Characterization of purified 241Am for common impuritiesby instrumental neutron activation analysis

Arijit Sengupta • V. C. Adya • R. Acharya •

P. K. Mohapatra • V. K. Manchanda

Received: 24 June 2010 / Published online: 20 July 2010

� Akademiai Kiado, Budapest, Hungary 2010

Abstract Americium is an important actinide element

having versatile applications based on its alpha and gamma

emissions. Multi-element determination of radioactive

samples using ICP-AES technique may be affected by the

presence of americium due to its rich emission spectra.

With a view to characterize plutonium based fuels con-

taining americium for trace metals by ICP-AES technique

accurately, a high purity 241Am (using a separation pro-

cedure developed in our laboratory) was prepared. To

ascertain its chemical purity it is essential to determine its

impurity contents accurately. Instrumental neutron activa-

tion analysis (INAA), being a sensitive multi-elemental

technique, was employed to determine the concentrations

of impurities in purified 241Am. Detection limits for the

common elements and rare earth elements have also been

determined. Comparison is made with the analytical data

obtained by the ICP-AES method.

Keywords Americium � Neutron activation analysis �Detection limit � ICP-AES

Introduction

Americium is an important radiation source for a, X-ray

and c-ray spectrometry owing to its long half life (432

years) [1, 2]. 241Am-Be neutron sources are widely used for

prompt gamma ray neutron activation analysis (PGNAA)

work for routine analysis of coal, cement and for oil well

logging operations [3–5]. Am is also used as a smoke

detector and is well known for its industrial gauging

applications [1, 6]. 241Am is formed by the b decay of241Pu which has a half life of 14.4 years. The amount of241Am formed, depends on the storage time of the fuel and

the fraction of 241Pu present in Pu.

In the quality control step of Pu based fuels for the

analysis of trace metal contents by ICP-AES, the presence

of 241Am is likely to interfere leading to the possibility of

inaccurate determinations for analytes due to the emission

rich spectra of Am [7]. For understanding the extent of the

spectral interference of Am on other analyte channels

during ICP-AES analysis, high purity Am is required. Am

was purified from analytical waste generated during

chemical quality control of Pu based fuel samples using a

three-step separation procedure developed in our labora-

tory [8]. However, though close to 99% purity of the

product was expected, it was essential to ascertain the

levels of impurities in the purified Am before its use for

the purpose mentioned above.

Impurity analysis in the purified Am was required to be

done for a number of elements as given in Table 1. ICP-

AES technique was usually used for the impurity analysis.

The elements whose concentrations are affected the most

by the presence of Am by ICP-AES method of analysis are

the rare earth elements. However, high dilution factors led

to poor detection limits (LD) for these elements and they

are often found below the detection limits when analyzed

by the ICP-AES method. Therefore, an alternative analyt-

ical method was required. Neutron activation analysis is a

powerful isotope specific nuclear analytical technique for

simultaneous determination of elemental composition of

major, minor and trace elements in diverse matrices. The

A. Sengupta � V. C. Adya � R. Acharya �P. K. Mohapatra � V. K. Manchanda (&)

Radiochemistry Division, Bhabha Atomic Research Centre,

Trombay, Mumbai 400085, India

P. K. Mohapatra

e-mail: mpatra@barc.gov.in

123

J Radioanal Nucl Chem (2011) 287:281–285

DOI 10.1007/s10967-010-0675-5

high sensitivity is due to irradiation at a high neutron flux

(1011 to 1015 cm-2 s-1) available from the research reac-

tors and measurement of radiations like c-rays from the

sample using high efficiency high resolution high purity

germanium detector (HPGe) coupled to a multi-channel

analyzer (MCA). Neutron activation analysis (NAA)

technique being a sensitive, multi-elemental and non-

destructive analytical technique [9] was used for the anal-

ysis of impurities in the purified Am stock. Due to the

negligible matrix effects in the samples of different origins

it is used for both large and small samples [10].

In the present method, a multi-element standard is co-

irradiated with the sample and the activities from both

sample and standard were measured. Detection limits were

given for the expected impurities in the purified Am.

Comparison of the results obtained with the NAA method

was done with those obtained by the ICP-AES method. To

our knowledge, this is the first report on the impurity

analysis of Am by INAA method.

Experimental

Purification of americium

Americium was purified from waste sample as reported in a

previous publication [8]. Separation of bulk uranium was

accomplished by a solvent extraction method using 30%

TBP in n-dodecane [11]. Impurities like Fe and Na were

separated by solvent extraction using 0.1 M TODGA

(N,N,N0,N0-tetraoctyldiglycolamide) in n-dodecane which

also contained 0.5 M DHOA (di-hexyl octanamide). The

co-extracted Ca was separated subsequently using an

extraction chromatography column containing CMPO

(octylphenyl-N,N-diisobutyl carbamoylmethyl phosphine

oxide) and TBP as the extractants.

Sample preparation and neutron activation analysis

The purified Am solution was dried on a filter paper and

was doubly sealed in PVC pouches in such a way that it

was kept inactive from outside. Standard solutions for

rare earth elements and other common metallic elements

were prepared from spec-pure readily available solutions

(E-Merck) by proper dilutions with 0.5 M HNO3. Supra-

pure HNO3 and quartz double distilled water were used for

making 0.5 M HNO3. Various aliquots of standards in the

range of 10–100 lg were dried on filter papers and were

doubly sealed in PVC pouches. Samples and standards

were sealed together and were tested for leak tightness.

These samples were irradiated in APSARA reactor,

Trombay for the determination of common impurities and

for rare earth elements. Additionally, for the determination

of Mn and rare earth elements (Eu, Sm, Gd, Dy) which

are forming short-lived isotopes, another set of experi-

ment was carried out at DHRUVA reactor, Trombay for

1 min using pneumatic carrier facility (PCF) in a flux of

5 9 1013 n/cm2/s. The geometry with respect to size and

shape of the sample, standards and blank were kept alike.

After 24 h cooling of samples and standard for APSARA

Table 1 Impurity analysis of

purified Am by NAA

Note: BDL means below the

detection limit (detection limits

are given in Table 2)

Element Target

isotope

Formed isotope t1/2 Ec (keV) c-Abundance Amount (lg)

in 17 lg of Am

Fe 58Fe 59Fe 44.5 days 1099.3 56.5 BDL

Ca 46Ca 47Ca 4.536 days 1297 74.9 BDL

Na 23Na 24Na 10.96 h 1368.8 100 BDL

Eu 151Eu 152Eu 13.54 years 344.3 26.6 0.026 ± 0.001152mEu 9.3 h 841.6 14.5

963.4 11.9

Sm 152Sm 153Sm 46.5 h 70 5.25 BDL

Gd 158Gd 159Gd 240.4 days 97.4 0.73 0.21 ± 0.02

Dy 164Dy 165Dy 2.334 h 94.7 3.58 0.012 ± 0.001

Cr 50Cr 51Cr 27.7 days 320.1 9.83 BDL

Ag 109Ag 110mAg 249.8 days 884.7 72.9 BDL

Co 59Co 60Co 5.271 years 1332 100 BDL

Mn 55Mn 56Mn 2.579 h 846.8 98.9 0.030 ± 0.001

1810.7 27.2

Ni 64Ni 65Ni 2.517 h 366.3 4.61 BDL

Cd 114Cd 115Cd 53.46 h 527.9 27.5 BDL

Zn 64Zn 65Zn 244.3 days 1115.5 50.7 BDL

Cu 63Cu 64Cu 12.7 h 1345.8 0.48 BDL

282 A. Sengupta et al.

123

irradiation and 2 h for DHRUVA irradiation, the irradiated

samples and standards were mounted on PVC plates and

were counted for activity contents using HPGe detector

connected to multi-channel analyzer.

ICP-AES analysis

A JY ULTIMA high resolution ICP emission spectrometer

with radial viewing configuration was used in the present

studies. Suprapur grade nitric acid (Merck, Germany) was

used throughout this work and spec pure chemicals were

used for making the standard solutions for ICP-AES

analysis. The analysis results are given with a maximum

error limit of 10%.

Results and discussion

Using the mass of the element in standard (mx,std) and count

rates of the standard (cpsx,std) and sample (cpsx,sample), the

mass of the element in sample (mx,sample) was determined by

the following equation.

mx;sample ¼ mx;std � ðcpsx;sample=cpsx;stdÞ � ðDstd=DsampleÞ ð1Þ

where Dstd and Dsample represent respective decay factors

(e-kt) for the standard and the sample. The analytical results

obtained during the impurity analysis of purified Am matrix

are given in Table 1. From the experiment, it was found that

for the common impurities, which are having stringent

specification limits for the Pu based fuel (e.g. Ca, Fe, Na, Cr,

0 500 1000 1500

0.0

2.0x106

4.0x106

6.0x106

8.0x106

1.0x107

1.2x107

1.4x107

Cou

nts

Energy (Kev)

Fig. 1 Gamma ray spectrum of the purified Am before irradiation

0 500 1000 1500-2.0x105

0.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

1.4x106

1.6x106

242Am

241Am

Co

un

ts

Energy (KeV)

Fig. 2 Gamma ray spectrum of the purified Am after neutron

irradiation without magnification

1000500

0

500

1000

1500

2000

2500

3000

3500

4000

152mEu(963.4)

56Mn(846.8)152mEu(841.6)152Eu(344.3)

Co

un

ts

Energy (KeV)

Impurity gamma ray peakes in the Am matrix

Fig. 3 Magnified gamma ray spectrum of the purified Am indicating

the neutron activated impurities

500 1000 1500 2000

0

200

400

600

800

1000

1200

1400

1600C

ou

nts

Energy(KeV)

Blank, 0.01 M HNO3

Fig. 4 Gamma ray spectrum of blank (pH-2) after neutron irradiation

under identical condition as the sample

Characterization of purified 241Am 283

123

Ag, Al, Cd, Zn, Cu), the values were below the detection

limits. The c-spectra of the purified americium before and

after irradiation are shown in Figs. 1 and 2, respectively

while the impurities in Am are shown in Fig. 3 after mag-

nifying the spectra. Since the purified Am sample was made

in 0.01 M HNO3 solution, the c-spectrum of the blank

0.01 M HNO3 was also recorded in an identical manner after

same irradiation time as for the sample and standards

(Fig. 4). During the quantitative analysis of the elements, all

the values obtained are background corrected. Appropriate

expansion of the c-spectrum after irradiation suggested that

Eu, Gd, Mn, Dy were present as impurities in the purified

americium solution in the concentration range of

0.01–0.2 lg in the irradiated Am sample (17 lg).

Calculation of the detection limits

Detection limits (LD) of the elements (in micrograms) were

calculated using sample background under characteristic

peaks using the formula,

LD lgð Þ ¼ 3ffiffiffiffiffiffi

Cb

p

=S � LT ð2Þ

where Cb represents the background counts, LT is live time,

S is the sensitivity (defined as cps/lg) and cps is the counts

per second of the standard. In Am matrix, higher detection

limit values were obtained (Table 2) which was attributed to

the high specific activity of Am. Large amount of Am may

lead to the significant contribution in the c spectrum of

purified Am due to the formation of appreciable amount of

fission products restricting the sample size.

Interference due to 242Am

A peak at 103.8 keV for 153Sm was interfered by 242Am,

which is an activation product of 241Am. After appropriate

expansion of the irradiated c spectrum of Am, it was found

that there were four peaks at the energies 99.6, 103.8, 116.9

and 120.6 keV respectively with the approximate relative

intensity ratios *4:6:2:1 (Fig. 5). This observation is in

good agreement with the literature [12] (Table 3) which

Table 2 Determination of

detection limits and sensitivity

of the elements in purified Am

sample

Element Irradiation time

(ti in min)

Neutron flux

(n/cm2/s)

LT (s) Sensitivity

(cps/lg)

Detection

limit (lg)

Eu 1 1013 1000 268 0.001

Gd 1 1013 1000 0.37 0.65

Sm 1 1013 1000 2.18 0.07

Dy 1 1013 1000 619 0.003

Mn 1 1013 1000 200 0.008

Na 420 1011 1500 0.53 0.12

Fe 420 1011 50800 0.003 0.68

Ca 420 1011 50800 0.005 0.76

Ag 420 1011 1800 0.024 2.38

Co 420 1011 1800 0.085 0.48

Ga 420 1011 1500 0.087 0.12

La 420 1011 50800 0.028 0.37

K 420 1011 1500 0.008 7.35

Cd 420 1011 1800 0.024 2.38

Zn 420 1011 1800 0.004 13.26

Cr 420 1011 1800 0.080 1.90

In 420 1011 1800 0.015 6.35

Cu 420 1011 1500 0.006 3.99

0 50 100 150 200 250-2.0x105

0.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

1.4x106

1.6x106

242Am

241Am

Co

un

ts

Energy (KeV)

Fig. 5 Activation of 241Am to 242Am as indicated by the gamma ray

peaks

284 A. Sengupta et al.

123

suggests that, the formation of activation product of 241Am

(i.e. 242Am) is identified by the four X-rays of the daughter

whose relative intensity ratios are as mentioned above

(Fig. 5). The evidence that there was no increase in the

intensity ratios for 153Sm indicates the absence of 153Sm.

Therefore, the 70 keV c ray line of 153Sm, which is at a

lower energy range i.e., at higher background, was chosen

for the determination of Sm. It was found that the value for

Sm in purified Am matrix was below the level of detection.

Comparison of results from NAA and ICP-AES

The purified Am was diluted to prepare a 100 lg/mL stock

for the ICP-AES analysis. The results (%) are indicated in

Table 4 along with those obtained from the NAA method.

The rare earth elements other than Gd were not detected by

the ICP-AES method. On the other hand, elements such as

Cu, Zn and Mg were detected by the ICP-AES method

while they were not detected by NAA method. The purity

of the Am stock was found to be 98.067% from the ICP-

AES method while it was 98.399% by the NAA method.

Conclusions

From the neutron activation analysis results of the purified

americium matrix, it was inferred that it contained 1.2% of

Gd, 0.15% of Eu, 0.07% of Dy and 0.18% of Mn as

impurities. These results indicated that the purity of

americium was [98%. Due to the presence of americium,

there is an increase in background leading to the higher

detection limits for the common elements and the rare earth

elements. The purified americium spectra was complicated

due to the formation of 242Am, the activation product of241Am.

References

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Table 3 Nuclear data for242Am-the activation product of241Am

Activation

product

t1/2 Target

isotope

Nuclear reaction

responsible for formation

Energy Relative

intensity [12]

242Am 152 years 241Am 241Am (n, c) 242Am 99.6 keV, X, D 3.60

103.8 keV, X, D 5.77

116.9 keV, X, D 2.21

120.6 keV, X, D 0.72

Table 4 Comparison of the analytical results obtained by NAA with

ICP-AES

Element NAA (%) ICP-AES (%)

Fe BDL BDL

Ca BDL BDL

Na BDL BDL

Eu 0.155 ± 0.003 BDL

Gd 1.2 ± 0.2 1.08 ± 0.07

Sm BDL BDL

Dy 0.070 ± 0.003 BDL

Cr BDL BDL

Ag BDL BDL

Al BDL BDL

Co BDL BDL

Mn 0.176 ± 0.006 BDL

Mg BDL 0.38 ± 0.02

Ni BDL BDL

Cd BDL BDL

Zn BDL 0.38 ± 0.04

Cu BDL 0.103 ± 0.003

Characterization of purified 241Am 285

123