NIST efficiency tracing method

NIST efficiency tracing method

Nuclear Instruments and Methods in Physics Research A312 (1992) 124-131 North-Holland NUCLEAR INSTRUMENTS &METHODS IN PHYSICS RESEARCH Section A LS...

847KB Sizes 0 Downloads 1 Views

Nuclear Instruments and Methods in Physics Research A312 (1992) 124-131 North-Holland

NUCLEAR INSTRUMENTS &METHODS

IN PHYSICS RESEARCH Section A

LSC standardization of "Mn in inorganic and organic samples by the CIEMAT/NIST efficiency tracing method L. Rodriguez, J.M . Los Arcos and A. Grau

Instituto de Incestigaciôn Bâsica, CIEMAT Audit. Complutense, 22 Madrid 28040, Spain

The standardization of '4 Mn by the CIEMAT/NIST LSC efficiency tracing method using a commercial solution of 54MnCl, and three specially synthetized organic salts of the dimethyl-butyric, decanoic and palmitic acids, in six different scintillators, is described . Samples over a wide range, 1-5, of figures of merit, were used and an overall uncertainty between 0.71% and 1.3%, depending on the scintillator used, was estimated for the activity concentration o the radioactive solutions . 1. Introduction Direct standardization of 54 Mn is usually performed by 41r(LS)(X, e)--y or 4-rr(PC)(X, e)--y coincidence methods which need a careful extrapolation of the coincidence-gamma ratio to unit value [1,2]. Indirectly, it can also be standardized by measuring the 534.8-keV photon by -y-spectrometry [3] with scintillation or semiconductor detectors, with typical detection efficiencies around or lower than 20%, or by 4-rr-LSC methods which offer a higher efficiency and avoid self-absorption effects. Recently, the CIEMAT/NIST LSC standardization method [41 based on the figure of merit of the detection system has been extended to any radionuclide decaying by beta or electron-capture processes, no matter if it has a gamma emission in coincidence [5-8]. In the case of 54 Mn, efficiencies around 50% can be reached for selected scintillators and the main drawbacks come from color and chemical quench, microprecipitation or vial adsorption that could appear during preparation and measurement of samples . Counting losses due to quenching effects are easily taken into account and the efficiency corrected for by using well established procedures [9,101 . The main difficulty lies in ensuring a good solubility of radiolabelled compounds commercially available, mainly inorganic salts, when added to liquid scintillators which have a toluene-based, or more generally speaking, organic composition . To that aim, radiolabelling of organic compounds with the radionuclide of interest has allowed a direct incorporation to the scintillator solution, and led to successful standardizations in previous cases [11-151. This paper presents the results of standardization of 54 Mn by the CIEMAT/NIST LSC method, using a

commercial solution of 54 MnC1 2 and three different organic manganose salts of the dimethyl-butyric (C,,H,202), decanoic (CiuH2n02) and hexadecanoic (C, 6H ;202 ) (palmitic) acids, diluted in toluene . First, the details of sample preparation are explained, followed by the,charactérization of samples in terms of their quench, count-rate stability and time evolution of spectra . Then the CIEMAT/NIST method is applied to the standardization of the four radioactive solutions and the results are discussed . 2. Experimental equipment and materials'" The synthesis of manganose compounds was made using a BUCHI rotovapor and the measurements of samples were carried out in a computer-controlled LIMB-WALLAC liquid scintillation spectrometer, model 1219 Rackbeta Spectral, with two EMI 9829QB bialkali, quartz window, phototubes operating in sumcoincidence and a 22'Ra source for quench determination by the external standard method . In all measurements, more than 400 000 counts were recorded, thus reducing the counting uncertainties to 0.16%. Three commercial scintillators, Hisafe II, Instagel and PCS as well as three others prepared in our laboratory with toluene, uioxane-naphtnaiene and toluene-alcohol were used. The composition of the homemade scintillators is shown in table 1 . Glass vials with low potassium content were used in all cases and each vial filled with 15 ml of a scintillator Mention of commercial products does not imply recommendation or endorsement by the authors or the CIEMAT and is included only for information purposes.

Olh8-9002/92/$05 .(10 ( 1992 - Elsevier Science Publishers B.V. All rights reserved

L. Rodriguez et al. / LSC standardizatioir of 'Un

125

Table 1 Composition of noncommercial scintillators Toluene toluene PPO dimethyl POPOP

Dioxane-naphtalene 1 I 5 g 0.3 g

dioxane naphthalene methanol ethylenglycol PPO dimethyl POPOP

Toluene-alcohol 880 ml 60 g 100 ml 20 ml 4g 0.4 g

solution, both for stability and standardization measurements. The scintillator volumes were released by a Brand dispenser with uncertainty lower than 1%. The gravimetric determination of the amount of radioactive solution added to each vial was performed with an electronic scale Sartorius 1712MP8, accurate to 0.01 mg. Chemicals used were MnCl, - 4H ,O, absolute ethanol, toluene, dimethyl-butyric, decanoic and palmitic acids, all ACS reagent grade, and a colorless radioactive solution of S4MnCl, in HCl OAM, with 3700 Bq/mg (100 p,Ci/g), all of them commercially available . 3. Sample preparation 3.1 . Inorganic samples

The inorganic samples were prepared from the colorless radioactive solution of '4 MnCl,, diluted to 1 : 100 in HCl OAM . Every vial was previously treated to saturation in a O.1M inactive solution of MnCl, in HCI OAM, for 24 hours . Then each vial was drained and filled with 15 ml of scintillator solution of Hisafe II, PCS, Instagel, dioxane-naphtalene or toluene-alcohol . The PCS solution was modified to have 1 .2% of water. Toluene-based scintillator was not used for this aqueous solution . Two different types of samples were prepared for each scintillator, by putting either directly the radioactive solution (about 100 mg) or adding previously 28 !Lg of inactive carrier, into each vial. No differences were observed in the behaviour of both types of samples . 3.2 . Organic samples

It has been widely reported [16-19] that carboxylic acids are very effective for extraction of metals from aqueous solutions by means of organic solvents, usually long-chain hydrocarbons, benzene or chloroform. Taking this into account and the high solubility in organic solvents, the manganose salts of dimethyl-butyric, de-

toluene 6(Nl ml ethanol 378 ml PPO 2.4 g dimethyl POPOP 0.9 g

canoic and palmitic acids were selected to be labelled with 54 Mn. Prior to labelling, inactive carrier was added and water eliminated from the radioactive solution by solving 0.1 mmol of MnCl,-4H,O, 5 ml of absolute ethanol and 370 kBq (10 WCi) of :54 MnCI, in a distillation flask. The ethanolic solution was vacuum-distilled, keeping the temperature below 40'C and adding absolute ethanol several times . The manganose salts were obtained in a similar way, adding to the ethanolic solution 0.2 mmol of the monocarboxilic acid diluted in toluene, distillating it under vacuum and diluting again the residual in toluene . The organic samples were prepared from these manganose solutions, by depositing them directly into 15 ml of six different scintillators : toluene . HisafeII, PCS, Instagel, dioxane-naphthalene and toluene-alcohol, without carrier addition nor previous treatment of vials. 4. Characterization of samples

Both inorganic and organic samples were checked for quenching effects, count-rate stability and time evolution of spectral shapes before accepting them for standardization . 4. I. Querreh effect

The chemical quench was studied through the variation of the quenching parameter provided by the equipment as a function of the DVIn" concentration in each scintillator and the results are shown in the fig . 1 . Inorganic samples were studied in HisafeII . PCS Instagel and toluene-alcohol . A strong green coloration, probably due to the presence of Mn04 formed by redox reactions between Mn" and peroxides accompanying the dioxane, caused us to discard the dioxane-naphthalene samples . From 0 to 280 l.Lg of Mn" in 15 ml of scintillator, the quenching parameter changed less than 2.5% for HisafeII, PCS and III(b) . LIQUID SCINTILLATION

L . R(xirigttc-z et al. / LSC standardization of 14Mn

la)

4500)

400 350900

C~

0

Clpr

w au 0

06

o

Tolusne-Alcoh41 PCS, Instagel **++m 8isate 11

+

250 -

~HMr

Un Cls samples 15 ml scintillator

200

e0

160

'160

260

pg of Mn++

250

300

(b)

450 400 350 300 i

o

s~

El

04

0 250 200

0

50

100

jug

er a a a -a Ue mToluene-Alcohol, a6eÉlsr Dia:me-Naphlalene ON" Inetapel «e+W Hbate û orlania earplee 15 anl ecin ator 150

200

of Mn++

250

''360

Fig . 1 . Influence of Mn" concentration on the quenching parameter for several scintillators. (a) Inorganic samples . (b) Organic samples .

Instagel and less than 3.3% for toluene-alcohol, as shown in fig. la. For organic samples no variation of the quenching parameter was observed in any scintillator in the same interval of Mn" concentration, as can be seen in fig. ib. Therefore, taking into account the specific activity of 54Mn and the amounts of carrier added, chemical quench was considered not to be significant under practical working conditions, either for organic or inorganic samples. 4.2. Count-rate stability

The stability of samples was checked along several weeks, keeping them inside the spectrometer at 15 ° C and periodically measuring them. A degree of stability was derived by recording the time evolution of the normalized N(t)/(N(t )) ratio between the total count rate after decay correction, N(t), and its average value

(N(t )) . Unlike them, counting losses in unstable samples were characterized by the N(t)/N(t = 0) ratio. s4 MnCl , inorganic samples were followed for 20 days in HisafelI, PCS, Instagel and toluene-alcohol, discarding dioxane-naphthalene due to the strong green coloration . Fig, 2a shows that only Instagel and toluene-alcohol samples remained stable, with no systematic, trend. Only statistical fluctuations with standard deviations 0.2% and 0.4% respectively, in good agreement with the practical conditions of measurement, were apparent . For PCS, the samples were stable for 15 days, with statistical standard deviation 0.2%. HisafelI samples were unstable, with count losses up to 2.5% in three days. The figs. 2b, c and d show the time evolution of the normalized count rates for the three organic compounds. Manganose dimethyl-butyrate samples were unstable in Toluene and Instagel with count losses greater than 20% and 10%, respectively, in 5 days, after which its measurements were stopped. Count rates in HisafelI, PCS, dioxane-naphthalene and toluene-alcohol showed a good stability for 30 days, with standard deviation 0.3%. Manganose palmitate and decanoate samples had a similar behavior in the same scintillators with standard deviations 0.1% for decanoate and 0.2%-0.3% for palmitate, during 20 days. 4.3. Spectral behavior The stability of samples was also checked through the time evolution of their logarithmic spectra. They confirmed that 54 MnC1 2 were unstable in HisafelI and that, although the count-rates in Instagel and toluenealcohol were apparently unaffected, a slight degradation of 54 MnC1 z samples in Instagel and toluene-alcohol was observed, and also indicated the superior stability of organic samples along several weeks, as indicated in the smoothed spectra shown in figs . 3 and 4. 5. Standardization 5.1. CIEMAT/NIS T efficiency tracing method The best samples obtained, 54MnCl, samples in PCS and the organic samples in HisafelI, PCS, dioxane-naphthalene and toluene-alcohol, were selected to standardize the inorganic and the three organic solutions, according to A = N/e( 54Mn)m, (1)

where A is the activity concentration of the solution, N is the total count rate, e is the counting efficiency and m is the mass of the radioactive solution .

L. Roddgue., et al. / LSC standardization of `"'Mh The counting efficiency e( 5°Mn) was computed by the CIEMAT/KIST efficiency tracing method [4], vihich uses an experimental efficiency curve for 1H to determine the figure of merit and the corresponding efficiency of beta or electron-capture radionuclides. 54 Mn decays by electron capture followed by a gamma transition to the ground level of 54Cr and its efficiency was computed by the CEGA2 [20] version of the VIASKL code [21], which takes into account the atomic rearrangements following the capture process as well as the gamma transition and the interaction of the 834 .8-keV photon with the scintillator. Nuclear data needed were taken from ref. [22]. In order to check the consistency of the procedure, a set of quenched samples was prepared for each scintillator and type of radioactive compound, by adding a variable amount of CCl 4 to each vial, and the count rate and the quenching parameter were measured with the LS spectrometer. Once the figure of merit was determined through the experimental 3H curve of each scintillator, the associated efficiency was computed for each vial and the activity concentration estimated from the set of quenched samples.

The experimental efficiency of each sample: was also estimated by this self-standardization procedure and both the experimental and the computed value were plotted in fig . 5 for organic and inorganic samples, versus the figure of merit in the interval used 1--5. equivalent to 1H efficiencies 50%-14% . In organic samples, figs. Sb, c and d, some efficiency discrepancy exceeded 1 % at single vials only occasionally but the computed values followed the experimental trend very closely over the whole set of quenched samples. In fact, the precision to which the activity concentration was determined from each scintillator set had standard deviations between 0.35% and 0.9%. For inorganic samples, fig. 5a, a worse precision, 1 .4%, was obtained in the full interval 1 .5-3.5 of figure of merit, although it reduced to 0.52% in the interval 1.5-2.5, thus reflecting that severe efficiency discrepancies were present in the strong quench region. An estimation of the different uncertainty factors of the computed efficiency is given in table 2, according to the BIFM Recommendation INC-1 (1980) [23]. The overall uncertainty varies between 0.69% in toluene-alcohol and 1 .3% in Hisafell . It should be noted that a

00.95 00 .93 V0.91 _ , 00 .89 0.87 C0.85 É0 .83 -0 .61 0.79

1

Time ( days )

~ 1.01 W40 .99 "P0 .97 :30 .95 .93 U0 ,00.91 00.89 P-40.87 P.85 $40 .83 -0.81 0.79

127

(d) 0 *a001.01 ~0 .99 ~0.97 :C0 .95 0.93 ***44 Hisate 11 Gesso Diozane-naphtalene ,00.91 BOEOG Toluene-alcohol 00 .89 +N m * 11 Toluene *041** Instagel ,40.87 00.85 "Mn CeHit0a )a samples N 0.83 -+ 15 mi scintillator 0 zo .81 +*0.79 5 0

gisare II WSW Dlozane-napttalene oaeep Toluene-alaaLal Toluene et 0 p "Ynt C, n.M)s Plea tillator 15

U

")lu 15

)s samples ~~sfor

Fig. 2. Count-rate stability of inorganic and organic samples in several scintillators. (a) "MnCl, samples. (b) '4Mn-dimethyl-

butyrate .

(C)-54

Mn-decanoate. (d) s4 Mn-palmitate .

1II(b). LIQUID SCINTILLATION

L. Rodriguez et ai. ,/ LSC staitdardizatiorl af 54Mrt

a

a

w a w

w a w

organic samples Hisafe It (15 ml )

;-0,10 and 20 d

Ä k Q w

Ä w. Q

-7 100

.od 2o d

200

T 400

300

600

SOD

700

1300

organic samples I)iaxone-Naphthalene (15 ml )

S4

Mn Cl 2 sample Instagel (15 mi )

a a~. a

w

a

_0,10 and 20 d

Ä Q

1â0

2â0

od 20d

300

400

5â0

600

Boo

700

Organic samples Toluene-Alcohol ( 15 ml 1

Si. Mn Ct2 sample Toluene-Alcohol (15 ml )

.a Q 1

1a0

200

300

400

Soo

600

700

800

Channel Number pig. 3. Spectral evolution of "MnCl 4 samples in several scintillators.

Fig. 4. Spectral evolution of organic samples in several scintillators .

major contribution comes from the ionization quench, whose influence was evaluated according to Birks' model [241 as upper bounds compared to toluene-based

scintillators, and further studies could significantly reduce the overall uncertainty. The uncertainties affecting nuclear constants, specially the K-capture probabil-

Channel Number

Table 2 Contribution of diverse uncertainty factors (I standard deviation) to the .54 Mn efficiency computed by the CIEMAT/NIST method Factor

Original uncertainty [%] and type

Resulting uncertainty [%] on e( 54 Mn) I

1- H Standardization Quench determination for 1H standard "H Counting PMT efficiency loss (-H, fig. of mer. 1-15) Gravimetry (1H) Quench determination for 54 Mn sample Nuclear constants Phototube assymetry Ionization quench

HSII

PCs

IAN

TA

0.67 0.10 0.16 2-6 0.01 0.10 ,-. -

0.67 0.06 0.05

0.60 0.05 0.04

0.56 0.04 0.04

0.54 0.04 0.04

0.06

0.05

0.04

0.04

0.75

0 .36

1 .01

0.14

1.3

0.81

1 .2

0.69

Overall uncertainty I

A A B B A B B B

HSII : Hisafell, DN : dioxane naphthalene; TA : toluene-alcohol .

0.10 0.003 0.3 0.25

L. Rodrlgaez et al. / LSC standardization of s'Mn

129

Table 3 Standard deviations [%] of activity concentration for organic samples in each scintillator "

Table 4 Estimated uncertainty factors (I standard deviation) associated with the activity concentration of 54 Mn

Compound

HSII

PCs

DN

TA

Factor

Dimethylbutyrate Decanoate Palmitate

0.36 0.45 0.35

-0.53

0,70 0.90 0.50

0.80 0.70 0.80

54

`' HSII: Hisafell ; DN : dioxane-naphthalene ; TA : toluene-al cohol.

ity PK are also important and a combined contribution about 0.3% has been estimated from several computations carried out for different values of the nuclear constants. 5.2. Assessment of activity concentrations

As indicated in section 5.1, an activity concentration estimate was obtained from each single vial using expression (1) and an average value was computed from each set of quenched vials for each scintillator. The standard deviations within each scintillator set is shown

Uncertainty [°k] 1'

Type

HSII PCs DN

TA

Mn computed efficiency Uravimetry( 5''Mn samples) Counting (54 Mn samples)

1 .3

0.81 1 .2 0.01 0.16

0.69 B B A

uncertainty

1.3

0.83

0.71

1 .2

HSII : Hisafell ; DN : dioxane-naphthalene ; TA : toluene-alcohol.

in table 3 for organic samples and vary between 0.35% for Hisafell and 0.9% for dioxane-naphthalene . For 54 MnCI z samples, the standard deviation was 1.4% due to the anomalous discrepancies observed at high quench . The results obtained after decay normalization are summarized in fig. 6 for all the samples measured . The estimations of the uncertainty factors affecting each single activity concentration estimate are given in

50

45 ô 40 .

"Macis

ô 35

oocxx> enper. (PCS) computed

>, 30

O

25

20 WW 1q

0

,r-r.

1.5 2.0 2.5 3T0 3.5 i 4.0 4.5 5.0 Figure of Merit 50

50 45

45

-140 .

Ô 40 ô 35

1

0 35

30

w 25

0

W 15

W 15~

101

.0

"Mn(CISHei0s)s 44444

P PCCS~eû 00=Dioaane-Naphthalene Toluene-Alaohal computed

25 :

1 .5 2.0 2.5 2.0 3.5 4.0 4.5 5.0

Figure of Merit

54 Mn-dimethylFig. 5 . Experimental and CIEMAT/NIST computed efficiencies for several scintillators. (a) 54 MnCl, samples. (b) 54 butyrate . (c) 54 Mn-decanoate . (d) Mn-palmitate . 111(b). LIQUID SCINTILLATION

130

L. Rodriguez et al. / LWslandardization of -`Mh

"Mn(CsH,&0s),

'l-1893 0

Hisafell PCs eee" Dioxane-Naphtha lene ~w Toluene -Mcbhoi "Un(C.04.0at

>b *1 19

= 12 ~~

1.5

270'

2.5 ' 00 ' 375 Figure of Merit

4.0

4.15

'

5-10

Fig . 6. Activity concentration estimates in each scintillator set . table 4 and are consistent with the standard deviations observed with organic samples, but do not agree for the inorganic one. Since each organic compound was measured in more than one scintillator, a test of means was successfully performed between each pair of scintillator sets and the final activity concentration of each radioactive solution was estimated as the global average of all the samples, regardless of the scintillator used to measure it. The external standard deviations of means were U.113% (20 degrees of freedom) for dimethylbutyrate, 0.20% (20 d.o.f.) for decanoate and 0.13% (35 d.o.f.) for palmitate . 6. Conclusions A method to prepare LSC samples from manganose palmitate, decanoate and dimethylbutytate radiolabelled with 54 Mn has been successfully set up as an alternative to conventional 54 MnCl, solutions commercially available. All three organic compounds produce negligible quench for a wide range 0-20 l.Lg/ml of Mn" cones ccntration, and have an excellent stability of count rates and spectral shape in Hisafell ., PCS, dioxanenaphthalene and toluene alcohol, during several weeks, Inorganic samples of 5'MnC12 have been stabilized only in PCS, since the spectral analysis revealed a slight degradation in Instagel and toluene-alcohol . The high stability samples thus obtained were used to standardize the radioactive solutions of "MnC1 2, "'Mri-dimethylbutyrate, -decanoate and -palmitate, by the CIEMAT/NIST efficiency tracing method to an

overall uncertainty between 0.71% and 1 .3%, depending on the scintillator used, over a wide range of figures of merit, I to 5. References [1] J . Steyn, in : Standardization of Radionuclides, Vienna, 1966 (IAEA, Vienna, 1967) p. 35. [2] R. Vaninbroukx and 1 . Stanef. Nucl. Insu. and Meth. 112 (197-3) Ill . [3] R. Vaninbroukx and G. Grosse, Int . J. Appl, Radiat. Isot. 17 (1966) 41, [4] A. Grau and E. Garcia-Torafio, Int . J. Appl. Rad . Isot. 33 (1982) 249. [5] J.M. Los Areas, A. Grau and E. Garda-Torafio, Proc. Int . Conf. on New Trends in Liquid Scintillation Counting and Organic Scintillators, eds. H. Ross, J.E. Noakes and J.D. Spaulding (Gatlinburg/Lewis . Chelsea, 1991) p. 611 . [6] E. Garcia-Torafio, A. Grau and J.M. Los Arcos, Comp. Phys. Comm. 50 (1988) 313 . [7] J.M. Calhoun . B.M. Coursey, D. Gray and L. Karam, in ref . [5] p, 317. [8] E. Garcia-Totaho, M.T. Martin-Casallo, L. Rodriguez, A. Grau and J.M. Los Arcos, in ref. [5] p. 307. [9] N. Kaczmatezyk and 1. Ruge, Int . J . Appl. Radiat . Isot. 20 (1969) 283.

[10] C.T. Peng, in : The Current Status of LSC, ed. E.D. Bransome Jr (Grune and Stratton, New York 1970) p. 283 . [11] L. Rodriguez, A. Grau, J.M. Los Areas and C. 5uarez, report CIEMAT 623 (Madrid, 1988). [12] L. Rodriguez, A. Grau, J.M. Los Areas and C. Suarez, Anales de Fisica B85 (1989) 131 . [13] L. Rodriguez, J.M. Los Areas and A. Grau, in ref. [5] p. 593.

L. Rodrîguez et al. / LSC starrdardîzatïorr of 1'Mn

[141 L. Rodriguez, J.M . Los Arcos and A. Grau, report CIEMAT 644 (Madrid, 1990) . [151 L. Rodriguez, J.M . Los Arcos and A. Grau, report CIEMAT 645 (Madrid, 1990). [161 M. Tanaka, N. Nakasuka and H. Yamada, J . Inorg. Nucl . Chem . 32 (1970) 2759 . [171 A.W . Ashbrook, J. Inorg. Nucl . Chem . 34 (1972) 3523 . [181 C.K. Schwitzer and F.C. Clifford, Anal . Chem . Acta 45 (1969) 57 . [191 P.W . West, T.G. Lyons and K.J . Caritoon, Anal . Chem. Acta 6 (1952) 400.

[201 CEGA2, J.M . Los Arcos, A. Grau and E . Garda-Toraho, submitted to Comp. Phys. Comm. [211 J.M . Los Arcos, A. Grau and A. Fernandez, Comp . Phys. Comm . 44 (1987) 209. [221 F. Lagoutine, N . Coursol and J. Legrand, Table des Radionucléides (Laboratoire de Metrologie des Rayonnements Ionisants, Gif-sur-Yvette, 1983). [231 R. Kaaris, Proces Verbaux du Comité International des Poids et Mésures, tome 49, A1-AI2 (1981). [241 J.M. Los Arcos and C. Borräs, report CIEMAT 646 (Madrid, 1990).

1I1(b). LIQUID SCINTILLATION