Procedure for the continuously monitoring of artificial total gamma activity concentration in seawater

Transcription

1 Procedure for the continuousy monitoring of artificia tota gamma activity concentration in seawater D--GESAMT-MWASS-01 Authors: H.-J. Gabrie I. Goroncy J. Herrmann H. Nies Ch. Wedekind Federa coordinating office for sea water, suspended particuate matter and sediment (Leitstee für Meerwasser, Meerschwebstoff und sediment) ISSN

2 D--GESAMT-MWASS Procedure for the continuousy monitoring of artificia tota gamma activity concentration in seawater 1 Scope The procedure is appied in a measuring network for the continuous monitoring of seawater for its concentrations of artificia gamma emitters. This measuring network forms part of the German nationwide network in the framework of the Integrated measuring and information system for the monitoring of radioactivity in the environment (IMIS) and conforms to the requirements of the Precautionary adiation Protection Act. Using measurement periods of one hour, the procedure periodicay records the tota gamma radiation in seawater. The contribution by radiation originating from artificia radioactive emitters is then cacuated from the proportiona contribution of count rates within predetermined energy ranges of the gamma radiation spectrum. Artificia radionucides with an activity concentration of as itte as 0,5 Bq -1 can thus be detected. The procedure furthermore faciitates the measurement and anaysis of gamma spectra over extended periods of time. Stationary monitoring units are mounted on oceanographic measuring buoys on the open sea whie those on the coast are mounted to permanent structures (e. g., moes, piers), suspended freey into the seawater. Moreover, some vesses of the Bundesamt für Seeschifffahrt und Hydrographie (BSH) serve as mobie measuring stations and are equipped with corresponding measuring equipment that uses a radiation detector mounted in a recess in the bottom of the vesse (trunk). 2 Samping No samping is required for this measuring technique. 3 Anaysis 3.1 Principe of the method The procedure continuay meters the tota gamma radiation in seawater in a direct manner. adiochemica processing of sampes is not required. 4 Measuring the activity 4.1 Measuring probe The measuring probe shown as a schematic diagram in Figure 1 was deveoped by the Bundesamt für Seeschifffahrt und Hydrographie (BSH) specificay to meet the particuar requirements of oceanic appications in the framework of IMIS. It is empoyed at the measuring stations of the nationa BSH measuring network. At fixed stations, the probes are mounted at a water depth of about 3 m. A probe consists of a waterproof detector with a 3 x 3 NaI(T)-detector whose integrated eectronic measuring equipment is wired to a contro unit above the surface. The eectronic measuring equipment is stabiized against temperature and ong-term drifts. Coasta stations that are affected by varying water eves use sensors that are ad-

3 D--GESAMT-MWASS ditionay fitted with a pressure sensor. These record the variabe water cover in order to provide data on changes in the shieding from cosmic radiation that wi be required for subsequent cacuations. The datasets are usuay transmitted at houry intervas to the centra data processing centre of the BSH via teephone ines (coasta stations) or sateite (offshore stations). They are graphicay potted (Figure 2) on the measuring network PC at the BSH once per working day in IMIS norma operations, and every two hours in IMIS intensive operations. After the resuts have been interactivey anaysed and checked for pausibiity, they are forwarded to IMIS (1, 2). 4.2 Principe of the measuring system This is an in situ-measurement technique that detects the gamma radiation of artificia radioactive substances in the sea. If increased activity concentrations are noted, the simutaneous anaysis of gamma radiation spectra enabes them to be associated with accident scenarios. The monitoring of activity concentrations of artificia radioactive substances is generay impeded by the fact that the natura background radiation does not remain constant. In particuar, heavy precipitation wi often cause high concentrations of natura radionucides in seawater. These mainy comprise daughter nucides of n-222 from the U-238 decay chain. The artificia emitters differ from natura ones in the ranges of their reevant gamma radiation energies. The gamma radiation spectrum of natura radionucides extends over a wide energy range, exceeding 2 MeV. In contrast, the major gamma radiation energies of artificia radionucides are found at eves beow 900 kev, as has been demonstrated through the anaysis of various accident scenarios (2). Fig. 1: Gamma detector with above-surface contro unit

4 D--GESAMT-MWASS Fig. 2: Measuring sequence from Station Kühungsborn, week 41 of 1997; (a) tota count number N g, (b) quotient q g, and (c) reative water height z w The monitoring technique expoits this difference by recording the count rates in two channes, one of which wi register the gamma radiation with energies > 900 kev whereas the energies < 900 kev are registered in the other channe (Fig. 3). If natura radionucides from the uranium decay chain are present, the count rates in the two channes wi increase in a proportion that is characteristic of the spectrum of the n-222 decay products. In contrast, the activity of artificia radioactive substances wi be registered amost excusivey in the ower energy channe. The activity concentration of artificia radioactive substances can then be cacuated from the tota count rate and the quotient of contributions by the two channes. Monitoring data therefore comprise the tota count number, N g, measured houry for one hour each, as we as the quotient q g that is cacuated from the contributions of the two channes A and B and are transmitted at houry intervas to the BSH for anaysis. Figure 3 iustrates this type of activity measurement schematicay. The symbos used therein transate as foows: A continuousy measured count rate in channe A in the energy band < 900 kev; B continuousy measured count rate in channe B in the energy band > 900 kev;

5 The count rates of the two channes contain contributions by: D--GESAMT-MWASS A0 background count rate in channe A, originating from cosmic and environmenta radiation; B0 background count rate in channe B, originating from cosmic and environmenta radiation; AN count rate in channe A, originating from natura radioactive substances; BN count rate in channe B, originating from natura radioactive substances; count rate in channe A, originating from artificia radioactive substances; The gross count rate, g, is cacuated from the tota count number, N g, per measurement period, t, according to equation (1): g Ng A B (1) t In routine measurement operations, g wi equa the background count rate, 0, in both channes (equation (2)): g A0 B0 (2) Fig. 3: Schematic iustration of count rates in a two-channe measurement If natura radioactive substances from the uranium decay chain end up in sea water, the count rate g wi increase by the amounts of AN and BN. If there are aso contributions by artificia radioactive substances, the count rate g wi be increased further by the amount of (equation (3)): g A0 AN B0 BN (3)

6 D--GESAMT-MWASS To cacuate the activity concentration of artificia radioactive substances, equations (4) through (6) need to be incuded: atio q 0 for norma background radiation (equation (4)): q 0 A0 B0 (4) atio q g in continuous measurement with possibe contributions to the gamma radiation as a resut of natura and artificia radioactive substances (equation (5)): q g A B A0 AN B0 BN (5) atio q N the contribution of gamma radiation that is excusivey referabe to natura radioactive substances (equation (6)): q N AN BN (6) Using equations (1) through (6), equation (7) can be derived, which specifies the count rate,, originating from artificia radioactivity: 1 qn 1 q n g qg 1 q (7) 0 In addition to measured vaues of N g and q g, the anaysis of the current dataset requires that the parameters 0 and q 0 be known from norma operations. Their vaues are determined as means from the period of the previous week. The ratio q N is determined once with the aid of equation (7) from data taken after a substantia contribution by natura radioactive substances, for exampe, foowing rain during a thunderstorm. Fuctuations in the thickness of the water ayer covering the detector wi cause fuctuations in the shieding of the detector from above-surface radiation and thus infuence the background count rate, 0. In order to be abe to eiminate such changes in the background effect by mathematica means, the current water eve, z w, is continuousy recorded with the aid of a pressure sensor at coasta stations affected thus and compared to the mean water eve, z WN. Appying an empiricay determined inear attenuation coefficient, μ, the vaue of 0 is corrected according to equation (8): ' - z 0 0 e WN z W (8) To cacuate under the impact of fuctuating water eves therefore requires equation (9): g 1 qn -z 1 WN z q W n 1 0 e 1 1 qg 1 q0 (9) Figure 2 provides an exampe of a measuring sequence.

7 4.3 Caibration of the measuring probe D--GESAMT-MWASS The tota gamma measurement wi not normay provide information on the type of the radionucides present and their activity. To identify these woud require them to be caibrated for every radionucide individuay (Tabe 1). If the composition of radionucides is unknown, no nucide-specific caibration vaue can be specified, however (1). In order to obtain such a caibration factor for the tota gamma activity concentration in practice, the quantity artificia tota gamma, in Bq -1, was introduced in anaogy to the caibration for the tota beta measurement. It is an auxiiary quantity that refers to a fictitious radionucide that emits one gamma quantum per radioactive decay event. The attribute artificia is derived from the anaytic procedure described in section 4.2. estricting the energy-dependent caibration of this type of probe to just one run is justified by the major effort required. It has been described by Wedekind (3). The vaidity of the caibration to the individua probe requires that they are of the same configuration and needs to be verified reguary on the basis of their characteristics. The caibration of the measurement setup detector suspended in water party requires arge sampe voumes. Caibration runs (Figures 4 and 6) with Ce-144 (E = 0,13 MeV), u-106 (E = 0,56 MeV) and Mn-54 (E = 0,84 MeV) used measuring vesses with a voume of 1 m 3. The range of K-40 radiation (E = 1,46 MeV) in water necessitates a poo with a voume of approx. 30 m Energy-dependence of the tota gamma measurement The gamma energy-dependent, tota gamma detection efficiency, t(e ), depends on the procedure used and is defined foowing equation (10): E E nt t (10) c p It has been derived from the activity concentration, c, of individua radionucides with ony one gamma ine each with an associated gamma emission probabiity, p i, and appies to the tota gamma measurement, that is, the net count rate, nt(e ), integrated over the entire gamma energy range (0,01 MeV to 2 MeV). Figure 4 iustrates this simpe tota gamma detection efficiency. The energy range 0,3 MeV to 1,5 MeV hosts a pateau of ca. 3 s -1 per Bq -1. This means that caibration with K-40 (E = 1,46 MeV) is sufficient and adequatey representative of the quantity artificia tota gamma Bq -1. Fig. 4: Tota gamma detection efficiency, t(e ), according to (3)

8 D--GESAMT-MWASS The procedure-reated tota gamma detection efficiency, t(e ), for a specific radionucide woud be of interest in a contamination scenario that mainy is due to this radionucide. This can be assessed with the aid of the graph in Figure 4 foowing equation (11). It adds the individua contributions, ti, of a gamma ines, i, of the radionucide in question and takes into account the associated emission probabiities, p i, as foows: n ti p (11) tn i 1 Tabe 1 provides an overview of the procedure-reated tota gamma detection efficiency, tn(e ), for a seection of reevant radionucides. Tab. 1: Detection efficiencies for seected radionucides, tn tota gamma measurement, and Sp gamma spectrometry; Measurement setup: detector freey suspended in water adionucide tr in a E in MeV p tn in Bq -1 s -1 Sp in Bq -1 s -1 K-40 1, ,46 0,11 0,33 0,011 Mn-54 0,855 0,84 1,0 3,3 0,13 Co-60 5,27 1,33 1,0 6,3 0,10 1,17 1,0 0,11 Zn-65 0,669 1,11 0,5 1,5 0,055 u-106 1,02 0,51 0,21 1,5 0,038 0,62 0,1 0,015 1,06 0,15 0,018 I-131 0,022 0,36 0,82 2,8 0,19 0,64 0,07 0,011 Cs-134 2,07 0,60 0,98 7,1 0,16 0,79 0,89 0,13 0,57 0,16 0,027 0,80 0,10 0,013 0,56 0,08 0,014 Cs ,2 0,66 0,85 2,7 0,13 Ce-144 0,780 0,13 0,11 0,2 0, Caibration procedure for measuring artificia tota gamma Measurement setup: Detector freey suspended in water The caibration of this setup is based upon the foowing parameters: Monitoring is effected in seawater using a 3 x 3 NaI(T)-detector incorporated into a network measurement probe of the type described in section 4.1. Caibration is based upon the quantity artificia tota gamma, Bq -1 as defined by IMIS. The caibration for measuring the tota gamma activity concentration empoys K-40 as a reference nucide (E = 1,461 MeV, p = 0,11). The poo used for the caibration measures 3 m x 3 m x 2,7 m (studies on the range of K-40 gamma radiation in water have demonstrated that the probe needs to be positioned at east 1 m from the was of the chamber). The detector is positioned in the centre of the poo. Natura K-40 is present in potassium choride sat with a specific activity of Bq kg -1 (cf. chapter IV.6.4 of this procedures manua). For the purpose of

9 D--GESAMT-MWASS caibration, a soution is produced from 7,0 g -1 of potassium choride that possesses an activity concentration, c K-40, of 113,4 Bq -1. Using the background count rate, 0, of 25 s -1, the gross count rate of the caibration soution, b, of 59 s -1, and the emission probabiity, p, of 11 %, the simpe tota gamma detection efficiency, t, of a 3 x 3 NaI(T)-detector for the measurand artificia tota gamma, Bq -1 is cacuated according to equation (10) as foows: t Bq 113,4 0,11 s 2,73 Bq The tota gamma detection efficiency, tn, for the radionucide K-40 is then given by equation (11): tn 2,73 0,1 Bq 1 s 1 0,30 Bq 1 s s Measurement setup: Probe mounted in bottom of vesse A trunk of 400 mm in diameter and 120 mm deep is buit into the bottom of a vesse. It is open to the seawater, and the 3 x 3 NaI(T)-detector extends into it, protected by a titanium cap. Considering the impossibiity to dupicate this setup in a aboratory, the caibration was performed with a probe instaed on the research vesse FS Gauss at sea and by comparing resuts at various degrees of sainity of the water. The activity concentration in the sea is determined by the potassium content of seawater (4). The potassium content in seawater in the open North Sea corresponds to an activity concentration, c K-40, of 12 Bq -1. For the background effect in the iver Ebe, the potassium content in its freshwater is negigibe. The count rate, g, measured in the North Sea at a sainity of 35 PSU (0,74 g -1 ) amounted to 10 s -1. The background effect count rate, 0, measured in the freshwater of the iver Ebe was 8,5 s -1. The tota gamma detection efficiency, tk, for a trunk-housed 3 x 3 NaI(T)-detector and the measurand artificia tota gamma, Bq -1 is cacuated using equation (10): tk 10 8, Bq 11 0,12 s 1,14 Bq s Caibration procedure for a gamma spectrometric measurement Measurement setup: Probe suspended in water Aside from measuring the artificia tota gamma activity concentration, the monitoring network probes aso record gamma radiation spectra. Even though the ow energy resoution capabiities of NaI(T)-detectors imit the nucide-specific anayses, radionucide compositions that are characteristic of accidents can we be recognized and distinguished from natura radioactive substances (2). For exampe, Figure 5 shows not ony the gamma radiation spectra of natura radionucides of the U-238 decay chain and the contribution of K-40 contained in the water of the Batic Sea, but aso the 662 kev-ine of Cs-137 that originates from Chernoby faout.

10 D--GESAMT-MWASS Fig. 5: Weeky spectrum of week 41 of 1997 metered at Station Kühungsborn In order to derive nucide-specific information from ceary identifiabe gamma ines ike that, the gamma spectrometric detection efficiencies of the probe deserves some attention. The detection efficiency, Sp(E ), for a specific radionucide with an activity concentration, c, with the puse count, N Sp, in its ine during a measurement period, t, is cacuated foowing equation (12) as foows: Sp E N Sp c p t (12) Figure 6 iustrates the energy-dependent detection efficiency, Sp(E ), for gamma spectrometry. It has been determined on the basis of the setup described in section 4.3 and on the radionucides mentioned there, taking into account the associated gamma emission probabiity, p i. Tabe 1 summarizes the spectrometric detection efficiency, Sp, of a seection of reevant radionucides. Fig. 6: Energy-dependent detection efficiency, Sp(E ), for gamma spectrometry according to (3).

11 D--GESAMT-MWASS Cacuation of the resuts The artificia tota gamma activity concentration in seawater, c, is cacuated from the count rate of the contribution by artificia radiation,, given in equations (7) and (9) in section 4.2, by rearranging equation (10) to equation (13): c t (13) The radioactive decay does not need to be taken into account in this procedure. If in a specific case ony one radionucide is to be monitored, the activity concentration, c N, of this nucide may be determined by equation (13) but by repacing t with the associated procedure-reated summated tota gamma detection efficiency tn (Tabe 1). The standard deviation, s, of the artificia count rate contribution,, is derived from the approach by equation (7) and can be computed by equation (14): qn s tm Ng 1 1 q (14) g In accordance with equation (13), the uncertainty, s(c ), of the artificia activity concentration, c, is obtained from equation (15): s s c (15) t Chapter IV.1 of this procedures manua is referred to in conjunction with the anaysis of recorded gamma radiation spectra. The activity concentrations of individua radionucides from the puse counts, N Sp, in the ine within the spectrum are computed according to equations (12) and (13) in anaogy to the computation of the tota gamma activity concentration. 5.1 Worked exampe The caibration of the measurement setup and the measurement itsef have produced the foowing vaues: t = 2,73 Bq -1 s -1 ; t m = 3600 s; N g = ; 0 = 8,89 s -1 ; q g = 6,55; q 0 = 3,39; q n = 14. The count rate,, due to artificia radioactive substances is quantified foowing equation (7): s ,55 8,89 1 s 1 3,39 4,95 s

12 D--GESAMT-MWASS Using this resut, equation (13) wi determine the artificia tota gamma activity concentration, c, in seawater: c 4 1 1,95 2,73 Bq 1,81 Bq whie the uncertainty, s(c ), of the artificia tota gamma activity concentration, c, is computed according to equations (14) and (15): s c 2, Bq 1 6,55 1 0,139 Bq The artificia tota gamma activity concentration, c, in seawater therefore amounts to: c 1 1,81 0,14 Bq 5.2 Consideration of uncertainties The evauation of the standard deviation, s(c ), needs to take into account the errors associated with the parameters q g, q N and q 0 used in equation (14). This can be iustrated on the basis of the cacuation exampe. The vaue of q g = 6,55 in the worked exampe was derived from the quotient between the two channe contributions A = to B = 7987 (A + B = N g = puses). The approach of equation (14) does not contain a difference for q g which is why the reative standard deviation of q g of 1,56 % derived from the channe contributions A and B hardy contributes to the standard deviation s(c ). The quotient q 0 originates from the average of 24 measurement periods of one hour each during the seven 24 hour-periods metered during the course of one week, i. e., from 168 individua measurements. The contribution by q 0 to the overa uncertainty is therefore significanty smaer than that by q g. For its part, the quotient q N is based on a singuar measurement with a high count rate contribution by natura radioactivity that has caused the measured gross puse count, N g, to take on a very high vaue. Compared to the contribution by the error in q g, the respective contribution by q N is therefore aso rather insignificant. Both q 0 and q N are considered as constants in the cacuation and do therefore not contribute to the tota uncertainty. 6 Characteristic imits of the procedure The detection imit, g, for the activity concentration in a singe one-hour measurement is derived from equation 14 and cacuated according to equation 16 (see chapter IV.5 of this procedures manua): g k 1 k t 1 t 0 m 1 1 qn 1 q 0 2 (16) Using the worked exampe in section 5.1 as we as the quanties k 1- = 3 and k 1-β = 1,645, equation (16) wi yied a detection imit of the procedure for the activity concentration of:

13 g 1,645 2, ,89 1 Bq ,39 3 D--GESAMT-MWASS ,61 Bq This detection imit for activity concentrations ony appies if the background count rate, 0, remains sufficienty constant. As equation (7) indicates, fuctuations in the count rates of 0 wi have a direct impact on. As has emerged from day-to-day practice, 0 must be expected to fuctuate. Anaogous to equation (13), a detection efficiency of t = 2,73 Bq -1 s -1 resuts in a reaistic detection imit of g = 0,74 Bq -1 in practica measuring network operations. 7 Cataogue of chemicas and equipment 7.1 Chemicas Standard soutions (activity standards) from the Physikaisch-Technische Bundesanstat, Brunswick; Potassium choride, technica quaity. 7.2 Equipment Described here is an underwater probe manufactured by the BSH for practica network measurements that empoys a 3 x 3 NaI(T)-scintiation counter and a micro-computer contro unit (1). For aboratory purposes, the procedure can be dupicated with commerciay avaiabe eectronic measuring equipment. eferences (1) Wedekind, Ch.; Schiing, G.; Grüttmüer, M.; Becker, K.: Neues Messverfahren zur Überwachung der adioaktivität des Meeres im Bundes-Messnetz des BSH. 10. Fachgespräch Überwachung der Umwetradioaktivität, Apri 1998, Hamburg. Bundesamt für Seeschifffahrt und Hydrographie, S , (1998). (2) Wedekind, Ch.; Schiing, G.; Grüttmüer, M. and Becker, K: Gamma radiation Monitoring Network at Sea. App. adiat. Isot. Vo. 50, No. 4, S , (1999). (3) Wedekind, Ch. (1973): -ay Spectrometer Probe for the Measurement of adioactive Poution in the Sea. Heath Physics, Vo. 25, S , (1973). (4) Nies, H.; van Eck, G.T. M. and de Jong, E. X: adionucides. In: Background Concentrations of Natura Compounds in ivers, Sea Water, Atmosphere and Musseis. Ed. Laane,. W. P. M., eport DGW , Den Haag/NL.S , (1992).