High-speed gas neutron detector for thermometry of thermonuclear plasma

Nuclear Inst. and Methods in Physics Research, A 945 (2019) 162633

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High-speed gas neutron detector for thermometry of thermonuclear plasma S.G. Lebedev ∗, V.E. Yants Institute for Nuclear Research of Russian Academy of Sciences, 60th October Anniversary prospect, 7a, 117312, Moscow, Russian Federation

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Keywords: Neutron detectors Thermometry Thermonuclear plasma Gaseous proportional counter Nuclear reactions Bragg’s effect

ABSTRACT The article deals with the problem of measuring the temperature 𝑇𝑖 of ions in D–T plasma. To measure 𝑇𝑖 , it is proposed to use a gaseous proportional counter operating in the current mode. For the selection of current pulses it is proposed to use the property of products of nuclear reactions which are the alpha-particle and the residual nucleus to form the braking peak of ionization when stopped (so called Bragg’s effect). Neutron energy distribution consists of events characterized by current pulses with two peaks — due to the alpha particle and the residual nucleus. This selection method allows excluding the so-called ‘‘wall effect’’, i.e. events that do not fully fit into the volume of the counter. As has been shown such kind of neutron detector can achieve an energy resolution of about 10−3 .

1. Introduction One of the urgent problems in controlled thermonuclear fusion, for example, in the ITER project [1], is the problem of temperature measuring of the ions in D–T plasma [2]. Temperature control is necessary for operational (in real time) control of the process of plasma heating by external energy sources. The requirements for the measurement method are very strict — they must be measured with a resolution of about 50 ms with the accuracy of 𝛥T/T ∼ 0.1. Direct information about plasma temperature is contained in the width of the neutron spectrum of 𝐷 + 𝑇 reaction in hot plasma. It was shown [3] that in the case of Maxwell plasma the neutron spectrum have a Gaussian shape. In this case, the width of the energy distribution of a neutron pulse 𝛥𝐸√ 𝑛 is related to the temperature of plasma ions 𝑇𝑖 by a ratio 𝛥𝐸𝑛 = 178 𝑇𝑖 [3], where 𝛥𝐸𝑛 and 𝑇𝑖 is measured in keV. The broadening of the spectrum has a Doppler’s nature and is related to the thermal motion of the center of mass of deuterium and tritium in the laboratory coordinate system. Thus, by measuring the half-width of the Gaussian distribution of the neutron spectrum, one can also obtain the temperature of the plasma ions. Before placing of the detector in thermonuclear plasma it should be tested in neutron generator with monochromatic neutron flux where the broadening of 14 MeV mono line 𝛥E𝐺 should be measured. The value 𝛥E𝐺 can be considered as detector energy distribution. It is expected this broadening will be small compared with that of thermonuclear plasma 𝛥E𝑛 due to high temperature of the latter. The methods of plasma diagnostics can be divided on active and passive kind. Under active detection methods, probes and sensors and also laser and particle radiation are introduced into the plasma. Thus, in these methods, it is initially assumed to interact with the structure ∗

under study, and, consequently, change the initial state of the plasma. The most famous active method for measurement of ion temperature is Charge eXchange Recombination Spectroscopy (CXRS) [4], which is linked with injection of beam of neutral atoms into the plasma. Some of the plasma ions will gain electrons from the beam via a process of charge exchange, and will subsequently emit visible light due to the electron transitions in the re-neutralized atom. This method allows measuring the distribution of ion temperature along the cross section of the plasma cord. The spread of the wavelength of this light gives a measurement of the ion velocity distribution, and hence ion temperature. The presence in the chamber of a mixture containing tritium requires that the equipment in this case should be in a vacuum box, excluding the ingress of tritium into the installation room. Passive plasma diagnostics involves the analysis of neutron fluxes emitted by the plasma itself. Such methods can be referred as nondestructive diagnostics. The most actively used types of neutron diagnostics are detectors on recoil protons, fission chambers, diamond detectors, gaseous counters and silicon detectors. Previously, a radiochemical neutron detector was proposed for thermometry of a thermonuclear reaction [5–7]. In this detector, under neutron irradiation, a radioactive gas is formed and then its activity and neutron spectrum are measured. The main disadvantage of this method is the time delay of the signal due to the transport of radioactive gas from the core to the remote proportional counter. The closest in technical essence and the achieved result is a method of measuring of 𝛥𝐸𝑛 and ion temperature 𝑇𝑖 in a D–T plasma using a diamond neutron detector [8] using the reaction n + 12 C → 𝛼 + 9 Be −5.7016 MeV. This method based on registering of neutrons from D–T plasma with a diamond crystal, using detector’s signals for producing

Corresponding author. E-mail address: [email protected] (S.G. Lebedev).

https://doi.org/10.1016/j.nima.2019.162633 Received 6 June 2019; Received in revised form 14 August 2019; Accepted 25 August 2019 Available online 28 August 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.

S.G. Lebedev and V.E. Yants

Nuclear Inst. and Methods in Physics Research, A 945 (2019) 162633

Fig. 1. Sketch of installation for D–T plasma temperature measurement. Designations: 1 — quartz case, 2 — pyrographite cathode, 3 — neutron flux, 4 – tungsten filament – anode, 5 — spectrometric gas, 6 – tracks of nuclei – products.

which was used in the SAGE experiment [10] for recording of the solar neutrino flux. Various spectrometric gases (CO2 , N2 , Ne and others, as well as their mixtures) can be used in gas detectors and, accordingly, the following reactions are used for neutron registration:

the neutron energy spectrum, determining its width at half-height 𝛥𝐸𝑛 and calculating the ion temperature 𝑇𝑖 . With an effective charge collection in a diamond detector, it is possible to achieve an energy resolution of the order of 0.15% at energy of 14 MeV [8]. Then it will be measured 𝛥𝐸𝑛 with an accuracy of about 1.5%, and the temperature 𝑇𝑖 with an accuracy of 3%–4%. In reality, the requirements of high load on the counting rate ∼ 106 s−1 (to ensure acceptable statistics for 50 ms) and a large working resource — tens and hundreds of days, there is the limitation the workability of solid-state detectors, having in mind the accumulation of structural defects in the chip, and consequently, charge traps. The disadvantages of the diamond detector described above are also its high cost, limited working lifetime of the crystal used, the existence of nuclear reaction channels other than the 12 C(𝑛, 𝛼)9 Be channel used to record neutrons, which reduce the reliability of the results, as well as incomplete charge collection. The proposed method [9] for measuring of the temperature of ions 𝑇𝑖 in D–T plasma suggests using of proportional gas counter as a neutron events recorder. Below it will be shown that this increases the reliability of the results, expands the capabilities of the detector and increases its efficiency due to a significant reduction in the cost and increase of its working lifetime and giving rise to the possibility of its effective use in conditions of strong electromagnetic interference. In addition, the correct choice of the detector parameters and the signal selection circuit for the formation of the neutron energy spectrum makes it possible to optimize its efficiency.

12

𝐶(𝑛, 𝛼)9 𝐵𝑒,

16

𝑂(𝑛, 𝛼)13 𝐶,

17

𝑂(𝑛, 𝛼)14 𝐶,

22

19

𝑁𝑒(𝑛, 𝛼) 𝑂

13

𝐶(𝑛, 𝛼)10 𝐵𝑒,

18

𝑂(𝑛, 𝛼)15 𝐶,

14

20

𝑁(𝑛, 𝛼)11 𝐵,

15

𝑁(𝑛, 𝛼)12 𝐵,

𝑁𝑒(𝑛, 𝛼)17 𝑂,

21

𝑁𝑒(𝑛, 𝛼)18 𝑂, (1)

Although the formation energy of a charged pair in the brake gas Xe is greater than in diamond, but when using nitrogen (𝑁2 ) as a spectrometric gas, for example, this disadvantage is fully compensated by the lower threshold of (𝑛, 𝛼) — reaction on nitrogen (0.169 MeV). When using CO2 , it is necessary to take into account the reactions on all isotopes of carbon and oxygen. Of the 5 possible reactions, only two can make a significant contribution to the resulting signal: 𝑛 + 12 𝐶 → 𝛼 + 9 𝐵𝑒 − 5.7016 MeV and 𝑛 + 16 𝑂 → 𝛼 + 13 𝐶 − 2.215 MeV (2) This means two pulses will appear around 8 and 12 MeV. As can be seen these pulses can be separated effectively. Each of these pulses allows defining the plasma temperature independently. Usually the main difficulty in using of gas proportional detectors for precision neutron spectroscopy is the so-called ‘‘wall effect’’ [11]. If the tracks of the reaction (1) products are outside the sensitive volume of the counter (‘‘fall into the wall’’), then the so-called ‘‘tail’’ on the left side of the spectrum appears in the amplitude distribution of the detector’s charge signals. With such a distortion of the Gaussian distribution, accurate measurements of the temperature broadening are impossible. To solve the wall effect problem, it is proposed to use the feature of reactions (1) that in the final state two highly ionizing product nuclei appear — 𝛼 particle and a heavy nucleus (for example, 9 Be in reaction with 12 C). If these nuclei are completely inhibited in the gas volume of the counter, then at the ends of the tracks of the alpha particle and the nucleus areas with an increased ionization density are formed (the Bragg’s effect). If the projection of the track relative to the field is such that electrons from these ionization seals come to the anode of the detector with some time difference (tens of nanoseconds), then two characteristic peaks will appear in the current signal of the detector (see Fig. 2). The presence of such two peaks in the current signal is a sign that the energy of the product nuclei is completely absorbed in the sensitive volume of the counter. The presence of gas amplification is necessary to both suppress the contribution to the current from the primary ionic component and also additional advantage of the gaseous proportional counter in conditions of strong electromagnetic interference. Selecting for the formation of the charge distribution only the signals, characterized by two current peaks, and discarding all the others, one can obtain the distribution, cleared from the ‘‘wall effect’’. This will be the true energy distribution. At the same time, some loss

2. Description of the gaseous neutron detector The method described for measuring of the ions temperature 𝑇𝑖 in D–T plasma is based on the registration of D–T plasma neutrons by a gas proportional counter. Electric signals from the counter are used to produce the neutron energy spectrum in the form of a Gaussian distribution. Then, its half-height width is measured which is converted into the ions temperature 𝑇𝑖 . The gas proportional counter is filled with a mixture of braking and spectrometric gases. The (𝑛, 𝛼) nuclear reactions occur in the spectrometric gas under neutron irradiation. At the same time, current signals from the counter are used to form the neutron energy spectrum, from which only those signals are selected whose shape is characterized by two peaks. The peaks are formed by fully ionizing nuclei — the 𝛼-particle and a heavy nucleus, obtained as a result of a (𝑛, 𝛼) nuclear reaction of spectrometric gas nucleus with a neutrons, that were completely slowed down in the sensitive gas volume of the counter. Fig. 1 shows D–T plasma thermometer setup used a gas proportional counter for neutron detection. As a neutron detector, it is proposed to use a gas proportional counter filled with a mixture of brake gas (for example, Xe) and a spectrometric gas, for which (𝑛, 𝛼) reactions with its isotopic components are possible under D–T plasma neutron irradiation. Counter design is similar to that developed at the Institute for Nuclear Research RAS, 2

S.G. Lebedev and V.E. Yants

Nuclear Inst. and Methods in Physics Research, A 945 (2019) 162633

Fig. 3. Charge distribution of detector signals. Fig. 2. Current pulse shape in the counter.

of efficiency related with such kind of cutting is not critical and the optimization of efficiency can be achieved by choosing the size of the detector and the gas pressure. The process of ions temperature measuring in the D–T plasma is as follows (see Fig. 1). Neutrons 3 in the detector’s case 1 interact with the spectrometric gas 5. Let us consider, for example, CO2 as a spectrometric gas and the specific 12 C target nucleus, which undergo to the 12 C(𝑛, 𝛼)9 Be nuclear reaction. When the nucleus — products 𝛼 particle and 9 Be scatters from the point of interaction of the neutron with the 12 C nucleus they produce tracks 6 of ionization in the gaseous medium 5 of the counter 1 and are inhibited to form a highdensity electron cloud (Bragg’s condensation) at the end of its path. The electrons of the primary ionization of the track drift towards the anode 4 along the lines of the electric field; on the other hand the ions drift towards the cathode 2. At the same time, the current associated with the movement of charges is induced in the external circuit. The current signal from extended primary ionization is a superposition of the contributions from each element of track 6 and is a fairly smooth function (see Fig. 2). When the electrons from the Bragg’s seals reach the anode two characteristic bursts in the form of a current signal appear which are the Bragg’s peaks (see Fig. 2) This peaks arise due to the high density ionization in the seals and small distance between the seals and the anode. The separation in time is due to the difference in distance from the seals to the anode 4. The full duration of the current signal is about several microseconds and depends on the density and composition of the gas, the voltage applied to the counter, and its size. The signals from the counter 1 are arrive to the input of the current preamplifier and further digitized by an amplitude digital converter or by digital oscilloscope. Then they are analyzed in an on-line analyzer, where the identification of tracks 6 occurs, by means of checking if nuclear reaction products have completely lost their energy in the gaseous filling of the counter 5, and whether the common current signal is characterized by the presence of two Bragg peaks. The analysis of the signals may consist in search for the events when two current peaks appear above the discrimination level which is equal to the largest current amplitude for the smooth part of the signal. The selected signals from the analyzer is transmitted to the integrator, where the current signal is integrated, then the signal goes to the computer to form the charge distribution (see Fig. 3), half-width at half height of which after calibration corresponds to the half-width of the neutron spectrum. The obtained half-width of the neutron spectrum 𝛥𝐸𝑛 is used to determine the temperature 𝑇𝑖 of plasma ions (see Fig. 4). For real D–T plasma, the neutron pulse width will be large due to the high temperature 𝑇𝑖 , and the line will be wide. By varying the counter parameters such as the composition of the mixture, and the voltage applied to the counter (which defines the gaseous amplification), one can achieve the smallest width of the spectrum of charge pulses and the best resolution. The gas proportional neutron counter is an all-quartz sealed construction. The detector cathode is pyrographite layer, obtained by decomposition of isobutane at a temperature of ∼ 950 ◦ C. The thickness of the pyrographite layer is ∼ 0.1 μm. After deposition of the layer

Fig. 4. The dependence of the ions temperature of the D–T plasma vs. the width of the neutron pulse.

some excess pyrographite where it is not needed is removed by burning in oxygen flow. Contact with the cathode is provided through a side capillary branch with a pyrographite inner coating. Molybdenum foil with the thickness of 10 μm, and width of 1 mm welded into quartz glass provides an electrical contact with inner coating. A similar foil acts as an anode output contact. The anode itself is a tungsten wire with a diameter of 20 μm. Detector dimensions are as follows the length is about 250 mm; internal diameter is about 18 mm. Filling of the counter is as follows the Xe and CO2 gases at 2 atm. The signals from the detector are enter to a current preamplifier (𝛥𝑓 = 800 MHz) and then to a digital oscilloscope with discretization of 2 ns. The digitized on-line signal is analyzed for the presence of two peaks and, if any, is recorded and integrated. From the integrals of such signals the energy distribution is composed where 𝛥𝐸𝑛 and 𝑇𝑖 are determined every 50–100 ms. The on-line analyzer program works on the principle of pattern recognition. Initially, the bank of reference Bragg signals based on visual selection is formed. Further, the program selects, on the basis of a bank of standards, the characteristic features of the Bragg peaks, from which the proper signals in counter are then selected. The detector contains about 100 cm3 CO2 or 1.6 × 1021 carbon atoms. According to data of ENDF/B-VII [12] the cross section of the (𝑛, 𝛼) reaction at 14 MeV is about 80 mb. Then the neutron counting rate will be N = (1.6 × 1021 ) × (8 × 10−26 ) ≈ 0.0001 counts/(neutron/(cm2 s)). With a neutron flux F ∼ 106 neutron/(cm2 s), the counting rate I will be ∼ 100 s−1 . At the reaction energy Q ∼ 6 MeV, 8 MeV remains for ionization and ∼ 4 × 105 pairs of ions are formed at a pair’s energy formation ∼ 20 eV. Then the energy resolution will be ∼ 10−3 . 3. Discussion The conditions of ITER such as high neutron and gamma fluxes significantly limited the workability of neutron detectors in measurements of ion temperature. The disadvantage of organic scintillators 3

S.G. Lebedev and V.E. Yants

Nuclear Inst. and Methods in Physics Research, A 945 (2019) 162633

such as stilbene and other neutron detectors based on registration of the recoil protons consists in non-Gaussian energy distribution of response function on a monoenergetic line which gives rise to problems under ion temperature measurement. The fission chambers are influenced by gamma background and subjected to electrical and mechanical interferences [13]. The diamond detector have small radiation resistance — maximum acceptable fluence (up to which it maintains its spectrometric properties,) is about only 5 × 1014 n/cm2 [14], that is only 100 ITER discharges, or a few hours of work. Another disadvantage of diamond detector is its low detection efficiency due to the small active volume. The silicon detectors have even lower radiation resistances than diamond. Quartz gaseous proportional counter has a unique radiation resistance [15] which allows using it for monitoring of thermonuclear plasma temperature.

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4. Conclusion Thus, the use of the described method for measuring of the temperature of D–T plasma ions provides increased reliability of the results, increased capabilities and measurement efficiency due to a significant reduction in the cost of the detector and an increase in its working life, as well as the possibility of using it in conditions of strong electromagnetic interference. References [1] F.W. Perkins, D.E. Post, N.A. Uckan, M. Azumi, D.J. Campbell, N. Ivanov, et al., Chapter 1: Overview and summary, Nucl. Fusion 39 (1999) 2137–2174. [2] V.T. Voronchev, V.I. Kukulin, Y. Nakao, Use of 𝛾-ray-generating nuclear reactions for temperature diagnostics of DT fusion plasma, Phys. Rev. E 63 (2001) 026413–026416.

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