Detector Mesh for Nuclear Repositories P.Finocchiaroa, A.Pappalardoa, V.Bellinib, L.Cosentinoa, V.Febbraroa,c, S.Scirèa,c

a INFN Laboratori Nazionali del Sud, via S.Sofia 62, 95125 Catania, Italy b Dipartimento di Fisica, Università di Catania, via S.Sofia 64, 95125 Catania, Italy c Ansaldo Nucleare S.p.A., c.so Perrone 25, 16161, Genova, Italy

within the framework of the INFN- Ansaldo Nucleare agreement

A new system for the on-line monitoring of short and medium term radioactive waste repositories, has been designed. It is distributed, fine-grained, robust and reliable. It employes a new family of cheap and powerful microsensors, to be placed in shape of a fine grid around each waste drum. An efficient data transmission network guarantees the monitoring of the drums, even during their displacement.

1. INTRODUCTION

One of the main problems related to the disposal of short and medium term radioactive waste, is the storage inside suitable sites, where a high level of safety must be guaranteed. The IAEA agency recommends the adoption of real time monitoring systems, in order to detect possible leaks of radioactive material, and to efficiently reduce the risks of contamination for the operators and for the environment [1].

To our knowledge, among the current storage sites distributed worldwide, nowhere such a kind of system has been implemented. What is usually done, is a periodic check by operators (or robots) plus some overall monitoring of fixed locations inside the repository.

We have recently started a research and development activity, named DMNR (Detector Mesh for Nuclear Repositories), aimed to carrying out an integrated solution for on-line monitoring of the radiation field, providing a 3-dimensional map of the radioactivity produced by the waste. The possible radioactive leaks from the waste drums, due to structural weakness or accident, can be recognized as an increase of the local activity.

This could be performed by means of traditional Geiger-Müller counters, but the cost of such a solution would be prohibitive. By borrowing from the high energy physics experiments, we have opted for a mesh of low cost radiation detectors, arranged as a grid of sensors around each waste drum. These

allow to record continuously the measured activity, in order to control its instant rate and also the history around each drum. The installation of a number of more conventional detectors in prefixed locations, is also foreseen.

2. THE DETECTORS

The mesh of detectors has to be suitably distributed, fine-grained, robust, reliable, and should be based on low-cost components. Moreover, they have to be operated in Geiger mode (on/off), since they just need to count the ionizing radiation events.

Figure 1. a) Grid of fibres around each drum; b) schematic view of the single detector.

Each detector consists of a scintillating optical fibre, 1-2 m long and 1mm diameter, arranged

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around each drum in longitudinal and ring geometry, fig. 1a. Both geometries are adopted, as the most likely leak position has to be determined. Each fibre can intercept radiation coming off the drum wall, mostly gamma rays, so that the energy released inside its active volume is converted into scintillation light, that propagates to both ends of the fibre itself. The scintillation efficiency of the fibres is of the order of 104 photons/MeV. Considering that (i) the expected gamma rays from nuclear waste have energy below 2 MeV; (ii) the average energy released in such a fibre is roughly of the order of 10 keV; (iii) about 3% of the produced scintillation light is collected at each fibre end; as a consequence we have that the number of photons to be detected can be as low as 2-10, too low to be managed by ordinary photomultiplier tubes. Moreover, photomultipliers would also pose additional issues like cost, non-ruggedness, fragility, high-voltage power supply, damage from accidental exposure to ambient light. Therefore we need a rugged photodetector to be coupled to each fibre end, with a sensitivity down to the single photon and capable of fast timing, in order to select events in strict coincidence between the two fibre sides, fig. 1b. When a pulse is detected on both sensors within few nanoseconds, it is recognized as an ionising event, counted and recorded to compute the overall activity.

2.1 Radiation Hardness From the point of view of radiation hardness,

fibres made with glass scintillator would be preferred, but the long decay time of their scintillation light (τ ∼ 3ms) represents a limitation for activity measurement. For such a reason, the faster plastic fibres (τ ∼ 2ns) are most suitable, even though their resistance to radiation is lower: as the overall efficiency is low, the damage will be kept low. By taking into account the realistic dose rate of 10-100mGy/h, by preliminary calculations it is possible to estimate a life for the detector of the order of at least 100 years. As an additional benefit, the plastic fibres are also cheaper than the glass ones.

2.2 Detection of Thermal Neutrons With standard plastic scintillating fibres, fast

neutron, beta and gamma rays can be detected (other high-energy charged particles, even though

detectable with high efficiency, never reach the fibres in a radiation waste environment). In order to make them sensitive also to thermal neutrons, such fibres should be suitably doped or covered with some material that is able to convert low energy neutrons (E < 1 eV) into higher energy radiation, such as 6Li. It captures thermal neutrons by means of the reaction 6Li + n → t + α + 4.78 MeV, whose cross section is about 1000 b. The triton and alpha particles can lose their energy inside the fibre, generating a detectable light pulse.

3. THE SiPM PHOTOSENSOR.

The high sensitivity photodetector we have decided to use is the newly born SiPM (Silicon PhotoMultiplier), a promising cheap detector that is reliable and easy to handle. A SiPM is made of a 2-dimensional array of Single Photon Avalanche Diodes (SPAD), that we call pixel, whose outputs are connected in parallel [2]. Each SPAD works in the Geiger regime (on/off), basically producing the same pulse (rise time < 1 ns) when fired by a photon or because of thermal generation. When a fast light pulse is detected by the SIPM, its output is the sum of the identical pulses produced by each fired pixel. Its sensitivity can be as low as the single photon, but such a lower limit is strongly affected by dark noise.

The reverse bias voltage needed to operate the SIPM is slightly above the breakdown voltage, of the order of 30-70V, depending on the sensor type and brand, low enough to reduce the cost of power supplies. Due to the low power consumption, several sensors can be connected to the same power supply unit.

3.1 Measurements with SiPM We have tested two SiPM modules, produced

respectively by Hamamatsu photonics (S10362-11-100U, commercially available) and STMicroelectronics (not yet available on the market) [3-4]. Both feature 10x10 cells, with overall size of 1 x 1 mm2

and 0.5 x 0.5 mm2 respectively. To investigate the response to light for both

devices, we have used a laser source to produce fast pulses (FWHM = 50ps) in the λ=650 nm region. In fig. 2, for each of the two devices we show the charge spectrum obtained by illumination with light pulses of variable intensity (basically a ramp) [5-6]. By comparing them, it is evident how the STM device has a photon resolving power about twice as

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large as the Hamamatsu (31 vs 17). Moreover, if we fix the laser intensity and fit the spectra with a function derived assuming a Poisson distribution for the photons, it is easily seen that the Hamamatsu device is strongly affected by cross talk between pixels, as shown in [6]. This means that when one pixel fires, often also one or more of the neighbouring pixels fire, following the first. The cross talk for the Hamamatsu device can also be put into evidence, by plotting the dark noise rate versus the threshold expressed in number of equivalent pixels, fig. 3. We expect that the dark noise drops when the threshold is set beyond 1 pixel, since it is unlikely that two pixels fire simultaneously without any correlation. In such a case the count rate of the STM device shows an effective drop of the curve, while for the Hamamatsu SiPM it still remains high, as a consequence of the cross talk effect.

Figure 2. Spectra acquired with ramping light intensity for both devices.

Preliminary tests for possible configurations of our application, have been performed by coupling two Hamamatsu SiPMs to a scintillating fibre 240 cm long, with 1mm diameter. Two radioactive sources have been used, in order to estimate the total efficiency for such a detector. A 60Co as ≈ 1MeV gamma source and a AmBe as ≈ 50 keV gamma source. For both sources we have measured an

efficiency of roughly 0.5%, a value that is strongly affected by the presence of correlated noise, that forced us to operate with a threshold corresponding to 6 fired pixels.

As known, the impact position along the fibre can be determined by means of the difference between the arrival times of the light pulses at the two fibre ends, provided that the time resolution is sufficient. In fig. 4 the position plots acquired with the 60Co source on three different points along the fibre are shown. Unfortunately we could not make any test with STM devices so far, because it was not possible to couple the device with the fibre, for lack of suitable mechanical adapters.

Figure 3. Noise rate vs. threshold. The Hamamatsu is affected by a high cross talk between pixels.

A possible extension to detect also thermal neutrons (E < 1eV), can be achieved by using a 6Li thermal neutron converter, in form of a layer in front of the fibres. At ISIS (UK) laboratory we have recently performed a series of measurements, using

ST

Hamamatsu Cross talk

ST

Hamamatsu

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a thin layer of 6LiF, deposited on a small piece of glass in contact with the fibre. We have exploited a high intensity neutron beam, and the data are still under analysis.

Simulations, which take into account the expected energy spectra, are going to be performed by using the GEANT MonteCarlo code. They should allow us to design the most suitable configuration of the mesh geometry, the number of fibres per drum, the relative positions of drums, in order to maximize the overall system efficiency.

Figure 4. Position plots acquired with the 60Co source at three different positions, along the 240 cm fibre.

The mesh of sensors needs front-end electronics and a data acquisition system, that are capable of

handling data produced by a high number of channels (∼ 104). The front-end electronics has to be efficient, simple and cheap, in order to keep the cost of the overall system as low as possible. For each fibre two leading edge discriminators are needed, connected to a coincidence module that drives a counter, for the activity measurement related to the fibre. Data from each drum are packed and sent to a computer system, for the hystorical data base and data handling. For every platform (4 drums), a programmable electronic board, based on an FPGA component, implements the circuits for coincidence modules, counters and for data transmission protocol. The data transmission is twofold, via a fixed cable infrastructure and a cheap wireless network, in order to achieve a high level of redundancy and to have the capability of monitoring the drums, even during their displacement. The topology of the network guarantees a high routing flexibility, in order to allow to move the platforms without any loss of data. Such a wireless network is based on the ZigBee protocol, that guarantees the desired flexibility and has a very low power consumption. A remote control system allows to connect the platforms into a predefined grid, whereas a special purpose remotely operated robot, useful for ordinary and extraordinary operations, is currently under study.

4. CONCLUSIONS

The R&D activity on the possible use of on-line systems for monitoring of nuclear waste storage, has shown us that the construction of a small-size complete prototype is feasible and reasonable. We plan to achieve this goal quite soon, as the single parts of the project have been proven operational.

REFERENCES

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(2003) 918. 4. M. Mazzillo et al., NIM A 591 (2008) 367–373. 5. P. Finocchiaro et al., IEEE Trans. on Elect. Dev,

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Science, SORMA 2008, to be published.

P. Finocchiaro et al. / Nuclear Physics B (Proc. Suppl.) 197 (2009) 35–3838