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Measurement of neutron spectra in a silicon filtered neutron beam using stilbene detectors at the LVR-15 research reactor
Author’s Accepted Manuscript Measurement of Neutron Spectra in a Silicon Filtered Neutron Beam Using Stilbene Detectors at the LVR-15 Research Reactor Michal Košťál, Jaroslav Šoltés, Ladislav Viererbl, Zdeněk Matěj, František Cvachovec, Vojtěch Rypar, Evžen Losa www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(17)30173-2 http://dx.doi.org/10.1016/j.apradiso.2017.06.026 ARI7930
To appear in: Applied Radiation and Isotopes Cite this article as: Michal Košťál, Jaroslav Šoltés, Ladislav Viererbl, Zdeněk Matěj, František Cvachovec, Vojtěch Rypar and Evžen Losa, Measurement of Neutron Spectra in a Silicon Filtered Neutron Beam Using Stilbene Detectors at the LVR-15 Research Reactor, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.06.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Measurement of Neutron Spectra in a Silicon Filtered Neutron Beam Using Stilbene Detectors at the LVR-15 Research Reactor Michal Košťál*1; Jaroslav Šoltés1, Ladislav Viererbl1, Zdeněk Matěj2, František Cvachovec3, Vojtěch Rypar1, Evžen Losa1 1
Research Center Rez Ltd, 250 68 Husinec-Rez 130, Czech Republic Masaryk University, Botanická 15, Brno 612 00, Czech Republic 3 University of Defence, Kounicova 65, Brno 612 00, Czech Republic 2
Email: [email protected] Telephone: +420266172655
Key words: LVR-15; Neutron spectrometry; Stilbene, Detector calibration; Si neutron filter; Reactor dosimetry
Abstract A well-defined neutron spectrum is an essential tool for calibration and tests of spectrometry and dosimetry detectors, and evaluation methods for spectra processing. Many of the nowadays used neutron standards are calibrated against a fission spectrum which has a rather smooth energy dependence. In recent time, at the LVR-15 research reactor in Rez, an alternative approach was tested for the needs of fast neutron spectrometry detector calibration. This process comprises detector tests in a neutron beam, filtered by one meter of singlecrystalline silicon, which contains several significant peaks in the fast neutron energy range. Tests in such neutron field can possibly reveal specific problems in the deconvolution matrix of the detection system, which may stay hidden in fields with a smooth structure and can provide a tool for a proper energy calibration. Test with several stilbene scintillator crystals in two different beam configurations supplemented by Monte-Carlo transport calculations have been carried out. The results have shown a high level of agreement between the experimental data and simulation, proving thus the accuracy of used deconvolution matrix. The chosen approach can, thus, provide a well-defined neutron reference field with a peaked structure for further tests of spectra evaluation methods and scintillation detector energy calibration. 1 Introduction Several references on possible neutron sources with a peaked structure in their energy spectra can be found. The simplest are represented by radioisotope sources containing beryllium and an alpha emitter. The major disadvantage of such sources is that they cannot be considered as standard because the beryllium and alpha emitter content and their distribution in the source are often not well known. Well defined sources are often based on particle accelerator systems. Basically, accelerator induced quasi mono-energetic neutron lines can be tuned in a broad energy range, even up to 200 MeV, as referred in Nolte et al 2004. This energy range is of special interest namely in fields of medical physics and accelerator driven systems. These lines are produced via (p,n) reactions, and can also serve as a standard for low and intermediate energy neutrons by modifying the energy of the primary protons and the scattering angle. Alternative methods, using selected nuclear reactions to produce low energy mono lines (below 50 keV), suitable for dosimeter tests are mentioned by Lamirand et al 2010.
Another method is based on neutron field modification by a certain material or combination of materials with resonant cross sections. Such neutron filters are capable to create welldefined tailorable neutron spectra having energies in a fairly narrow band from a white or fission spectrum in cases of nuclear reactors. Most commonly, these sources and filters are aimed at intermediate neutron energies convenient for boron neutron capture therapy, where a significant reduction of doses from gammas, thermal and fast neutrons is desired as well. Suitable materials convenient for the creation of such neutron beams in intermediate energies have been already proposed in the past and are mentioned e.g. by Nascimento et al 2007. If the transmission parameters are well known, derived from the material cross section data, even algorithms for quasi mono-energetic lines design based on a certain material combination can be developed (Mansy et al 2015). The neutron source described in this article is based on filtering of a reactor fission spectrum by a 1 m thick silicon layer. The described arrangement was primarily designed as a thermal neutron beam used for neutron radiography (Soltes et al 2016). Since 2011, neutron radiography is experiencing a renaissance in the Czech Republic as it has been reintroduced after nearly 20 years at the LVR-15 research reactor in Rez. The filtered neutron beam was tested for utilization in the calibration of organic scintillators, used in neutron spectrometry, thanks to its suitable properties and a peak character in the resonance and fast energy regions. The first series of tests shows excellent results and demonstrates the suitability and ability of this field to be used as a reference field for further calibration and tests of neutron instrumentation.
2 The LVR-15 Reactor The LVR-15 (see Figure 1) is a multipurpose research reactor representing a flexible neutron source for applied research and commercial activities (Koleska et al 2015). It is a light water cooled and moderated, tank type research reactor operated at a thermal power of 10 MW. The reactor core consists of rectangular IRT-4M fuel elements. It is tube type fuel made of aluminum cladding with UO2 dispersed into an aluminum matrix. For use of fuel with control rods, a central tube is removed and replaced with an absorber guide with a neutron absorption rod inserted (see Figure 2). Together with fuel there are also beryllium reflector blocks, water and air zones. It can be easily modified for specific purposes and equipped by vertical irradiation channels or another experimental infrastructure. The core is relatively well described through experimentally validated models in MCNP (Koleska et al 2016) and NODER (Ernest, 2006) codes. The reactor is utilized for fundamental and extended material research (together with the low power LR-0 reactor they provide a technical and experimental base and support for the Czech nuclear industry), sample irradiation and isotope production. The standard production capabilities of the reactor cover 99Mo from HEU and LEU targets, 153 Sm, 192Ir, 177Lu, 203Hg and 127Xe. The sample irradiation and isotope production can be performed in several vertical channels also including the pneumatic rabbit system and a silicon doping facility (a rotating vertical channel). Irradiation loops under specified conditions (pressure, temperature, sample tensility) can be installed inside the core for material tests and irradiation. The reactor is equipped with 9 horizontal channels providing neutron beams for further utilization like neutron scattering experiments, prompt gamma activation analysis, neutron radiography, etc. (see Figure 1) and one additional special purpose epithermal neutron beam of the former experimental boron neutron capture therapy facility.
Figure 1: Upper and side view of LVR-15, a red oval shows position of HK-1 beam.
Figure 2: IRT-4M fuel type, loaded in LVR-15 research reactor 3
Experimental and calculation methods
3.1 Measuring arrangement Neutron spectra in the 0.8 to 20 MeV energy range were measured via the proton-recoil method using stilbene scintillator (with dimensions 10×10 mm) with neutron and gamma pulse shape discrimination (see Figure 3) (hereinafter stilbene). The two-parameter spectrometric system (Veskrna et al 2014) is fully digitized and able to process up to 300 000 impulses per second. This is possible due to the use of an active voltage divider for photomultiplier. The input analog signal from the photomultiplier is divided in the DC (direct current) coupled preamplifier into two branches. Each branch is amplified differently in a ratio of 1:8 and digitized by separate analog to digital converters with a 12-bit resolution. Such a difference in the amplification increases the dynamic range of particle energies so that the spectrometer is capable of processing and increasing the signal-to-noise ratio. Two fast analog to digital converters (ADC) working at a 500 MS/s sampling frequency are used and the digital signal processing is implemented into a field-programmable gate array (FPGA). Therefore, it is able to process all the data flow from both analog to digital converters without any dead time.
Figure 3: Illustration of the integration method used in the discriminating particle response (more information on the range of parameters may be found in Matej 2014)
The digital signal i(t) from the ADC is filtered by a software low-pass averaging filter. It is necessary to solve the problem related with the DC coupling offset voltage from the output of the photomultiplier. For this purpose, the normal voltage level (zero voltage, iout) is calculated as (1). This algorithm processes the sampled signal (iout(t)) before each triggered impulse in the time tt, and averages 50 registered samples computing the value of iout. In order not to count part of the measured pulse, the average is calculated by 20 samples before the triggered time tt. Before the discrimination of the neutrons and gamma pulses the iout (t) value is subtracted from all samples. tt 70
Vout ( iout (t )) / 50
(1)
tt 20
The discrimination algorithm works with two parameters. The first parameter of the spectrometric system is the type of the detected particle (impulse). For this purpose, pulse
shape discrimination is used. The pulse shape discrimination (D) is realized inside the FPGA by an integration method which uses as a working principle the comparison of the area delimited by a part of the trailing edge of the measured response (Q1) limited by the whole response (Q2). Q1 and Q2 areas as the integrals over time are expressed in (2) and their illustration is shown in Figure 3. t2
t2
t1
t0
Q1 i(t )dt , Q2 i(t )dt , D E
Q1 Q2
(2)
t2
i(t )dt
(3)
tt 16
Energy [channel]
The computational complexity of the implemented integration method is linear, and therefore, it is suitable for online measurements processing a high number of impulses per second. The second parameter (E), of the spectrometric system, is the energy of the detected particle. The energy is evaluated from the integral (3) of the whole response where (tt) is time when the impulse was triggered. As is shown in Figure 2 the impulse begins before the time tt, therefore, the lower limit of the integration is set to tt-16 to cover the whole area of energy range. The diagram of the discrimination parameter vs the channel energy for the detection system is plotted in Figure 4. The yellow line means discrimination level.
Discrimination parameter [u.a.] Figure 4.: Separation between neutrons and gamma
The next step in the neutron spectrum evaluation is the deconvolution of the recoiled proton spectra. When evaluating experimental data, we meet equation 1 which describes the process
of measurement using various devices. It can be solved in discrete form (see (2)). The linear system (2) cannot be solved by usual procedures since the matrix A is usually ill-conditioned.
g ( x) =
A( x, y) f ( y)dy
(4)
(I )
g = Af
(5)
g is the measured proton (resp. electron) spectrum, i.e. experimental data. (Neutrons are
detected by means of protons and photons by means of electrons.) A is the detector response function. It is determined by the Monte Carlo method and measurement of mono-energetic sources of neutrons and photons. f is the resulting neutron (resp. gamma) spectrum to be found (in the units of m-2.s-1MeV-1 ) The above-mentioned system of equations can be solved by more approaches. One of them is the maximum likelihood estimation, which is a standard statistical tool for point estimations. For the maximizing of the likelihood function, we use a general iterative algorithm called Expectation Maximization (EM). The iteration formula for the EM algorithm can be get easily (see Cvachovec et al 2002):
g j a ji
n
f i ( k 1) = f i ( k ) j =1
n
f
(k ) l
(6)
a jl
l =1
The uncertainties of the evaluated neutron spectrum should, above all, include uncertainties originating from the measured proton spectrum, where the following contributions were identified: Uncertainties resulting from the stochastic nature of the measurement; Uncertainties resulting from operation of the measuring device; Uncertainties resulting from output monitoring; Uncertainties caused by energetic calibration. More can be found also in Cvachovec J. et al 2007.
Figure 5.: Gamma spectra of 60Co (blue line) with it derivation used for energy calibration.
The energy calibration of the scintillator response to neutrons is realized indirectly via gamma rays by knowledge of the gamma to neutron conversion ratio and is mainly performed using 60 Co and 137Cs radioactive sources. The Compton edge energy is measured due to the absence of the full absorption peak in spectra, which is a property of a low Z of organic detector. Its energy can be simply determined by the Compton formula, and its position in the spectra can be found by differentiation of the spectra, as the Compton edge is in an inflection point. Figure 5 presents the spectrum of 60Co photons together with its derivative, with marked Compton edges at 963.2 keV and 1117.6 keV. The dependence between the peak channel numbers and the gamma ray energies was observed to be linear, confirming the linearity of the detection system (scintillator, photomultiplier and detection electronics). The scintillator light output for gamma and neutrons which are important in determination of response function and calibration of experimental data were obtained in (Kuchtevic et al 1971) and was modified and later verified by testing in five mono-energetic lines (1.2 MeV, 2.5 MeV, 5 MeV, 14.6 MeV and 19 MeV) in Physikalisch-Technische Bundesanstalt, Braunschweig (see Cvachovec et al 2002).
3.2 Experimental set up In the presented experiments (Figure 6), neutron spectra were measured at the end of the horizontal beam port HK-1 indicated in Figure 1. The beam port is filled with 100 cm thick plug of high purity silicon which is a combination of 4 cylindrical Si monocrystals (diameter 7.8 cm, length of 50 cm, 20 cm, 20 cm and 10 cm, respectively) encapsulated in aluminum (outer diameter 9.95 cm). The primary focus of such an arrangement is to obtain a high thermal neutron flux in the order of 2×108 for the needs of neutron transmission radiography. For this reason, the fast neutron flux was minimized to around 5×104 m-2·s-1. Although fairly low, this intensity is almost ideal for 10×10 mm stilbene scintillation crystal measurements. It is worth noting, that in the case of fast neutron spectrometry, thermal neutrons are not the primary issue, but they increase the photon flux by radiative captures in the volume of the
detector and the surrounding structures, thus they may significantly increase the background count rate in the detector. Thermal neutron flux had to be attenuated by the instalment of additional thermal neutron filters into the neutron beam – a 6Li (bounded in LiCO3) filter and a nat.Cd filter. The first filter is realized by a thin hollow plastic disc containing 6LiCO3 powder, because it has a lower photon production rate compared to cadmium. Behind the 6Li plate a 4 cm thick Bi polycrystalline cylindrical ingot was placed for additional shielding of the fast gammas from the core, followed by a 1 mm Cd plate for additional thermal neutron shielding and another 8 cm of polycrystalline Bi for the suppression of both primary gammas from the core and secondary gammas emitted by neutron capture on the cadmium filter. Bismuth was used due to its suitable properties, namely a strong gamma attenuation and a low neutron attenuation. The relative transmissivity of parallel neutron beam calculated for various components of beam port are plotted in Figure 7. In several measurements, a 10 cm thick Plexiglas filter (PMMA hereafter) was added inbetween the cadmium plate and the second bismuth block. PMMA can be used for suppression of the neutron flux, especially in the lower energies (up to 1 MeV). Such attenuation is important, as it allows tests of bigger detector crystals, for which the recent filtered flux rate is still too high. Moreover, due to the profile of PMMA’s cross section, reflected in its transmisivity (Figure 7) the higher energy peaks become more notable in Si filtered spectra.
Figure 6.: Radial cross-section of the HK-1 horizontal channel, dimensions in [cm], the diameter of beam tube is 10.0 cm.
1E+0 1E-1 1E-2
Permeability [-]
1E-3 1E-4 1E-5 1E-6 1E-7 Permeability of 10cm plexiglass block
1E-8
Permeability of 12cm Bi block
1E-9
Permeability of 100cm Si block 1E-10 0
1
2
3
4
5
6
7
8
9
10
Energy [MeV]
Figure 7.: Calculated neutron permeability of various filter components used in the neutron beam of the HK-1 channel
3.3 Calculation methods Along with the measurements, calculations were performed using the MCNP6 Monte Carlo code (T. Goorley, et al 2012) with the ENDF/B-VII.0 (Chadwick et al 2006) data library. The calculations performed with rigorously defined model of the whole reactor core, as a neutron source, together with its surroundings show, however, slow convergence in cases where the neutron spectra are calculated distant from the core. Thus, for the neutron transport though the HK-1 beam tube a stand-alone model of a section of the channel tube was created, with a divergent beam emitted from a point source substituted for the reactor core (similar to Figure 6) (see Viererbl et al 2012). A similar approach was described in (Kostal et al 2011). Neutron spectra for the beam source input were calculated using the precise core model with MCNP 6. In the separate channel model, the neutron spectra, corresponding to each separate experimental setup, were calculated at the beam exit after the series of filters, corresponding to the detection position. The calculation uncertainty is in all cases well below 10 % (from 0.5 % in peaks to 10 % in minima), thus meeting the convergence criteria. The resulted calculated fluxes were broadened with regard to the experimentally determined stilbene resolution (Cvachovec et al 2002) presented in Table 1. The effect of various filter components on the shape of the neutron spectrum at the beam exit can be seen in Figure 8.
Table 1.: Energy resolution of a stilbene detector used for neutron spectra measurements (Osmera and Zaritsky 2008) Elow [MeV] 0.498 0.608
FWHM % 20.49 19.73
Elow [MeV] 2.37 2.47
FWHM % 10.97 10.67
0.743 0.821 1 1.35 1.65 1.92 2.23 2.35
19.07 18.3 17.02 15.05 13.63 12.5 11.42 11.02
2.73 3.01 3.68 4.97 6.07 7.41 8.61 10
9.96 9.3 8.05 6.59 5.91 5.46 5.25 5.13
Empty channel (no filters)
1E+17
Si filter (radiography configuration) 1E+15
Si-Li-Cd-Bi Si-Li-Cd-Bi-PMMA
Flux density [a.u.]
1E+13 1E+11 1E+9 1E+7 1E+5 1E+3 1E+1 1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
1E+1
Energy [MeV]
Figure 8.: Calculated energy neutron spectra at the exit of the beam port
3.4 Proton recoil spectra The neutron spectra are evaluated from proton spectra using a suitable deconvolution method. The applied method uses a maximum likelihood estimation algorithm for the deconvolution (Cvachovec J. et al 2007) of the measurement evaluation performed with a stilbene spectrometer. The proton recoil spectra are presented in Figure 9. Additionally, a proton recoil spectrum of a 235U fission spectrum field measured at a special 3.3% enriched LR-0 reactor core (see Kostal et al 2017) is presented in the plot. The comparison shows that in the 235U fission spectrum the shape of the proton recoil spectrum is smooth, reflecting the smooth shape of fission spectrum, while the silicon filtered spectra show distinguishable edges in the structure, due to the peak structure of these neutron spectra.
1E+7
Pulses in channel
1E+6
1E+5
1E+4
1E+3 Proton recoil spectrum behind 1m Si + 10cm PMMA 1E+2
Proton recoil spectrum behind 1m Si Proton recoil spectrum in fission spectra [Kostal et al 2017]
1E+1 0
1
2
3
4
5
6
7
8
9
10
Recoil proton energy [MeV]
Figure 9.: Proton recoil spectrum of fission spectrum and silicon filtered spectra
4 Results 4.1 Neutron spectra The resultant spectrum is evaluated as an average of 10 measurements which are assumed as independent because they were performed with various stilbene crystals and also photomultipliers. The measurements were realized with various voltage on photomultiplier, which is the only parameter simply varied by user. The linearity of each setting was tested by means of linearity in Compton edge positions. Various crystals were tested on same photomultiplier and same setting. All measurements were performed with same spectrometric system, and calibration constants for all setting were found (see chapter 3.1). The neutron spectra are in Figure 10, and the corresponding data are listed in Table 2. The presented experimental uncertainty is obtained as a standard deviation obtained by considering all the partial experiments. When taking into account the independent uncertainty estimation using its main sources, the value is comparable with this standard deviation. The main assumed uncertainty sources are the uncertainties of deconvolution, the energy calibration and also the detector resolution. These major sources connected with experiment can contribute by 2 – 7 % to overall uncertainty. The statistical counting uncertainties are in all cases below 1 %, thus, they can be neglected in the evaluations. An uncertainty of 10 – 20 % is observed in the region of the peaks in few groups, and is caused mostly by numerical problems in the iteration process during the neutron spectrum deconvolution. Based on the agreement between several independent experiments, it can be deduced, that the used experimental system gives reliable results. It appears that the spectrum shape is in agreement with measurements. This might indicate correct energetic distribution of silicon cross sections. However the peak magnitudes between calculation and measurement vary. This state can be attributed mainly to incorrect angular distribution of neutrons scattered on silicon. Some effect might also play an incorrectly
determined stilbene resolution. But this fact would not explain all discrepancies visible in peak magnitudes. The calculated spectrum plotted in Figure 10 was broadened with respect to stilbene resolution (Table 1). The peak structure in detail is notable from Figure 11. It is notable, that many of the peaks are formed by the superposition of two or more lines. Despite the poor resolution of stilbene detectors, the shape of the experimental spectra indicates that the broad peak in the 3.2 MeV – 4.7 MeV region is actually result of the mixing of two finer peaks. The calculation shows, its origin by merging of two peaks at 3.65 MeV and 4.185 MeV. This result also shows that the resolution of a stilbene detector is relatively acceptable also for peak structure spectra measurements.
1 Experiment Calculation
Flux density [a.u.]
0.1
0.01
0.001
0.0001 0
1
2
3
4
5
6
7
8
Energy [MeV]
Figure 10.: Calculated and measured neutron spectra at the exit of the beam port
9
10
1E+2
1.2
1E+1
0.6
1E+0
0
1E-1 0
1
2
3
4
5
6
7
8
Energy [MeV]
Calculated broaden neutron spectra
Calculated spectra in smooth structure
Figure 11.: Calculated broadened and raw (un-broadened) neutron spectra at the exit of the beam port
Table 2.: Measured neutron spectra behind Si filter Eup [MeV] 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1
Group flux [a.u] 7.68E-2 1.40E-1 2.73E-1 1.43E-1 7.13E-2 3.47E-2 4.38E-2 5.65E-2 7.01E-2 6.04E-2 5.06E-2 3.89E-2 3.00E-2 2.55E-2 2.15E-2 1.91E-2 1.72E-2 1.74E-2 1.78E-2 1.79E-2 1.77E-2 1.62E-2
Rel. Unc. [%] 22.2% 15.5% 14.8% 28.8% 18.8% 22.8% 9.0% 6.7% 13.2% 10.2% 7.9% 7.1% 7.7% 6.8% 5.9% 6.2% 5.5% 7.0% 8.8% 10.2% 11.1% 12.3%
Eup [MeV] 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 6.1 6.2
Group flux [a.u] 3.19E-2 2.95E-2 2.20E-2 1.63E-2 1.15E-2 8.15E-3 6.76E-3 5.77E-3 6.08E-3 6.48E-3 8.95E-3 1.30E-2 2.10E-2 3.37E-2 3.30E-2 3.10E-2 2.17E-2 1.53E-2 1.35E-2 1.21E-2 1.37E-2 1.61E-2
Rel. Unc. [%] 6.3% 6.9% 7.8% 12.0% 10.3% 10.6% 8.1% 14.4% 14.4% 17.2% 14.2% 18.5% 13.7% 10.5% 3.9% 13.9% 12.9% 12.6% 10.4% 10.7% 9.8% 13.7%
Eup [MeV] 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2 9.3
Group flux [a.u] 5.05E-3 4.29E-3 3.71E-3 3.66E-3 3.58E-3 3.55E-3 3.52E-3 3.35E-3 3.24E-3 3.15E-3 2.99E-3 2.86E-3 2.75E-3 2.56E-3 2.38E-3 2.14E-3 1.88E-3 1.68E-3 1.54E-3 1.54E-3 1.51E-3 1.40E-3
Rel. Unc. [%] 15.4% 13.3% 17.1% 17.5% 16.6% 13.5% 10.1% 9.9% 12.5% 15.3% 13.7% 10.1% 8.8% 8.7% 9.2% 11.1% 12.3% 11.2% 12.2% 11.8% 9.2% 7.5%
Spectra in smooth structure [a.u]
Broaden spectra [a.u]
1.8
3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4
1.50E-2 1.80E-2 2.28E-2 4.02E-2 6.98E-2 6.77E-2 6.41E-2 4.67E-2 3.44E-2
13.9% 18.2% 22.6% 20.5% 20.7% 6.1% 13.1% 11.3% 7.8%
6.3 6.4 6.5 6.6 6.7 6.8 6.9 7 7.1
2.19E-2 2.91E-2 2.83E-2 2.70E-2 2.11E-2 1.63E-2 1.23E-2 9.20E-3 6.79E-3
12.5% 9.6% 4.3% 8.8% 8.3% 8.5% 11.1% 13.9% 13.9%
9.4 9.5 9.6 9.7 9.8 9.9 10 10.1 10.2
1.31E-3 1.20E-3 1.11E-3 9.54E-4 8.03E-4 6.60E-4 5.40E-4 4.47E-4 3.72E-4
7.7% 7.1% 11.1% 15.4% 18.5% 15.8% 15.6% 16.8% 19.6%
4.2 Neutron spectra filtered by PMMA The intensity of the neutron spectrum behind the Si filter is suitable for tests of small crystals of the dimensions ~10×10 mm. Bigger crystals are more sensitive, thus, the flux of thermal and resonance neutrons at the beam exit might be too high for such crystals. From this reason, the use of PMMA filters, which suppress the neutron flux level at lower energies, however, still preserve the peaked structure of the neutron spectrum in the fast energy region, were also tested. Plexiglas was tested due to suitable behavior while placed in neutron fields (see Figure 7). The influence of adding the PMMA filter into the beam is shown in Figure 11. The comparison shows that hydrogen rich compounds are suitable for suppression of the lower energy part of the silicon filtered spectrum for bigger crystals tests. The application of the PMMA filter will increase the relative count rate at higher energies, which is essential for stilbene detector tests.
1E+4
Measurement Calculation Measurement plexi
Flux density [a.u]
1E+3
Calculation plexi
1E+2
1E+1
1E+0 0
1
2
3
4
5 Energy [MeV]
6
7
8
9
10
Figure 12.: Calculated and measured neutron spectra at the beam port exit with and without the PMMA filter
5 Conclusions A Si filter was found to be valuable for realizing proper neutron field with peak structure applicable in neutron detector testing. This property makes it an ideal tool for calibration and testing of detector devices. Both, measured and calculated, neutron spectra show a peak structure of the silicon filtered neutron spectrum in the region of 1 – 10 MeV. The results were reproduced in 10 independent measurements with different setups of the spectrometric system and with different components (organic crystals, photomultipliers). Result indicates possible problems in definition of energetic distribution of silicon cross sections. This conclusion is based on observed incoherent magnitudes of calculated and measured peaks. The practical use of Plexiglas for the attenuation of the lower energy part of fast and resonance neutrons was verified with the same rate of C/E agreement as without this attenuator. Thanks to such characteristics, the resulting filtered neutron spectrum can be considered as a reference neutron field suitable for scintillation detectors calibration and tests. The comparison between the calculated and measured neutron spectra shows a very good agreement.
6 Acknowledgements The presented work was financially supported by the Ministry of Education, Youth and Sport Czech Republic Project LQ1603 (Research for SUSEN) within the SUSEN Project (established in the framework of the European Regional Development Fund (ERDF) in project CZ.1.05/2.1.00/03.0108) and with the use of infrastructures Sustainable energy – SUSEN and Reactors LVR-15 and LR-0, which were financially supported by the Ministry of Education, Youth and Sports - projects LM2015093 and LM2015074 and with the support of the mathematical and physical research project of Cvachovec F. et al 2006
7 References Cvachovec et al 2002 CVACHOVEC, F., CVACHOVEC, J. and TAJOVSKÝ, P. Anisotropy of Light Output in Response of Stilbene Detectors. Nuclear methods and Instruments in Physics Research A, 2002, vol. 476, pp. 200-202. Cvachovec J. et al 2007 Cvachovec J., Cvachovec F.,: Maximum Likelihood Estimation of a Neutron Spectrum and Associated Uncertainties, Advances in Military Technology, Vol.1, No. 2 January 2007 , pp. 5 – 28 Cvachovec F. et al 2006 F. Cvachovec, Z. Bures, M. Komarek at all, Support of Mathematical and Physical Research, Final Report of Specific Research in 2006, University of Defence in Brno, pp 2 – 6 Chadwick et al 2006 M.B. Chadwick, P. Obložinský, M. Herman et al ENDF/B-VII.0: Next Generation Evaluated Nuclear Data Library for Nuclear Science and Technology, Nuclear Data Sheets, Vol. 107, Issue 12, 2006, pp. 2931-3060 Ernest, 2006, Ernest, J., 2006, Abstrakt výpočtového programu NODER-Rev.1, Řež, Czech Republic, ÚJV Řež. Koleska et al 2015 J. Ernest, M. Vins, J. Stehno, Capabilities of the LVR-15 research reactor for production of medical and industrial radioisotopes, J. of Radioanal Nucl. Chem. (2015), Vol. 305, pp. 51–59
Koleska et al 2016 M. Koleška, L.Viererbl, M. Marek, J. Ernest, M. Šunka, M. Vinš, Determination of IRT-2M fuel burnup by gamma spectrometry, Appl. Rad. and Isot., Vol. 107, (2016), pp. 92-97 Kostal et al 2011 M. Košťál, F. Cvachovec, J. Cvachovec, B. Ošmera, W.Hansen, Determination of AKR-2 leakage beam and verification at iron and water arrangements, Ann. of Nucl. En., Vol. 38, (2011), pp. 157–165 Kostal et al 2017 M. Košťál, Z. Matěj, F. Cvachovec, V. Rypar, E. Losa, J. Rejchrt, F. Mravec, M. Veškrna, Measurement and calculation of fast neutron and gamma spectra in well defined cores in LR-0 reactor, Appl. Rad. and Isot., Vol. 120, (2017), pp. 45–50 Mansy et al 2015 M.S. Mansy, I.I. Bashter, M.S. El-Mesiry, N. Habib, M. Adib, Filtered epithermal quasi-monoenergetic neutron beams at research reactor facilities, Appl. Rad. and Isot., Vol. 97, (2015), pp. 78–83 Matej 2014 Digitalization of spectrometric system for mixed field of radiation. Saarbrücken: LAP LAMBERT Academic Publishing, 2014. 136 s. ISBN 978-3-659-59970-5. Nascimento et al 2007 F. Nascimento, A.R. Ramos, A.C. Fernandes, M. Felizardo, T. Morlat, J.G. Marques, c, F. Giuliani, T.A. Girard, J.A. Paixão, Optimization of filtered neutron beams for the calibration of superheated droplet detectors at the RPI, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 580, (2007), pp. 282–285 Nolte et al 2004 R. Nolte, M. S. Allie, R. Bottger, F. D. Brooks, A. Buffler, V. Dangendorf, H. Friedrich, S. Guldbakke, H. Klein, J. P. Meulders, D. Schlegel, H. Schuhmacher and F. D. Smit, Quasi-monoenergetic neutron reference fields in the energy range from thermal to 200 MeV, Radiation Protection Dosimetry (2004), Vol. 110, Nos 1-4, pp. 97-102 Lamirand et al 2010 V. Lamirand, V. Gressier, A. Martin, D.J. Thomas, Comparison of nuclear reactions for the production of monoenergetic neutron fields with energies below 100 keV, Radiation Measurements, Vol. 45, (2010), pp. 1112–1115 Osmera and Zaritsky 2008 Ošmera, B., Zaritski, S., 2008. WWER-1000 Benchmarks in LR-0 Experimental Reactor, UJV Rez Report No.12993-R, pp. 1–106. Soltes et al 2015 Soltes, J., Viererbl, L., Lahodova, Z., Koleska, M., Vins, M., Thermal Neutron Filter Design for the Neutron Radiography Facility at the LVR-15 Reactor. IEEE Transactions on Nuclear Science 63 (3), 1640-1544. Soltes et al 2016 Soltes, J., Viererbl, L., Vacik, J., Tomandl, I., Krejci, F., Jakubek, J., The New Facilities for Neutron Radiography at the LVR-15 Reactor. Journal of Physics: Conference Series 746. T. Goorley, et al 2012. T. Goorley, et al., "Initial MCNP6 Release Overview", Nuclear Technology, 180, pp 298-315 (Dec 2012). Kuchtevic et al 1971 V.I. Kuchtevic, O. A. Trykov, L.A. Trykov, Odnokrystalnyj scintilljacionnyj spektrometr, Atomizdat, Moscow 1971, (in Russian) Veskrna et al 2014 Digitalized two parametric system for gamma/neutron spectrometry, Martin Veškrna, Zdeněk Matěj, Filip Mravec, Václav Přenosil, František Cvachovec, Michal Košťál, 18th Topical Meeting of the Radiation Protection & Shielding Division of ANS, Knoxville, TN USA, 2014, Viererbl et al 2012 Viererbl, L., Soltes, J., Lahodova, Z., Kostal, M., Vins, M., Horizontal Channel for Neutron Radiography and Tomography in LVR-15 Research Reactor, presented at RRFM/IGORR 2012.
Highlights ► Neutron spectra behind 1m Si neutron filter ► Comparison of the experimental data with calculation ► Discrepancies between calculational and experimental neutron spectra
PII: DOI: Reference:
S0969-8043(17)30173-2 http://dx.doi.org/10.1016/j.apradiso.2017.06.026 ARI7930
To appear in: Applied Radiation and Isotopes Cite this article as: Michal Košťál, Jaroslav Šoltés, Ladislav Viererbl, Zdeněk Matěj, František Cvachovec, Vojtěch Rypar and Evžen Losa, Measurement of Neutron Spectra in a Silicon Filtered Neutron Beam Using Stilbene Detectors at the LVR-15 Research Reactor, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.06.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Measurement of Neutron Spectra in a Silicon Filtered Neutron Beam Using Stilbene Detectors at the LVR-15 Research Reactor Michal Košťál*1; Jaroslav Šoltés1, Ladislav Viererbl1, Zdeněk Matěj2, František Cvachovec3, Vojtěch Rypar1, Evžen Losa1 1
Research Center Rez Ltd, 250 68 Husinec-Rez 130, Czech Republic Masaryk University, Botanická 15, Brno 612 00, Czech Republic 3 University of Defence, Kounicova 65, Brno 612 00, Czech Republic 2
Email: [email protected] Telephone: +420266172655
Key words: LVR-15; Neutron spectrometry; Stilbene, Detector calibration; Si neutron filter; Reactor dosimetry
Abstract A well-defined neutron spectrum is an essential tool for calibration and tests of spectrometry and dosimetry detectors, and evaluation methods for spectra processing. Many of the nowadays used neutron standards are calibrated against a fission spectrum which has a rather smooth energy dependence. In recent time, at the LVR-15 research reactor in Rez, an alternative approach was tested for the needs of fast neutron spectrometry detector calibration. This process comprises detector tests in a neutron beam, filtered by one meter of singlecrystalline silicon, which contains several significant peaks in the fast neutron energy range. Tests in such neutron field can possibly reveal specific problems in the deconvolution matrix of the detection system, which may stay hidden in fields with a smooth structure and can provide a tool for a proper energy calibration. Test with several stilbene scintillator crystals in two different beam configurations supplemented by Monte-Carlo transport calculations have been carried out. The results have shown a high level of agreement between the experimental data and simulation, proving thus the accuracy of used deconvolution matrix. The chosen approach can, thus, provide a well-defined neutron reference field with a peaked structure for further tests of spectra evaluation methods and scintillation detector energy calibration. 1 Introduction Several references on possible neutron sources with a peaked structure in their energy spectra can be found. The simplest are represented by radioisotope sources containing beryllium and an alpha emitter. The major disadvantage of such sources is that they cannot be considered as standard because the beryllium and alpha emitter content and their distribution in the source are often not well known. Well defined sources are often based on particle accelerator systems. Basically, accelerator induced quasi mono-energetic neutron lines can be tuned in a broad energy range, even up to 200 MeV, as referred in Nolte et al 2004. This energy range is of special interest namely in fields of medical physics and accelerator driven systems. These lines are produced via (p,n) reactions, and can also serve as a standard for low and intermediate energy neutrons by modifying the energy of the primary protons and the scattering angle. Alternative methods, using selected nuclear reactions to produce low energy mono lines (below 50 keV), suitable for dosimeter tests are mentioned by Lamirand et al 2010.
Another method is based on neutron field modification by a certain material or combination of materials with resonant cross sections. Such neutron filters are capable to create welldefined tailorable neutron spectra having energies in a fairly narrow band from a white or fission spectrum in cases of nuclear reactors. Most commonly, these sources and filters are aimed at intermediate neutron energies convenient for boron neutron capture therapy, where a significant reduction of doses from gammas, thermal and fast neutrons is desired as well. Suitable materials convenient for the creation of such neutron beams in intermediate energies have been already proposed in the past and are mentioned e.g. by Nascimento et al 2007. If the transmission parameters are well known, derived from the material cross section data, even algorithms for quasi mono-energetic lines design based on a certain material combination can be developed (Mansy et al 2015). The neutron source described in this article is based on filtering of a reactor fission spectrum by a 1 m thick silicon layer. The described arrangement was primarily designed as a thermal neutron beam used for neutron radiography (Soltes et al 2016). Since 2011, neutron radiography is experiencing a renaissance in the Czech Republic as it has been reintroduced after nearly 20 years at the LVR-15 research reactor in Rez. The filtered neutron beam was tested for utilization in the calibration of organic scintillators, used in neutron spectrometry, thanks to its suitable properties and a peak character in the resonance and fast energy regions. The first series of tests shows excellent results and demonstrates the suitability and ability of this field to be used as a reference field for further calibration and tests of neutron instrumentation.
2 The LVR-15 Reactor The LVR-15 (see Figure 1) is a multipurpose research reactor representing a flexible neutron source for applied research and commercial activities (Koleska et al 2015). It is a light water cooled and moderated, tank type research reactor operated at a thermal power of 10 MW. The reactor core consists of rectangular IRT-4M fuel elements. It is tube type fuel made of aluminum cladding with UO2 dispersed into an aluminum matrix. For use of fuel with control rods, a central tube is removed and replaced with an absorber guide with a neutron absorption rod inserted (see Figure 2). Together with fuel there are also beryllium reflector blocks, water and air zones. It can be easily modified for specific purposes and equipped by vertical irradiation channels or another experimental infrastructure. The core is relatively well described through experimentally validated models in MCNP (Koleska et al 2016) and NODER (Ernest, 2006) codes. The reactor is utilized for fundamental and extended material research (together with the low power LR-0 reactor they provide a technical and experimental base and support for the Czech nuclear industry), sample irradiation and isotope production. The standard production capabilities of the reactor cover 99Mo from HEU and LEU targets, 153 Sm, 192Ir, 177Lu, 203Hg and 127Xe. The sample irradiation and isotope production can be performed in several vertical channels also including the pneumatic rabbit system and a silicon doping facility (a rotating vertical channel). Irradiation loops under specified conditions (pressure, temperature, sample tensility) can be installed inside the core for material tests and irradiation. The reactor is equipped with 9 horizontal channels providing neutron beams for further utilization like neutron scattering experiments, prompt gamma activation analysis, neutron radiography, etc. (see Figure 1) and one additional special purpose epithermal neutron beam of the former experimental boron neutron capture therapy facility.
Figure 1: Upper and side view of LVR-15, a red oval shows position of HK-1 beam.
Figure 2: IRT-4M fuel type, loaded in LVR-15 research reactor 3
Experimental and calculation methods
3.1 Measuring arrangement Neutron spectra in the 0.8 to 20 MeV energy range were measured via the proton-recoil method using stilbene scintillator (with dimensions 10×10 mm) with neutron and gamma pulse shape discrimination (see Figure 3) (hereinafter stilbene). The two-parameter spectrometric system (Veskrna et al 2014) is fully digitized and able to process up to 300 000 impulses per second. This is possible due to the use of an active voltage divider for photomultiplier. The input analog signal from the photomultiplier is divided in the DC (direct current) coupled preamplifier into two branches. Each branch is amplified differently in a ratio of 1:8 and digitized by separate analog to digital converters with a 12-bit resolution. Such a difference in the amplification increases the dynamic range of particle energies so that the spectrometer is capable of processing and increasing the signal-to-noise ratio. Two fast analog to digital converters (ADC) working at a 500 MS/s sampling frequency are used and the digital signal processing is implemented into a field-programmable gate array (FPGA). Therefore, it is able to process all the data flow from both analog to digital converters without any dead time.
Figure 3: Illustration of the integration method used in the discriminating particle response (more information on the range of parameters may be found in Matej 2014)
The digital signal i(t) from the ADC is filtered by a software low-pass averaging filter. It is necessary to solve the problem related with the DC coupling offset voltage from the output of the photomultiplier. For this purpose, the normal voltage level (zero voltage, iout) is calculated as (1). This algorithm processes the sampled signal (iout(t)) before each triggered impulse in the time tt, and averages 50 registered samples computing the value of iout. In order not to count part of the measured pulse, the average is calculated by 20 samples before the triggered time tt. Before the discrimination of the neutrons and gamma pulses the iout (t) value is subtracted from all samples. tt 70
Vout ( iout (t )) / 50
(1)
tt 20
The discrimination algorithm works with two parameters. The first parameter of the spectrometric system is the type of the detected particle (impulse). For this purpose, pulse
shape discrimination is used. The pulse shape discrimination (D) is realized inside the FPGA by an integration method which uses as a working principle the comparison of the area delimited by a part of the trailing edge of the measured response (Q1) limited by the whole response (Q2). Q1 and Q2 areas as the integrals over time are expressed in (2) and their illustration is shown in Figure 3. t2
t2
t1
t0
Q1 i(t )dt , Q2 i(t )dt , D E
Q1 Q2
(2)
t2
i(t )dt
(3)
tt 16
Energy [channel]
The computational complexity of the implemented integration method is linear, and therefore, it is suitable for online measurements processing a high number of impulses per second. The second parameter (E), of the spectrometric system, is the energy of the detected particle. The energy is evaluated from the integral (3) of the whole response where (tt) is time when the impulse was triggered. As is shown in Figure 2 the impulse begins before the time tt, therefore, the lower limit of the integration is set to tt-16 to cover the whole area of energy range. The diagram of the discrimination parameter vs the channel energy for the detection system is plotted in Figure 4. The yellow line means discrimination level.
Discrimination parameter [u.a.] Figure 4.: Separation between neutrons and gamma
The next step in the neutron spectrum evaluation is the deconvolution of the recoiled proton spectra. When evaluating experimental data, we meet equation 1 which describes the process
of measurement using various devices. It can be solved in discrete form (see (2)). The linear system (2) cannot be solved by usual procedures since the matrix A is usually ill-conditioned.
g ( x) =
A( x, y) f ( y)dy
(4)
(I )
g = Af
(5)
g is the measured proton (resp. electron) spectrum, i.e. experimental data. (Neutrons are
detected by means of protons and photons by means of electrons.) A is the detector response function. It is determined by the Monte Carlo method and measurement of mono-energetic sources of neutrons and photons. f is the resulting neutron (resp. gamma) spectrum to be found (in the units of m-2.s-1MeV-1 ) The above-mentioned system of equations can be solved by more approaches. One of them is the maximum likelihood estimation, which is a standard statistical tool for point estimations. For the maximizing of the likelihood function, we use a general iterative algorithm called Expectation Maximization (EM). The iteration formula for the EM algorithm can be get easily (see Cvachovec et al 2002):
g j a ji
n
f i ( k 1) = f i ( k ) j =1
n
f
(k ) l
(6)
a jl
l =1
The uncertainties of the evaluated neutron spectrum should, above all, include uncertainties originating from the measured proton spectrum, where the following contributions were identified: Uncertainties resulting from the stochastic nature of the measurement; Uncertainties resulting from operation of the measuring device; Uncertainties resulting from output monitoring; Uncertainties caused by energetic calibration. More can be found also in Cvachovec J. et al 2007.
Figure 5.: Gamma spectra of 60Co (blue line) with it derivation used for energy calibration.
The energy calibration of the scintillator response to neutrons is realized indirectly via gamma rays by knowledge of the gamma to neutron conversion ratio and is mainly performed using 60 Co and 137Cs radioactive sources. The Compton edge energy is measured due to the absence of the full absorption peak in spectra, which is a property of a low Z of organic detector. Its energy can be simply determined by the Compton formula, and its position in the spectra can be found by differentiation of the spectra, as the Compton edge is in an inflection point. Figure 5 presents the spectrum of 60Co photons together with its derivative, with marked Compton edges at 963.2 keV and 1117.6 keV. The dependence between the peak channel numbers and the gamma ray energies was observed to be linear, confirming the linearity of the detection system (scintillator, photomultiplier and detection electronics). The scintillator light output for gamma and neutrons which are important in determination of response function and calibration of experimental data were obtained in (Kuchtevic et al 1971) and was modified and later verified by testing in five mono-energetic lines (1.2 MeV, 2.5 MeV, 5 MeV, 14.6 MeV and 19 MeV) in Physikalisch-Technische Bundesanstalt, Braunschweig (see Cvachovec et al 2002).
3.2 Experimental set up In the presented experiments (Figure 6), neutron spectra were measured at the end of the horizontal beam port HK-1 indicated in Figure 1. The beam port is filled with 100 cm thick plug of high purity silicon which is a combination of 4 cylindrical Si monocrystals (diameter 7.8 cm, length of 50 cm, 20 cm, 20 cm and 10 cm, respectively) encapsulated in aluminum (outer diameter 9.95 cm). The primary focus of such an arrangement is to obtain a high thermal neutron flux in the order of 2×108 for the needs of neutron transmission radiography. For this reason, the fast neutron flux was minimized to around 5×104 m-2·s-1. Although fairly low, this intensity is almost ideal for 10×10 mm stilbene scintillation crystal measurements. It is worth noting, that in the case of fast neutron spectrometry, thermal neutrons are not the primary issue, but they increase the photon flux by radiative captures in the volume of the
detector and the surrounding structures, thus they may significantly increase the background count rate in the detector. Thermal neutron flux had to be attenuated by the instalment of additional thermal neutron filters into the neutron beam – a 6Li (bounded in LiCO3) filter and a nat.Cd filter. The first filter is realized by a thin hollow plastic disc containing 6LiCO3 powder, because it has a lower photon production rate compared to cadmium. Behind the 6Li plate a 4 cm thick Bi polycrystalline cylindrical ingot was placed for additional shielding of the fast gammas from the core, followed by a 1 mm Cd plate for additional thermal neutron shielding and another 8 cm of polycrystalline Bi for the suppression of both primary gammas from the core and secondary gammas emitted by neutron capture on the cadmium filter. Bismuth was used due to its suitable properties, namely a strong gamma attenuation and a low neutron attenuation. The relative transmissivity of parallel neutron beam calculated for various components of beam port are plotted in Figure 7. In several measurements, a 10 cm thick Plexiglas filter (PMMA hereafter) was added inbetween the cadmium plate and the second bismuth block. PMMA can be used for suppression of the neutron flux, especially in the lower energies (up to 1 MeV). Such attenuation is important, as it allows tests of bigger detector crystals, for which the recent filtered flux rate is still too high. Moreover, due to the profile of PMMA’s cross section, reflected in its transmisivity (Figure 7) the higher energy peaks become more notable in Si filtered spectra.
Figure 6.: Radial cross-section of the HK-1 horizontal channel, dimensions in [cm], the diameter of beam tube is 10.0 cm.
1E+0 1E-1 1E-2
Permeability [-]
1E-3 1E-4 1E-5 1E-6 1E-7 Permeability of 10cm plexiglass block
1E-8
Permeability of 12cm Bi block
1E-9
Permeability of 100cm Si block 1E-10 0
1
2
3
4
5
6
7
8
9
10
Energy [MeV]
Figure 7.: Calculated neutron permeability of various filter components used in the neutron beam of the HK-1 channel
3.3 Calculation methods Along with the measurements, calculations were performed using the MCNP6 Monte Carlo code (T. Goorley, et al 2012) with the ENDF/B-VII.0 (Chadwick et al 2006) data library. The calculations performed with rigorously defined model of the whole reactor core, as a neutron source, together with its surroundings show, however, slow convergence in cases where the neutron spectra are calculated distant from the core. Thus, for the neutron transport though the HK-1 beam tube a stand-alone model of a section of the channel tube was created, with a divergent beam emitted from a point source substituted for the reactor core (similar to Figure 6) (see Viererbl et al 2012). A similar approach was described in (Kostal et al 2011). Neutron spectra for the beam source input were calculated using the precise core model with MCNP 6. In the separate channel model, the neutron spectra, corresponding to each separate experimental setup, were calculated at the beam exit after the series of filters, corresponding to the detection position. The calculation uncertainty is in all cases well below 10 % (from 0.5 % in peaks to 10 % in minima), thus meeting the convergence criteria. The resulted calculated fluxes were broadened with regard to the experimentally determined stilbene resolution (Cvachovec et al 2002) presented in Table 1. The effect of various filter components on the shape of the neutron spectrum at the beam exit can be seen in Figure 8.
Table 1.: Energy resolution of a stilbene detector used for neutron spectra measurements (Osmera and Zaritsky 2008) Elow [MeV] 0.498 0.608
FWHM % 20.49 19.73
Elow [MeV] 2.37 2.47
FWHM % 10.97 10.67
0.743 0.821 1 1.35 1.65 1.92 2.23 2.35
19.07 18.3 17.02 15.05 13.63 12.5 11.42 11.02
2.73 3.01 3.68 4.97 6.07 7.41 8.61 10
9.96 9.3 8.05 6.59 5.91 5.46 5.25 5.13
Empty channel (no filters)
1E+17
Si filter (radiography configuration) 1E+15
Si-Li-Cd-Bi Si-Li-Cd-Bi-PMMA
Flux density [a.u.]
1E+13 1E+11 1E+9 1E+7 1E+5 1E+3 1E+1 1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
1E+1
Energy [MeV]
Figure 8.: Calculated energy neutron spectra at the exit of the beam port
3.4 Proton recoil spectra The neutron spectra are evaluated from proton spectra using a suitable deconvolution method. The applied method uses a maximum likelihood estimation algorithm for the deconvolution (Cvachovec J. et al 2007) of the measurement evaluation performed with a stilbene spectrometer. The proton recoil spectra are presented in Figure 9. Additionally, a proton recoil spectrum of a 235U fission spectrum field measured at a special 3.3% enriched LR-0 reactor core (see Kostal et al 2017) is presented in the plot. The comparison shows that in the 235U fission spectrum the shape of the proton recoil spectrum is smooth, reflecting the smooth shape of fission spectrum, while the silicon filtered spectra show distinguishable edges in the structure, due to the peak structure of these neutron spectra.
1E+7
Pulses in channel
1E+6
1E+5
1E+4
1E+3 Proton recoil spectrum behind 1m Si + 10cm PMMA 1E+2
Proton recoil spectrum behind 1m Si Proton recoil spectrum in fission spectra [Kostal et al 2017]
1E+1 0
1
2
3
4
5
6
7
8
9
10
Recoil proton energy [MeV]
Figure 9.: Proton recoil spectrum of fission spectrum and silicon filtered spectra
4 Results 4.1 Neutron spectra The resultant spectrum is evaluated as an average of 10 measurements which are assumed as independent because they were performed with various stilbene crystals and also photomultipliers. The measurements were realized with various voltage on photomultiplier, which is the only parameter simply varied by user. The linearity of each setting was tested by means of linearity in Compton edge positions. Various crystals were tested on same photomultiplier and same setting. All measurements were performed with same spectrometric system, and calibration constants for all setting were found (see chapter 3.1). The neutron spectra are in Figure 10, and the corresponding data are listed in Table 2. The presented experimental uncertainty is obtained as a standard deviation obtained by considering all the partial experiments. When taking into account the independent uncertainty estimation using its main sources, the value is comparable with this standard deviation. The main assumed uncertainty sources are the uncertainties of deconvolution, the energy calibration and also the detector resolution. These major sources connected with experiment can contribute by 2 – 7 % to overall uncertainty. The statistical counting uncertainties are in all cases below 1 %, thus, they can be neglected in the evaluations. An uncertainty of 10 – 20 % is observed in the region of the peaks in few groups, and is caused mostly by numerical problems in the iteration process during the neutron spectrum deconvolution. Based on the agreement between several independent experiments, it can be deduced, that the used experimental system gives reliable results. It appears that the spectrum shape is in agreement with measurements. This might indicate correct energetic distribution of silicon cross sections. However the peak magnitudes between calculation and measurement vary. This state can be attributed mainly to incorrect angular distribution of neutrons scattered on silicon. Some effect might also play an incorrectly
determined stilbene resolution. But this fact would not explain all discrepancies visible in peak magnitudes. The calculated spectrum plotted in Figure 10 was broadened with respect to stilbene resolution (Table 1). The peak structure in detail is notable from Figure 11. It is notable, that many of the peaks are formed by the superposition of two or more lines. Despite the poor resolution of stilbene detectors, the shape of the experimental spectra indicates that the broad peak in the 3.2 MeV – 4.7 MeV region is actually result of the mixing of two finer peaks. The calculation shows, its origin by merging of two peaks at 3.65 MeV and 4.185 MeV. This result also shows that the resolution of a stilbene detector is relatively acceptable also for peak structure spectra measurements.
1 Experiment Calculation
Flux density [a.u.]
0.1
0.01
0.001
0.0001 0
1
2
3
4
5
6
7
8
Energy [MeV]
Figure 10.: Calculated and measured neutron spectra at the exit of the beam port
9
10
1E+2
1.2
1E+1
0.6
1E+0
0
1E-1 0
1
2
3
4
5
6
7
8
Energy [MeV]
Calculated broaden neutron spectra
Calculated spectra in smooth structure
Figure 11.: Calculated broadened and raw (un-broadened) neutron spectra at the exit of the beam port
Table 2.: Measured neutron spectra behind Si filter Eup [MeV] 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1
Group flux [a.u] 7.68E-2 1.40E-1 2.73E-1 1.43E-1 7.13E-2 3.47E-2 4.38E-2 5.65E-2 7.01E-2 6.04E-2 5.06E-2 3.89E-2 3.00E-2 2.55E-2 2.15E-2 1.91E-2 1.72E-2 1.74E-2 1.78E-2 1.79E-2 1.77E-2 1.62E-2
Rel. Unc. [%] 22.2% 15.5% 14.8% 28.8% 18.8% 22.8% 9.0% 6.7% 13.2% 10.2% 7.9% 7.1% 7.7% 6.8% 5.9% 6.2% 5.5% 7.0% 8.8% 10.2% 11.1% 12.3%
Eup [MeV] 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 6.1 6.2
Group flux [a.u] 3.19E-2 2.95E-2 2.20E-2 1.63E-2 1.15E-2 8.15E-3 6.76E-3 5.77E-3 6.08E-3 6.48E-3 8.95E-3 1.30E-2 2.10E-2 3.37E-2 3.30E-2 3.10E-2 2.17E-2 1.53E-2 1.35E-2 1.21E-2 1.37E-2 1.61E-2
Rel. Unc. [%] 6.3% 6.9% 7.8% 12.0% 10.3% 10.6% 8.1% 14.4% 14.4% 17.2% 14.2% 18.5% 13.7% 10.5% 3.9% 13.9% 12.9% 12.6% 10.4% 10.7% 9.8% 13.7%
Eup [MeV] 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2 9.3
Group flux [a.u] 5.05E-3 4.29E-3 3.71E-3 3.66E-3 3.58E-3 3.55E-3 3.52E-3 3.35E-3 3.24E-3 3.15E-3 2.99E-3 2.86E-3 2.75E-3 2.56E-3 2.38E-3 2.14E-3 1.88E-3 1.68E-3 1.54E-3 1.54E-3 1.51E-3 1.40E-3
Rel. Unc. [%] 15.4% 13.3% 17.1% 17.5% 16.6% 13.5% 10.1% 9.9% 12.5% 15.3% 13.7% 10.1% 8.8% 8.7% 9.2% 11.1% 12.3% 11.2% 12.2% 11.8% 9.2% 7.5%
Spectra in smooth structure [a.u]
Broaden spectra [a.u]
1.8
3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4
1.50E-2 1.80E-2 2.28E-2 4.02E-2 6.98E-2 6.77E-2 6.41E-2 4.67E-2 3.44E-2
13.9% 18.2% 22.6% 20.5% 20.7% 6.1% 13.1% 11.3% 7.8%
6.3 6.4 6.5 6.6 6.7 6.8 6.9 7 7.1
2.19E-2 2.91E-2 2.83E-2 2.70E-2 2.11E-2 1.63E-2 1.23E-2 9.20E-3 6.79E-3
12.5% 9.6% 4.3% 8.8% 8.3% 8.5% 11.1% 13.9% 13.9%
9.4 9.5 9.6 9.7 9.8 9.9 10 10.1 10.2
1.31E-3 1.20E-3 1.11E-3 9.54E-4 8.03E-4 6.60E-4 5.40E-4 4.47E-4 3.72E-4
7.7% 7.1% 11.1% 15.4% 18.5% 15.8% 15.6% 16.8% 19.6%
4.2 Neutron spectra filtered by PMMA The intensity of the neutron spectrum behind the Si filter is suitable for tests of small crystals of the dimensions ~10×10 mm. Bigger crystals are more sensitive, thus, the flux of thermal and resonance neutrons at the beam exit might be too high for such crystals. From this reason, the use of PMMA filters, which suppress the neutron flux level at lower energies, however, still preserve the peaked structure of the neutron spectrum in the fast energy region, were also tested. Plexiglas was tested due to suitable behavior while placed in neutron fields (see Figure 7). The influence of adding the PMMA filter into the beam is shown in Figure 11. The comparison shows that hydrogen rich compounds are suitable for suppression of the lower energy part of the silicon filtered spectrum for bigger crystals tests. The application of the PMMA filter will increase the relative count rate at higher energies, which is essential for stilbene detector tests.
1E+4
Measurement Calculation Measurement plexi
Flux density [a.u]
1E+3
Calculation plexi
1E+2
1E+1
1E+0 0
1
2
3
4
5 Energy [MeV]
6
7
8
9
10
Figure 12.: Calculated and measured neutron spectra at the beam port exit with and without the PMMA filter
5 Conclusions A Si filter was found to be valuable for realizing proper neutron field with peak structure applicable in neutron detector testing. This property makes it an ideal tool for calibration and testing of detector devices. Both, measured and calculated, neutron spectra show a peak structure of the silicon filtered neutron spectrum in the region of 1 – 10 MeV. The results were reproduced in 10 independent measurements with different setups of the spectrometric system and with different components (organic crystals, photomultipliers). Result indicates possible problems in definition of energetic distribution of silicon cross sections. This conclusion is based on observed incoherent magnitudes of calculated and measured peaks. The practical use of Plexiglas for the attenuation of the lower energy part of fast and resonance neutrons was verified with the same rate of C/E agreement as without this attenuator. Thanks to such characteristics, the resulting filtered neutron spectrum can be considered as a reference neutron field suitable for scintillation detectors calibration and tests. The comparison between the calculated and measured neutron spectra shows a very good agreement.
6 Acknowledgements The presented work was financially supported by the Ministry of Education, Youth and Sport Czech Republic Project LQ1603 (Research for SUSEN) within the SUSEN Project (established in the framework of the European Regional Development Fund (ERDF) in project CZ.1.05/2.1.00/03.0108) and with the use of infrastructures Sustainable energy – SUSEN and Reactors LVR-15 and LR-0, which were financially supported by the Ministry of Education, Youth and Sports - projects LM2015093 and LM2015074 and with the support of the mathematical and physical research project of Cvachovec F. et al 2006
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Highlights ► Neutron spectra behind 1m Si neutron filter ► Comparison of the experimental data with calculation ► Discrepancies between calculational and experimental neutron spectra