Gamma Ray Counters to Monitor Radioactive Waste Packages in the MICADO Project

One of the goals of the MICADO Euratom project is to monitor the gamma rays coming out of radioactive waste drums in storage sites on a medium/long term basis. For this purpose 36 low-cost gamma ray counters were designed and built to act as a demonstrator. These counters, named SciFi, are based on a scintillating fiber readout at each end by a silicon photomultiplier, as-sembled in a robust arrangement in form of 80 cm long pipes. Several counters will be placed around radwaste packages in order to monitor the gamma dose-rate by collecting a continuous data stream. The 36 sensors were thoroughly tested with a 22Na, a 137Cs and an AmBe sources, the results are quite satisfactory and the next step will be the test in a real environment


Introduction
The MICADO (Measurement and Instrumentation for Cleaning And Decommissioning Operations) Euratom (EU) project is aimed at the full digitization of low-and intermediate-level radioactive waste management [1,2]. Following a complete active and passive characterization of the radwaste drums with neutrons and gamma rays, the project contemplates a longer-term monitoring phase in the Work Package 7 by means of low-cost dedicated detectors for neutrons (named SiLiF [3]) and for gamma rays (named SciFi). A continuous automatic monitoring of the radwaste drums after their characterization represents an added value in terms of safety and security, and the availability of continuous streams of counting-rate data around each drum would be a comfortable tool toward the transparency, which now more than ever is a relevant topic of the nuclear industry with respect to the common people environment-aware [1, [3][4][5][6][7].
The radiological monitoring of radwaste has to be based on the measurement of gamma rays and neutrons, because they are penetrating and thus more easily detectable out of the drums. Due to the foreseen mass deployment, the sensors have to be reasonably low-cost and configurable in a modular and scalable fashion, so that one can tailor the system to small, medium and large scale storage configurations. The proposed monitoring system is based on detectors which can be easily installed and/or reassembled in different geometrical configurations, as they are mechanically very simple and are based on commercial electronics.
The neutron monitoring detectors have already been described in reference [3], and in this paper we describe the SciFi monitoring detectors devoted to the gamma radiation: following a description of their operational principles and mechanical setup, some simulation results are described followed by the results of the characterization and a few tests of 36 SciFi detector units. Finally, an example is provided with the evaluation of the expected performance and sensitivity in a realistic case.

Materials and Methods
When facing the development of gamma ray monitoring detectors for a possible mass deployment around radwaste drums, we made the following considerations: • the sensors should be simple and robust; • they do not need to have spectroscopic features but can be simple counters; • they can have low intrinsic efficiency, as they can measure for long time spans and be sensitive enough; • low efficiency also implies the capability to stand in high radiation fluxes without being saturated; • they should possibly be spatially extended to cover a wide region of a drum; • they should likely be based on commercially available technology; • they should be reasonably inexpensive.
In light of all this we opted for a solution based on a length of plastic scintillating fiber read out at both ends by Silicon PhotoMultipliers (SiPM), which represents a low-cost solution based on commercial products. Moreover, such a solution has a high degree of modularity in terms of length, shape and number of detectors.

The scintillating fiber
The chosen scintillating fiber, whose main characteristics are listed in Table 1, is the BCF-20 produced by Saint Gobain [8]. We tested both 1 mm and 3 mm diameter fibers, and chose the latter which has a higher detection efficiency as the average energy deposited by the detected gamma rays is larger. Moreover, even though the 3 mm fiber is stiffer than the 1 mm one, it is still flexible enough for a possible future solution in a curved or round shape. The operating principle is the following: whenever a gamma ray interacts with the fiber it deposits a variable amount of kinetic energy which gives rise to a short flash of scintillation light. A fraction of these scintillation photons is trapped into the fiber and propagates toward both ends. If the number of these photons is large enough to produce a signal above a predefined threshold simultaneously on both fiber ends, such a coincidence event is considered as the detection of a gamma ray and can be counted. By using the GEANT4 code [9] we simulated the interaction of gamma rays of a few selected energies with fibers of 1 and 3 mm diameter, recording the energy deposited on the fiber event by event whenever the deposition was at least 50 keV. Indeed, below 50 keV the average number of photons detected on the SiPM would be < 4, as will be explained in sections 2.2 and 2.3. Then, for each impinging gamma energy we calculated the average deposited energy, and the results are plotted in Figure 1. Apart from very low energy cases where the photoelectric effect could take place, the interaction occurs through compton effect and the energy deposition is done by the scattered electron. The plateaux in Figure 1 are due to the electron escaping the fiber, and reflect the different fiber diameters. The same simulation allowed us to calculate the expected intrinsic detection efficiency as the ratio between the reported counts and the number of impinging gamma rays. The resulting plots, reported in Figure 2, fully justify the choice of the 3 mm diameter fiber for the SciFi detectors.

The SiPM photodetector
The SiPM photodetector, capable of counting single photons, is an intrinsically noisy device whose noise level can be dramatically reduced by enforcing a suitable threshold on its output signals [10][11][12][13][14][15][16][17][18][19][20]. The chosen SiPM is the MicroFC-30035-SMT, produced by ON Semiconductor [21], whose main characteristics are listed in Table 2. For all the tests described in this work we made use of a home made voltage amplifier, which features a 200x gain and a 4 GHz bandwidth. The SiPM bias, according to the manufacturer's specifications, was set at a 2.5 V overvoltage i.e. 27 V bias. In order to choose an appropriate threshold level for the SiPM we studied the noise counting rate as a function of the threshold, and the resulting typical staircase plot is reported in Figure 3. Each step corresponds to one detected photon, and we decided to use a threshold at 175 mV that corresponds to ≈ 3.5 photons and reduces the noise rate by three orders of magnitude down to about 350 counts per second. As the signals have a duration of about 30 ns, a duration of 100 ns for the coincidence window between the two SiPMs makes the probability of spurious coincidences negligible.

The SciFi detector assembly
In order to decide the fiber length we calculated the probability of detection as a function of the impact position, of the deposited energy and of the threshold selected on the SiPMs, taking into account the scintillation light yield, the light trapping efficiency, the fiber attenuation length, and the photon detection efficiency (PDE) of the SiPM. Using input values taken from Table 1 and Table 2, and assuming the threshold at ≥ 4 photons on the SiPMs, we produced the corresponding plots for fiber length of 80, 120 and 300 cm, in two cases of 100 and 200 keV energy deposited in the fiber. The plots are reported respectively in Figure 4, Figure 5 and Figure 6. One can immediately see that with 200 keV deposited energy the response is uniform for the 80 and 120 cm fibers, whereas it is acceptable for the 300 cm one. Conversely, with 100 keV deposited energy the 80 and 120 cm fibers respond almost uniformly, even though losing 15-20% efficiency; the 300 cm fiber in such a case is barely acceptable. We opted for the 80 cm length, even in light of the height of the standard 120 and 210 liters radwaste drums (76 and 88 cm). The scintillating fiber was allocated inside a 2 cm diameter and 1 mm thick aluminum pipe, and held in place by two cylindrical holders designed to host a circular PCB with the SiPM and its support circuitry. The holder has a central hole to allow for the fiber-to-SiPM alignment and optical coupling by means of a tiny grease drop, and two side grooves to allow for the passage of cables (for this prototyping phase we chose the grease for reversibility, an optical glue will be the final solution). Two light-tight rubber caps complete the setup, with three cables coming out of one single side for the common voltage bias and the two output signals. A sketch and three pictures of the SciFi detector components are shown in Figure 7. We remark that the attenuation of gamma rays when crossing the aluminum pipe is a few percent at very low energy and goes down to about 1-2% at higher energy ( Figure 8) [22].

Results
Thirty-six SciFi detectors were assembled as shown in the previous section, and in order to characterize them we made several tests with standard laboratory gamma sources and a high activity gamma and neutron source. The electronic setup, sketched in Figure 9, was quite simple: the outputs of the SiPM amplifiers were connected to two discriminators, with threshold equal to 175 mV. The two logic outputs were used as inputs to a coincidence unit with a 100 ns window, and the final output was sent to a counter.

Pointlike source at short distance
A first test to validate the detector behavior was done by means of a pointlike 22 Na source of activity A ≈ 42 kBq, by counting the detected gamma rays as a function of the distance in the range of few centimeters. The source was placed at several distances from the fiber, and the number of counts was recorded. The source emits gamma rays of 1274 keV (99.9% branching ratio) and also β+ particles (90.3% branching ratio) which immediately annihilate giving rise to a pair of 511 keV gamma rays. When the distance r between source and fiber is much smaller than the fiber length, this one can be assumed as infinite and the geometrical efficiency ε can be calculated as ε ≈ w/2πr, where w is the fiber diameter. The expected count rate C can be calculated, for instance at 1 cm, by means of the activity A and the simulated detection efficiency εdet at 511 and 1274 keV: .903 x εdet(511 keV) + 0.999 x εdet(1274 keV)] ≈ 100 cps (2) that is what we obtained and corresponds to an equivalent dose of about 118 µSv/h [23]. A 1/r fit reproduces perfectly the measured count rates, as shown in Figure 10. Figure 10. The observed count rate as function of the distance between the pointlike 22 Na source and the 3 mm diameter fiber. Also shown is a 1/r fit.

Detection efficiency measurement
For a better check of the simulation results, and in order to verify the uniformity of the response within the set of SciFi detectors, we assembled a simple setup with a pointlike 137 Cs gamma source, with activity A = 1.49 Mbq, and a detector holder that kept the midpoint of the fiber at distance d = 52 cm from the source (Figure 11). The counting rate of each detector was measured in 200 s, as well as the background rate and the signal-to-background ratio, and the results are plotted in Figure 12. The nonuniformity of the response is due to tiny differences between the individual SiPMs, which are amplified by their exponential multiplication mechanism, and slight imperfections of the optical coupling. However, the nonuniformity is quite limited and reasonable. Notice that the background count level is the same as shown by the blue line in Figure 3, as it represents the physical background due to cosmic rays and natural radioactivity.
The net measured counting rates, i.e. after subtracting the background, were used to calculate the detection efficiency with gamma rays of 662 keV energy. The geometrical efficiency ε in this configuration can be calculated starting from a barrel-like solid angle covering (Figure 13), subtracting the fraction covered by the two spherical caps, and then dividing by the number of fibers that would cover the entire cylinder. The relevant quantities are d = 52 cm, fiber length f = 80 cm, fiber diameter c = 0.3 cm, r = 65.6 cm, h = 25.6.
Ωcylinder / Ωtotal ≈ 1 -2 x Ωcap / Ωtotal ≈ 0.610, (4) Nfibers ≈ 2πr/c ≈ 1089, The decay branching ratio of 137 Cs into 662 keV gamma rays is 85%, and the attenuation in the aluminum pipe is ≈ 2.0%, therefore the expected number of impinging gamma rays on the fiber per second was Ngamma ≈ A x 0.85 x (1-0.02) x ε ≈ 694 (7) The average net counting rate from the 36 fibers was 11.43 counts per second (cps), therefore the average detection efficiency resulted εdet(662 keV) ≈ 11.43 / 694 ≈ 1.65%, (8) in reasonable agreement with the 1.8% value estimated from the simulation. The detection efficiency for all the fibers is reported in Figure 14.

Tests in a more realistic setup
A realistic setup resembling the emission from a radwaste drum was employed for a series of tests of the SciFi detectors. We made use of an intense AmBe neutron source, installed in an experimental hall at INFN Laboratori Nazionali del Sud (LNS), which emits 2.2x10 6 neutrons/s by exploiting alpha particles from the 241 Am decay to induce the 9 Be(α,n) reaction. In order to produce such an amount of neutrons one needs a considerable quantity of 241 Am, which is highly radioactive as it emits 59 keV gamma rays. Indeed the activity of our source is 34 GBq, and it is enclosed in a 95x75x85 cm 3 iron box along with its moderator (Figure 15). The source is surrounded by a first polyethylene case followed by 30 cm thick paraffin which slow down the high energy neutrons whose initial kinetic energy extends up to 10 MeV. The outer 5 cm of the shielding are made from borated paraffin that absorbs the vast majority of the outgoing thermalized neutrons. The energy spectrum of the gamma rays coming out of the box has several main components: • 59 keV from the decay of 241 Am; • 478 keV from neutron capture in boron (component of the borated paraffin); • 511 keV from e+e-pair production followed by positron annihilation; • 2200 keV from neutron capture in hydrogen (component of paraffin and polyethylene); • 4438 keV from the 9 Be(α,n) 12 C*; • a continuum due to the compton scattering in the source assembly materials.
All the SciFi detectors were tested on top of the source box, and the measured counting rate, the background and the signal-to-background ratio are reported in Figure 16. For each detector we calculated the ratio between the observed counting rates respectively with the 137 Cs and the AmBe sources. The results, plotted in Figure 17, indicate a quite reasonably constant behavior of this ratio.   The spectrum of the gamma rays exiting the source box was measured at several distances by means of a cylindrical Lanthanum Bromide scintillator (LaBr3, in short LaBr), 3.8 cm diameter and 5 cm length, placed at mid-height with respect to the box. LaBr naturally contains about 0.09% of the 138 La isotope, which is unstable and emits gamma rays at 1435 keV, and also impurities of alpha emitters which produce counts around 2-3 MeV equivalent energy. The spectrum measured at 4 cm distance from the box wall is plotted in Figure 18, along with the environment background measured far from the source. For all the following evaluations the background spectrum was subtracted from the measurements.
The LaBr spectra were numerically convoluted with the simulated detection efficiency of the SciFi (Figure 2) and rescaled for the different active area, in order to estimate the counting rate to be expected on the SciFi detectors at several distances. In Figure 19 we reported as an example the spectrum of Figure 18 after background subtraction and folding with the SciFi efficiency. The integral of such a spectrum is the expected count rate on a SciFi at the same distance. The same measurements were done using SciFi number 12, which has an average behavior ( Figure 12, Figure 14, Figure 16), and the net count rates (i.e. background subtracted) were reported as a function of the distance in Figure 20, along with the corresponding equivalent data inferred from the LaBr measurements.
The data were fitted with the inverse squared distance function where a is a scale constant, b is the background and the offset d is the distance between the box wall and the source inside. The fitting curves are also shown in Figure 20. Not surprisingly the results, listed in Table 3, are mutually consistent as they exhibit a very close scale factor and a null background. As for the offset we observe that the effective position of the source inside the box is not well known (the source cannot be easily han-dled due to safety restrictions because of its huge activity), and both values are realistic. However, such a difference arises from the different response at close distance from the box, due to the relevant shape difference between the two detectors. A final test was done by measuring the counting rate in 100 s for the SciFi detector n.12 in twenty-two positions, on top and around the AmBe source box as sketched in Figure 21. The measured count rates in the twenty-two positions are shown in Figure 22.    Table 3.

Discussion
In the previous sections we have shown that the SciFi detectors can fruitfully be employed to detect and count gamma rays, in a rather wide energy range. Indeed, by folding the spectra measured by a LaBr detector with the simulated SciFi detection efficiency one obtains count rates in quite a reasonable agreement with the data measured by the SciFi detectors. Apart from very low energy, where the detection efficiency drops below 1%, in the energy range between 150 and 5000 keV it is between 1% and 2%. On the one hand this means that the total number of counts one obtains if assuming an intermediate efficency value of ≈ 1.3% is reasonably correct. On the other hand such a low detection efficiency insures that the detector will not be saturated up to a high gamma flux. The equivalent dose rate in the configuration of Figure 11 is of the order of 0.6 µSv/h, with the count rate below 15 cps. A SciFi detector can easily withstand 10 5 -10 6 cps, that would roughly correspond to an equivalent dose rate of the order of 4-40 mSv/h. These results prove that SciFi detectors can successfully be employed in the MICADO project in order to monitor radwaste drums in a storage site in the medium/long term.
An interesting exercise has been done in such a framework, to investigate the possibility of monitoring a reference case of radwaste drum. Such a case consists of a Radioactive Waste Package assembly, enclosed in a standard 220 liter drum (86 cm height, 57 cm diameter) with a PVC (poly(vynyl-chloride)) matrix. The drum, sketched in All of these isotopes are assumed to be dispersed in the matrix, and to give rise to an equivalent dose rate of 2.4 µSv/h at 1 cm [24]. We observe that the majority of the dose rate is due to 60 Co and 152 Eu, which have much higher activity and emit gamma rays from 344 to 1330 keV. Assuming the same detection efficiency as in the case of 22 Na (Eq.2 and Figure 10), the cps to equivalent dose rate coefficient is ≈ 1.18 (µSv/h) / cps, and vice versa the equivalent dose rate to cps coefficient is ≈ 0.85 cps / (µSv/h) [23]. This implies that the expected net count rate in a SciFi is of the order of 2.4 x 0.85 ≈ 2 cps, which has to be summed to the background count rate which should be of the same order of magnitude.
In one minute the statistical uncertainty of the measured counting rate would be better than 10%, and in ten minutes better than 3%. This means that any initial counting asymmetry between the four fibers is quickly appreciable, as well as any change that might occur due to internal displacement of the waste and/or deterioration of the drum. As for the radiation damage, we assume it is neglibible as according to the test results reported in [25] the SciFi detectors would not be affected even after a hundred years of exposure at their highest dose rate counting. Possible failures in the long term will most likely be due to electronics.

Conclusions
The simulations, tests and measurements we have done allowed us to show that the SciFi technology is a good candidate for gamma radiation monitoring of radioactive waste. The 36 detectors we built have a reasonably uniform behavior, in light of their robustness, low cost, and simple construction based on commercial components. We are now planning to test them soon in a real radwaste storage site in the framework of the MICADO project.