FHT 3040 CCM Grid Box Monitor

Transcription

1 Technical Specification ZT E Sensitive measurement of Co-60 contamination also in presence of varying Co-60 ambient radiation No lead shielding necessary, easily mobile Tremendous improvement of signal-to-noise-ratio Nearly homogeneous sensitivity inside measurement cavity Efficient suppression of ambient artificial and natural doserate Parallel total gamma measurement Versatile Windows based software for operation and report generation

2 Page 1 / 8 Introduction By virtue of the newly developed CCM method (Cobalt Coincidence Method) extremely low detection limits in conjunction with short sampling times and an excellent spatial focussing can be achieved. The signal-to-noise-ratio can be improved by 1-2 orders of magnitude as compared to integral gamma measurements. The method can also be applied where conventional gross counting or spectroscopy are hampered by temporal variations of the external gamma field or by natural radioactivity of the measured object itself. Contrary to spectroscopy also Compton-scattered gamma quanta contribute to the measuring effect. Therefore the CCM method is very suitable for the Co-60 measurement in matrix material of low Z (aluminium, inflammable waste etc.). Other relevant gamma emitters, e.g. Cs-137, Sb-125 and Ru-106 can simultaneously be tracked at comparable detection limits, via an integral measurement (in absence of natural radiation of the measured object). The NBR method (Natural Background Reduction) to compensate for varying contents of natural radioactivity can also be principally applied as long as no incidence of artificial radiation from outside the measurement cavity occurs. Depending on sampling time, ambient radiation, size and distance of the detectors typical detection limits range between 100 Bq and 1000 Bq of Co-60. General Especially at older nuclear sites Co-60 is the leading nuclide, i.e. the total activity of a contaminated part is inferred from a Co-60 measurement and the known nuclide vector once determined by gamma spectroscopy. A nuclide specific measurement is usually carried out by high resolution Ge-detectors. The relatively high costs and the long spectrum acquisition times are major disadvantages of these detectors. In-situ measurements are generally degraded by ambient radiation: The detector cannot discriminate, whether the radiation arises from the object itself or from the environment, especially in case of the same nuclide. For reasons of weight and costs, lead shieldings can overcome this problem only for small-sized samples, but not for larger machine components or barrels. Low-priced detectors with poor spectroscopic properties (e.g. NaI(Tl) scintillation detectors and organic scintillation detectors) suffer from the additional problem that they cannot discriminate fission and activation products from natural radioactivity of the sample. Beta measurements are suitable for highly localized surface contamination detection, but suffer from absorption effects - in the object of interest, fragmented surface structures etc..

3 Page 2 / 8 Theory of Operation of CCM Upon each radioactive decay, Co-60 simultaneously emits two gamma quanta at 1173 kev and 1332 kev, respectively. Using large area detectors with high gamma efficiency, the appropriate electronics can measure both the single and the coincident count rate. Setting correct energy windows for both detectors avoids the acquisition of real coincidences caused by natural radionuclides (U-238, Th-232 with their respective progenies, K-40) or of cosmic irradiation. The expectation value of stochastic coincidences, S, accounts to S R 1 R 2 tc with R 1 and R 2 denoting the single countrates in the two energy windows and t c the coincidence time window (app. 75 ns). The real coincident countrate, C, results as C M S with M denoting the total measured coincidence. The relationship between C and the Co-60 activity, A, being looked for reads C 1 2 A The single detection probabilities 1, 2 (s -1 /Bq) have to be determined for the respective detector size and distance utilising a reference source. Virtual Gamma Shielding For Co-60 contaminations not residing in between the detectors the real coincident countrate is inversely proportional to the 4 th power of the distance. This can be put down to the known inverse distance square proportionality of the single detection probabilities 1 and 2. Any interfering source being apart by a distance of the order of magnitude of the detector size cannot systematically degrade the described CCM method. Only the stochastic coincidences reduce the level of statistical significance. Inside the measuring cavity outlined by the detector cross sectional area and mutual distance, a very homogenous sensitivity is achieved. Detector size and distance are about the dimension of the measured object. The detector thickness ranges between 5 and 10 cm, typically.

4 Page 3 / General The FHT 3040 CCM grid box monitor is designed for the clearance measurement of grid boxes filled with machine components, demolition materials etc.. The monitor is based on the CCM method which is Co-60 nuclide specific, spatially focussing and even has no cross-sensitivity against interference from ambient doserate. The grid boxes to be measured can simply be put onto the carriage by a forklift truck. The carriage itself is seated on bearings so that the grid boxes can easily be moved into and out of the cavity. Costly door mechanics and floor reinforcement usually necessary to cope with the heavy loads of the lead shielding can simply be omitted. The complete measurement assembly can easily be transferred to various sites of the facility. 1.2 Components The FHT 3040 essentially comprises the following components: support frame made of extruded aluminium profiles with cover panels carriage seated on bearings to move grid box into cavity measuring electronics FHT8000 A with detector input module II and fast multi coincidence board 4 ea. large area plastic scintillation detectors with the dimensions 1000 x 500 x 100 mm with embedded PMT and resistor chain 4 ea. preamplifier FHT670C with integrated HV bias supply Windows based software 1.3 Features nuclide specific measurement of Co-60 contamination low limit of detection, excellent signal-to-background-ratio spatial focussing of sensitivity in the detector near field insensitive against varying doserate levels of artificial and natural origin mobile use because lead shielding is not required integrated software for the evaluation of the raw countrates according to the CCM method self control of the monitor with signalisation calibration using standard Co-60 reference sources

5 Page 4 / Detector Mechanics support frame made of extruded aluminum profiles open cavity volume, width x height x depth: 1130 x 700 x 1300 mm cover panels made of stainless steel detector holders 6 ea feet, height adjustable with mounting brackets 1.5 Detector Unit Made up of 4 ea plastic scintillation detectors, 1000 x 500 x 100 mm, with integrated photo multiplier and voltage divider. One detector is mounted on each side of the assembly. The efficiency of the detector arrangement related to 4 geometry (measured under operation conditions, point source in centre position) typically accounts to: Co-60 CCM Co-60 window Cs-137 integral Background R o (s -1 ) at 100 nsv/h Efficiency (s -1 /Bq) Limit of detection = ß = 5 % t b = 60s Signal-to-background-ratio A Co-60 =1000 Bq at 100 nsv/h 50 % 5 % 0.5 % 1.6 Measuring Electronics FHT 8000 A Hardware The measuring channel FHT 8000 A is used as data evaluation electronics. The FHT 8000 A comprises all boards to supply the detectors and evaluate the measured values. The operation is fully automatic. All evaluation routines are stored as software modules and include control routines for all operational modes of the monitor. They especially allow a fully automatic restart of the monitor after return of power. data acquisition and evaluation system based on a PC-compatible computer integrated 8.5 LCD colour monitor, external monitor connectable operating system Windows XP hard disk CCM coincidence board for up to 6 ea detectors software based module configuration (amplifier thresholds, assignment of counter channels etc.)

6 Page 5 / Measuring Electronics FHT 8000 A Software The measuring electronics comprises all functions and program modules necessary for the fully automatic operation. Program modules are available for the following functions: start-up routine (setting of parameters) measuring and evaluation routines data visualisation control of alarm and limit values plateau measurement background and calibration measurement Upon return of power the monitor starts up automatically. The main menu appears showing the currently measured values: CCM channel Cs-137 window Integral gamma countrate both as raw countrate and in physical units (Bq). Fig. 1 Measured values

7 Page 6 / 8 Fig. 2 Gross count rates and background

8 Page 7 / Technical Data mechanics made of extruded aluminium profiles with cover panels made of stainless steel dimensions of standard grid box length 1270 mm width 1070 mm height: 675 mm detector unit 4 ea plastic scintillation detectors, 1000 x 500 x 100 mm, with integrated photo multiplier and voltage divider basic probe FHT 670 CHT background Co-60 CCM app. 10 s -1 Co-60 window app s -1 Cs-137 integral app s -1 efficiency Co-60 CCM app s -1 /Bq Co-60 window app s -1 /Bq Cs-137 integral app s -1 /Bq limit of detection acc. ISO Bq in 10 sec 5% probability for false alarm and for not detected contamination, respectively signal-to-background-ratio s/b s/b (CCM) > 100 s/b (integral) at 100 nsv/h ambient gamma doserate electronics measuring electronics FHT8000 fitted with CCM multi coincidence board and 9 LCD colour monitor signalisation unit lamp column with horn red: alarm orange: measurement abort limit of detection not reached green: status ok quit button for horn check button for lamps + horn measuring range activity 10 2 to 10 5 Bq ambient and operation temperature + 15 C to 40 C ambient pressure 900 hpa bis 1050 hpa climatic conditions 20 % to 80% rm., non condensing cross sensitivity against artificial no systematic influence up to 1 µsv/h and natural ambient doserate power consumption 1 AC 230 V / 50 Hz, max. 200 W weight (without grid box) app. 650 kg

9 Page 8 / 8 maximum allowed weight of grid box 1000 kg dimensions of CCM monitor length 3000 mm width 1600 mm height 2500 mm dimensions of cavity height 700 mm width 1130 mm depth 1300 mm 2. Height-adjustable Top Detector manual crank with spindle gear for the measurement of 200 l barrels adapter for carriage height of cavity mm external height of CCM grid box monitor mm The typical radiological data at 1000 mm internal cavity height are the following: Co-60 CCM Co-60 window Cs-137 integral Background R o (s -1 ) at 100 nsv/h Efficiency (s -1 /Bq) Limit of detection = ß = 5 % t b = 60s Signal-to-background-ratio s/b A Co-60 =1000 Bq at 100 nsv/h 40 % 4 % 0.5 % 3. OPTION: Weighing Cell For an individual identification of the weight of the sample a weiging cell can be used. Fig. 3 Display of the sample weight

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