0.007 of ctr from the decay of “3mSn(l .658 h) as given in the same reference (Lyttkens et al., 1981), the above factor comes out as 0.6493 &-0.0030. Taking the relative intensity of the 255.1 keV y-ray as measured in the

115.ld

638.0 (9.6 f 0.6) x 1O-4

255.1

646.8

Fig. 3. Decay scheme of ‘%n(l lS.ld). The intensities are per 100 disintegrations of the parent.

Intensity

of 255.1 keV gamma ray in ‘%n

present work (3.37 + 0.05) and the total internal conversion coefficient of this y-ray equal to 0.046 + 0.006 (Lyttkens et al., 1981) together with the relative intensity of the 638.0 keV y-ray equal to 0.00150 k 0.00009 as measured by Delucchi and Meyer (1976), the EC branchings to the three states of ‘131n, 1029.7, 646.8 and 391.7 keV are found to be 0.00096(6)%, 2.26(4)% and 97.73(4)% respectively, if we use the conversion factor equal to 0.6423. These branchings change to 0.00097(6)%, 2.29(4)% and 97.71(64)% if we use the conversion factor as 0.6493(30). Using Q + = 1039 + and Audi, - 4 keV (Wapstra 1985) and log f values from Gove and Martin (1971), the log fr values for 1029.7 keV(1/2+ ,3/2+), 646.8 keV(3/2-) and 391.7 keV(l/2-) are obtained as >7.5, 8.18 f 0.02 and 7.01 f 0.01 respectively. These values do not depend on the choice of two sets of EC branchings mentioned above. Figure 3 shows the decay scheme of ‘13Sn with %EC feeding being the average of these two sets.

III. Conclusions The intensity of the 255.1 keV y-ray relative to the 391.7 keV y-ray in the decay of “‘Sn has been accurately measured using high resolution and large volume HPGe detectors, carefully prepared sources and efficiency curve of these detectors with minimum uncertainty. The measured value of this intensity (3.37 + 0.05) in the present work is about 18% greater than that of Delucchi and Meyer (1976) but agrees nicely with

AR1 U/&-O

735

decay

earlier works by Inoue et al. (1973) and Fogelberg and Backlin (1968). Because of this new value of the intensity of the 255.1 keV y-ray, the EC branching and the log ft value, for the 646.8 keV state of ‘131n, z 1.7% and 8.5 as given by Delucchi and Meyer (1976) are changed to ~2.3% and 8.18 +0.02 respectively.

References Blachot J. (1991) Nuclear data sheets update for A = 108. Nucl. Data Sheets 62, 803. Bosch H. E., Simon M. C., Szichman E., Gatto L. and Abecasis S. M. (1967) Disintegration of Sn”“. Phys. Rev. 159, 1029. Dasmahapatra B. K. and Mukherjee P. (1973) “*Ag”’ as a calibrating source for the absolute efficiency of Ge(Li) detectors. Nucl. Instr. Methods 107, 611. Delucchi A. A. and Meyer R. A. (1976) Decay of ‘?Sn to levels of “‘In. J. Inorn. Nucl. Chem. 38, 2135. Fogelberg B. and Backlin A. (1968) Precision measurements in the decay of “‘Sn. NP-17722 (LP-23); as quoted by Raman S. and Kim H. J. (1971) Nuclear data sheets for A = 113. Nucl. Data Sheets B5, 181. Gove N. B. and Martin M. J. (1971) Log-f tables for beta decay. Nucl. Data Tables A i0, 265. Hajnal P. and Klusek C. (1974) Semiempirical efficiency equations for Ge(Li) detectors. Nucl. Instr. Methods 122, 559. Inoue H., Yoshizawa Y. and Morii T. (1973) Gamma-ray energies and relative intensities of ‘%e, ‘OarnAg, “%n, 13’1 and “)Ba. J. Phys. Sot. Japan 34, 1437. Lederer C. M. and Shirley V. S. (1978) Table ofIsotoDes, 7th edn. Wiley and Sons, New York. _ _ Lvttkens J.. Nilson K. and Ekstrom L. P. (19811 Nuclear -data she&s for A = 113. Nucl. Data Sheets 33,’ 1. Wapstra A. H. and Audi G. (1985) The 1983 atomic mass evaluation (I). Nucl. Phys. A 432, 1.