Interference corrections for some spectral peaks used in gamma spectrometric analysis of 238U and 226Ra in presence of thorium in samples

INTERFERENCE CORRECTIONS FOR

SPECTRAL PEAKS USED IN

- RAY

SPECTROMETRIC ANALYSIS OF

238

U

AND

226

Ra IN PRESENCE OF THORIUM

H. Yücel1, E. Köse2, A.N. Solmaz2, D. Bor2 1Turkish Atomic Energy Authority

The aim of this study is

 To quantify the contributions of spectral interferences

to the commonly used analytical peaks at 63.3 (234Th)

and 1001 keV (234m_{Pa) due to emissions from} 235_U, 238_U, 232Th and their decay products for the 238U activity

determination.

To quantify the spectral contributions to the 186 keV (226_{Ra) for the} 226 Ra activity determination by the -ray

emissions from 235U and 232Th in the presence of both

natural U and natural Th in the samples.

The first one is to calculate the percentage errors in the 238U and 226Ra activity determinations in samples

such as NORMs in case of ignoring the contributions of mainly 232_{Th and} 235_{U to the analytical peaks.}

The second one is to calculate the error (%) in the resulted effective dose due to these measured

activities in samples such as NORMs to meet the dose criteria (e.g., given in EC No.112 radiation Protection, 1999)

 The γ-Ray Spectrometer with a well-type HPGe detector

• P-type closed-ended coaxial

• Relative efficiency: % 44.8

• Energy resolution: 2 keV at 1332.5 keV of ₆₀Co

• Standard NIM electronics (Amplifier, Multiport ADC/MCA,

etc.)

• Genie 2000 gamma software

 Detector Shielding

• 10 cm Pb in 0.9 cm st. St. housing

• 1 mm Sn liner

• 1.6 mm Cu liner

Multinuclide Standard Source

(Isotope products Lab,traceable to PTB)

Energy range: 47 keV to 1836 keV

• 210_Pb, 109_Cd, 57_Co, 123m_Te, 51_{Cr ,} 113_Sn, 85_Sr, 137_{Cs ,} 88_Y, 60_Co

CRMs (purchased from CANMET,Canada) are

used for the method validation.

Material description Code

Concentration (%)

Certified activity values for radionuclides _{(Bq kg}-1)

Thorium

amount Uranium amount 232_Th 238_U 235_U 226_Ra

Rare Earth-Thorium ore OKA-2 2.893±0.058 0.02186±0.0008 117381±123 2700±101 128±5 Not reported

Uranium-Thorium ore DL-1a 0.0076±0.0004 0.0116±0.0003 308±16 1432±37 68±2 1400±40

Uranium-Thorium ore DH-1a 0.091±0.003 0.2629±0.0003 3692±123 32462±41 1541±3 31500±1100

Uranium tailing sample UTS-1 Not reported Not reported 680±16 3600±37 166±1.7 3670±38

Uranium tailing sample UTS-2 Not reported Not reported 880±18 4400±41 203±2 5600±46

Table 1. Recommended values for the radionuclides in the certified reference materials

• Certified Reference Materials (CRM) were obtained from CANMET-Mining and Mineral Sciences Laboratories, Canada. • All values are given in 95% confidence interval.

MEASUREMENT GEOMETRY

Two “close” counting geometries

(a) Cylinder beaker, 3.6 cm filling height, 4.3 cm internal diameter (b) Tube, 3.5 cm filling height, 1.4 cm internal diameter

The possible spectral interferences, from the nuclides of

235U, 238U and 232Th, and their decay products are

investigated using the currently available databases (NNDC, TOI).

SPECTRAL INTERFERENCE

CORRECTION METHODOLOGY

ANALYTICALLY USED PEAKS and THEIR INTERFERENCE (Overlapping) PEAKS ARE GIVEN IN TABLE 2

Origin Energy (keV) γ-ray emission probability

(%)

Possible overlapping X- and γ-rays

Nuclide , Energy (keV) and γ-ray emission probability (%)

γ-ray ( 238_{U /} 234_{Th) 63.29±0.02 4.84±048} 231_{Th 63.86 (0.023)} 232_{Th 63.81 (0.267)} 234_{Th 62.86 (0.0211)}

γ-ray ( 238_{U /} 234_{Th) 92.6 keV [doublet:}

92.38±0.01 and 92.80±0.02]

2.81±0.15;

2.77±0.15 Fluo. X-ray Th Kα₁ 93.35 (45.4) X-ray Pa Kα₂ 92.282 (28.3)

231_{Th 93.02 (0.0476)} γ-ray ( 235_{U) 143.76±0.02 10.96±0.08} 232_{Th 140.88 (0.018)} 228_{Ac 141.02 (0.0518)} 212_{Bi 140.15 (0.5047)} 234_{Pa 140.91 (0.309)} 234_{Pa 143.7 (0.3193)} 214_{Pb 141.3 (0.04)} 227_{Th 141.49 (1.23)} 223_{Ra 144.232 (3.2195)} 230_{Th 143.87 (0.0488)} γ-ray ( 235_{U) 163.33±0.02 5.08±0.04} 234_{Pa 164.94 (0.0515)} 234_{Pa 165.61 (0.0721)} 231_{Th 163.10 (0.155)} γ-ray ( 235_{U) 185.715±0.005 57.2±0.5} 228_{Ac 184.544 (0.0718)} 226_{Ra 186.10 (3.59)} 234_{Pa 186.15 (1.68x10)} -3 234_{Th 184.8 (0.0132)} γ-ray ( 235_{U) 202.11±0.02 1.08±0.02} 234_{Pa 203.12 (1.23)} γ-ray ( 235_{U) 205.311±0.010 5.01±0.05} 228_{Ac 204.028 (0.1197)} 228_{Th 205.93 (0.0203)} 214_{Pb 205.59 (0.015)} 227_{Th 204.27 (0.1968)} 227_{Th 205.03 (0.1476)} γ-ray ( 238_{U /} 234m_{Pa) 258.26±0.10 0.0728±0.0004} 228_{Ac 257.52 (0.030)} 234_{Pa 257.2 (0.0515)} 214_{Pb 258.79 (0.55)} 227_{Th 256.25 (7.011)} γ-ray ( 238_{U /} 234m_{Pa) 742.81±0.03 0.080±0.004} 214_{Bi 741.5 (0.0399)} γ-ray ( 238_{U /} 234m_{Pa) 766.36±0.02 0.294±0.012} 214_{Bi 768.4 (4.79)} 234_{Pa 764.8 (0.1957)} 214_{Bi 769.7 (0.0299)} 214_{Pb 766 (0.08)} 211_{Pb 766.51 (0.6170)} γ-ray ( 238_{U /} 234m_{Pa) 1001.03±0.03 0.837±0.010} 228_{Ac 1000.69 (5.32x10)} -3

Table 2. The potentially used γ-ray peaks for determining 235_{U and} 238_{U and their}

The spectral interference corrections were made for the following peaks:

 63.3 keV -ray peak of 234Th

 1001.0 keV -ray peak of 234m_{Pa and}

 186.1 keV -ray peak of 226Ra

for the quantification of 238U and 226Ra activities.

To do this, the contributions of the overlapping

emissions (from the nuclides such as 232_Th, 235_U, 238_U,

and their decay products ) were taken into account by the use of “reference” peaks.

CORRECTION METHODS FOR

SPECTRAL INTERFERENCE

Mixed peak 185.715 keV (57.2 %) 235_U + 186.1 keV (3.5%) 226_Ra Measured sum peak, N_p minus Reference peak, 214_{Pb (}226_Ra) 295.213 keV (18.5%) Measured peak,N_p

Measured sum peak, N_p minus N_p [185.715 keV 235_U] Np [186.1 keV 226Ra] Measured peak,N_p Mixed peak 63.29 keV (4.8378%) 234_Th + 62.86 keV (0.0211%) 234_Th + 63.86 keV (0.023%) 231_Th + 63.81 keV (0.267%) 232_Th N_p [63.86 keV 231_Th] minus Np [63.81 keV 232Th] N_p [63.29 keV + 62.86 keV 234_Th] Reference peak, 228_{Ac (}232_Th) 338.322 keV (11.2518%) or 911.205 keV (26.6%) or 968.971 keV (16.1728%)

THE CORRECTIONS OF THE 63 keV

Fig.1. A schematic representation of the method for correction of the spectral interference on the 63 keV peak of 234_Th.

Mixed peak 1000.69 keV (5.35x10-3_%) 228_Ac + 1001.03 keV (0.837%) 234m_Pa Reference peak, 228_{Ac (}232_Th) 338.322 keV (11.2518%) or 911.205 keV (26.6%) or 968.971 keV (16.1728%)

Measured sum peak, N_p minus

Np [1001.03 keV 234mPa]

Measured peak,N_p

Np [1000.69 keV 228Ac]

Fig.2 Schematic representation of the method for correction of the spectral interference on the 1001.0 keV peak of 234m_Pa.

The corrections for the interference of the 63 keV spectrally disturbed by the 63.81 keV -ray of 232Th and 63.86 keV -ray

of 231_{Th (}235_{U) can be performed using two following}

equations, The first one calculates the contribution of 63.81 keV -ray (0.267%) of 232Th to the mixed peak by the

following equation:









Ref Ac 228 keV, 338.3 Th keV, 63.81 _p Ref keV, 338.3 s COI p γ keV 63.81 s COI p γ p N F F ε I F F ε I N 232          

THE CORRECTIONS OF THE 63 keV

: background subtracted peak area : - ray emissin probability

: full energy peak efficiency

: the correction factor for TCS effects

: the correction factor for self absorption effects

γ I _γ p ε COI F s F p N

Secondly, the correction for the interference of 234Th at

the 63.3 keV peak by 63.86 keV -ray (0.023%) of 231_Th

can be corrected by using the 185.7 keV (57.2%) keV -ray of 235U as a reference peak, as follows:





_



_



1_f N F F ε I F F ε I N Ref 235 keV, 185.7 Th 231 keV, 63.86 _p Ref keV, 185.7 s COI p γ keV 63.86 s COI p γ p    U 

Where f is the ratio of the activities (disintegration rates) of A(238_{U) to A(}235_{U) in the sample containing natural}

uranium. (Natural abundance: 235U = 0.712%, 238U

= 99.2748%) 72 . 21  f

The net peak area for the 185.7 keV (57.2%) keV peak of 235U used in below equation as a reference peak was

deduced from the mixed peak 185.7 keV 235U+186.1

keV 226Ra by using a second-order interference

correction method (Fig.1). For this, the least spectrally disturbed peak at 295.2 keV of 214_{Bi (}226_{Ra) has been}

preferred as a reference peak to estimate the 226Ra

contribution via 186.1 keV (3.5%) -ray, as follows:





_



_



Ref 214 keV, 295.2 Ra 186.1keV, _p Ref 295.2keV, s COI p γ 186.1keV s COI p γ 226 p N F F ε I F F ε I N   _ Bi_

As is shown in Fig.2, the correction for spectral interference on the 1001.0 keV peak of 234mPa can simply made by a

first-order correction method because it interferes with the only 1000.7 keV -ray emission of 228_{Ac (}232_{Th). Therefore,}

the other suitable measured peaks of 228Ac can be used as

the reference peaks, which yields to contribution of 232Th in

equilibrium with 228Ac via the calculation of the

contribution of 1000.7 keV -ray by following equation:





_



_



p





f f keV s COI p keV s COI p p N keV Ac F F I F F I Ac keV N 228 _Re Re , 3 . 338 7 . 1000 228 _{338.3 ,} , 7 . 1000      

In the uranium analysis of a sample if it also contains thorium, the percentage error in the 238U activity

determination via the 63 keV peak of 234Th (238U) due

to the spectral interference of 232Th can be estimated

by the relation:



_{Th U}



238

_U

_activity

_7.5

_C

_C

in

Error

%





But when the 1001 keV peak of 234mPa (238U) was used, the percentage error in the 238U activity

determination via the 1001 keV peak due to the

contribution of 232Th can be estimated by the relation:



_{Th U}



238

_U

_activity

_0.65

_C

_C

in

Error

%





CONCLUSION

Material description Sample Code

238_{U activity*}

Contribution to 238_U

activity 238_{U activity*}

Contribution to

238_{U activity} 226_{Ra activity*} 232_{Th activity*}

Measured via 63.3 keV due to 232_{Th and} 231_Th(235_U) Measured via 1001. keV due to 232_Th (228_Ac) (Bq kg-1_{) % (Bq kg}-1_{) %} (Bq kg-1_{) (Bq kg}-1₎ Rare-Earth Thorium ore OKA-2 3159±79 97,54 27017±434 2,98 2777±198 76455±1604 Uranium-Thorium ore DL-1a 1846±35 0,92 1772±71 0.07 1441±103 366±10 Uranium-Thorium ore DH-1a 31230±344 0,93 35578±403 0.04 26933±1954 3578±69 Uranium tailing sample UTS-1 2352±29 2,57 911±39 0.25 3872±276 582±14 Uranium tailing sample UTS-2 3200±30 3,03 1068±63 0.27 5461±388 750±18

RESULTS

The activity results and their percentage errors in gamma spectrometric measurements for the validation materials (CRMs) are given below.