RU2365944C1 - Cherenkov's detector for registration of gamma emission impulses of nano- and subnanosecond duration - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention concerns equipment for registration of an ionising radiation flux of a unitary process of nano - and subnanosecond duration and can be used at registration of high-energy gammas at low fluence rate. The technical result in the Cherenkov's detector for registration of gamma emission impulses of nano - and subnanosecond duration, containing a converter (γ,e), a gas radiator, a rotary mirror and a light-receiving element, is reached because the light-receiving element of avalanche type, with the time resolution ~ 0.1 nc, not dependent on its photoresponsive area, is mounted after the rotary mirror, and the photoresponsive area S_pa of the avalanche type light-receiving element corresponds to the area of a cross section of the gas radiator S_rad., i.e. S_pa. ~ S_rad, electrical leads of the avalanche type light-receiving element are connected to the multichannel time analyser through a cable, and also that the light-receiving element of avalanche type is executed in a kind of the gas-filled spark counter containing mixed gas (C₂H₂F₄+10%SF₆) in gas volume, from a light source a translucent grid, a dielectric plate from glass with the photocathode (CsJ) placed on it, and a metal anode.

EFFECT: increase of Cherenkov's detector sensitivity.

2 cl, 4 dwg

Description

The invention relates to a technique for recording a stream of ionizing radiation of a single process of nano- and subnanosecond duration and can be used when registering high energy gamma quanta at a low flux density.

The well-known Cherenkov detector with a gas radiator (DCH) / 1 /, made in the form of a hollow cylinder filled with gas, along the axis of which (γ, e) are sequentially a Be converter, a radiator in the form of CO ₂ gas, a system for collecting light from plane and spherical mirrors and photodetector in the form of a photomultiplier tube (PMT).

The disadvantage of the detector / 1 / is the insufficient temporal resolution of ~ 0.35 ns at the required value of ~ 0.1 ns. To implement a time resolution of ~ 0.1 ns at high sensitivity, it is necessary to collect the optical quanta generated in a gas radiator with a diameter of ~ 10 cm into a spot of size

~ 1 mm ² , which makes it possible to use an electron-optical converter (EOC) with a time resolution of <10 ^-10 s. Due to the high anisotropy of Cherenkov radiation in the detector design / 1 /, the light collection system ensures its collection only into a spot with a diameter of ~ 15 mm, which led to the necessity of using a photo-electron multiplier (PMT) with a time resolution of ~ 0.35 ns as a photodetector. Thus, the detector given in / 1 / has a relatively low sensitivity and temporal resolution.

Known threshold gas Cherenkov detector / 2 / containing a gas radiator, a light-collecting system and a photoelectric converter. In the detector / 2 /, a mixture of carbon dioxide CO ₂ and SF ₆ gas with a partial pressure ratio of 1: 9 to 1:11 was used as a radiator, which made it possible to reduce the luminescence of the gas radiator and thereby led to a significant increase in the ratio of the number of Cherenkov optical quanta to the background quanta from gas scintillation per unit of absorbed energy of a fast electron and, as a result, to an increase in the sensitivity of JFG. For measurements with a time resolution of <0.1 ns, an image intensifier tube (with an input window with an area of ~ (1-5) mm ² ) was used as a photodetector (necessary). However, even in this work, the optical system for collecting Cherenkov radiation is inefficient due to the large anisotropy of Cherenkov radiation and, at a radiator cross section of ~ 100 cm ^2, cannot collect all Cherenkov radiation into a small spot. For this reason, the detector / 2 / cannot provide both high sensitivity and high temporal resolution.

The closest technical solution to this proposal is the Cherenkov detector for detecting pulses of ionizing radiation of the subnanosecond range, containing (γ, e) converter, gas radiator, rotary mirror and photodetector / 2 /. As noted above, the disadvantage of the detector / 2 / is its low sensitivity, which is determined by the inefficient collection of Cherenkov optical quanta (1 ~ 2%).

The technical result of the invention is to increase the sensitivity of the Cherenkov detector due to a significant increase in the collection efficiency of Cherenkov optical quanta generated in the entire volume of the gas radiator, while providing a temporal resolution of ~ 0.1 ns.

The technical result in the Cherenkov detector for detecting gamma radiation pulses of nano- and subnanosecond duration, containing (γ, e) a converter, a gas radiator, a rotary mirror and a photo detector, the outputs of which are connected through a coaxial cable to a multi-channel time analyzer, by the fact that the avalanche photodetector type with a time resolution of about 0.1 ns does not depend on its photosensitive area is set after the rotary mirror, and the photosensitive area S _fl photodetector of avalanche-type soot etstvuet cross-sectional area of the gas radiator S _rad, i.e. S _fl ~ S _glad .

The technical result is also achieved by the fact that the avalanche type photodetector is made in the form of a gas-filled spark counter containing a gas mixture in the gas volume (C ₂ H ₂ F ₄ + 10% SF ₆ ), from the side of the light source a translucent grid, a dielectric plate made of glass, on which a photocathode (CsJ) is applied, and a metal anode.

The essence of the proposed technical solution lies in the fact that the Cherenkov detector provides the most complete collection of optical quanta. Structurally, this was solved by using certain types of avalanche-type photodetectors and performing a photosensitive cross-sectional area S _{fl of the} photodetector corresponding to the cross-sectional area of the radiator S _fl ~ S _rad .

Schematically, the proposed device is presented in figure 1, and figure 2 presents one of the possible options for the avalanche type photodetector, made in the form of a gas-filled spark counter. Figure 3 presents the calculated impulse response of one channel of the avalanche type photodetector, made in the form of a gas-filled spark counter. The axis X shows the time T of the increase in the output signal from one virtual channel in fractions of 10 ^-9 s at a distance between the anode and cathode d = 0.2 mm and an avalanche speed of ~ 2 × 10 ⁸ mm / s, and the ordinate M is the coefficient multiplications in an avalanche; t = 0 is the moment of electron escape from the cathode. Figure 4 shows the calculated impulse response of the collection of light quanta of Cherenkov radiation in the plane of the avalanche type photodetector for the design of the proposed detector with a radiator length of 100 cm and a diameter of ~ 10 cm

Accepted designations. In Fig. 1: converter 1, gas radiator 2, rotary mirror 3, multichannel photodetector 4, multichannel time analyzer 5. In Fig.2: metal box 6, metal mesh 7, glass 8, photocathode 9, anode 10, gas (C ₂ H ₂ F ₄ + 10% SF ₆ ) is indicated by 11, coaxial cable 12.

The anode 10 can be made in the form of a dielectric plate with metal strip lines deposited on it, along which electrical pulses are removed from the channels of the photodetector 4.

As noted above, the flux of optical quanta of Cherenkov radiation is measured by an avalanche-type photodetector 4 from practically the entire area of the gas radiator, and the time resolution of the photodetector is independent of its area. A photodetector cannot be single-channel, since with a large photosensitive area, the time resolution determined by the RC value of the output circuit will significantly exceed ~ 10 ^-10 s. Therefore, it is proposed to use an avalanche type photodetector, for example, made in the form of a 2-electrode flat spark chamber, one electrode of which is made of glass / 3 /, with a CsJ layer deposited on it from the inside. When an optical quantum is incident on the photocathode, an electron with a low energy (~ eV) flies out of it, which enters a strong electric field with a voltage of E ~ 10 ⁵ V / cm, created by the corresponding voltages on the grid 7 and anode 10, and when the electron moves at a speed v ~ 2 · 10 ⁷ cm / s, there is an avalanche increase in its electric charge at a rate of α ~ 10 ¹⁰ l / s to a value of q ~ 10 ^-13 C, sufficient for its registration with a time stamp dispersion of <10 ^-10 s. Moreover, the avalanche cross section is <1 mm ² , which can be considered as the limiting size of the virtual channel of a multichannel photodetector. Thus, the avalanche type photo detector transform Channels light into an electric charge formed by the plurality of avalanches, the cross section of each of which has a small area <1 mm ^2/3 /. Each channel is an independent detector with a high time resolution. The photo detector 4 becomes a multi-channel detector. In this case, the sensitivity of the avalanche type photodetector 4 will be determined by the total photosensitive area, and the time resolution will be determined by the time resolution of one channel.

In the proposed technical solution, a countable method for measuring the flux of gamma rays Φ _γ (t) is implemented, i.e. measuring the temporal distribution of optical quanta, when the current pulse at the channel output - i (t) is its reaction to interaction with one quantum at time t _k :

where g (t) is the impulse response of photodetector 4, equal to the current pulse at its output after interacting with one quantum (particle), and _k is the pulse amplitude.

Hence, the requirements for the avalanche-type photodetector channels are determined:

- the amplitude of the current at the channel output - a _k , should be sufficient for its temporary fixation in a multichannel amplitude analyzer (MVA),

- the number of channels in the avalanche-type photodetector should be large so that when recording quanta, the change in the measurement sensitivity is insignificant. This means that the photosensitive area of one channel S _{fc is} much smaller than the photosensitive region of the multi-channel photodetector S _fd , i.e. S _FC <S _PD.

From these requirements it follows that the avalanche-type photodetector channel must have an internal reproduction coefficient — M ~ 10 ^{6 of the} primary charge q _i = 1.6 · 10 ^-19 C — when registering an optical quantum with a photodetector.

To date, two types of detectors are known that have a temporal resolution <10 ^-9 s, small channel sizes ~ (10 ^-2 -10 ^-4 ) mm ² and a multiplication factor M ~ 10 ⁶ :

- gas-filled spark meters, of which the most acceptable is a 2-electrode chamber / 3 /, one of the electrodes of which is made of a dielectric plate (RPC - resistive plate chamber) with a resistance in the range ρ ~ 10 ⁸ ÷ 10 ¹² Ohm · cm and the size of the virtual element ~ 10 ^-4 cm ² with a time resolution of ~ 10 ^-10 s when used in the avalanche mode of reproduction. Such spark counters are successfully used in experiments in high-energy physics to measure the energy of weakly ionizing particles (μ mesons, etc.),

- semiconductor silicon photodiodes (SiPM) / 4 /, operating in the Geiger mode of measurement with a size of ~ 1 mm, a temporal resolution along the pulse front of ~ 10 ^-10 s and multiplication of M ~ 10 ⁶ .

The proposed Cherenkov detector for detecting pulses of ionizing radiation of the subnanosecond range works as follows. From a source of pulsed gamma radiation (for example, from a DT target of an inertial thermonuclear installation) onto (γ, e) - converter 1, a parallel stream of gamma rays with an energy E _γ = 16.75 MeV is incident. Such gamma rays are produced as a result of the synthesis of (dt) nuclei in ~ 10 ^-4 cases relative to the production of DT neutrons with E _n = 14.1 MeV; therefore, their number is small and a high sensitivity detector is needed. As a result of gamma-ray scattering and pair production (electron-positron) reactions, fast electrons fly out of converter 1, mainly in the direction of movement of the primary gamma-quanta, which cause Cherenkov glow in the volume of the gas radiator 2 - the flow of optical quanta - hν in the direction of flow fast electrons. At a gas pressure (SF ₆ + CO ₂ ) ~ 1.5 atm, the threshold electron energy for the emission of Cherenkov radiation is ~ 10 MeV, which makes it possible to effectively cut off all background gamma quanta. The rotary mirror 3 changes the direction of the optical quanta of Cherenkov radiation by an angle of 90 °, which protects the avalanche-type photodetector 4 from direct exposure to its source gamma-quanta. The optical quanta hν that arose in the volume of the radiator 2, after scattering by the rotary mirror 3, fall on the avalanche-type photodetector 4, made, for example, as shown in / 3 /. The photodetector can be made with an area of ~ 3 cm ² , a time resolution of ~ 10 ^-10 s (at R = 10 Ω) and a multiplication coefficient of one photoelectron M = 10 ⁶ and works as follows: when an optical quantum falls on the photocathode 9, an electron flies out of it with low energy (~ eV), which enters a strong electric field with E ~ 10 ⁵ V / cm, created by the corresponding voltages on the grid 7 and anode 10, and when the electron moves at a speed of v ~ 2 · 10 ⁷ cm / s an increase in its electric charge at a rate of α ~ 10 ¹⁰ l / s to a value of q ~ 10 ^-13 C. When this charge moves at a speed of ~ 2 × 10 ⁷ cm / s in the interval d = 0.03 cm, a current signal i = (10 ^-13 × 2 × l0 ⁷ ) / 0.03 ~ 0 is induced on the electrodes of the strip line of the anode 10 7 × 10 ^-4 A, sufficient for its processing in MVA 5. From the calculated impulse response of photodetector 4 shown in Fig. 4, it follows that its width at half height is 1.2 × 10 ^-10 s.

The time resolution of the detector is determined by the following independent components:

proper time of light collection in the design of the DCHG (figure 3) 10 ps the dispersion of the time of flight in the RPC (figure 4) 50 ps scatter in the time of collection of signals to the output cable ~ 20 ps error of response time of the input discriminator of a multichannel time analyzer ~ 100 ps

As a result, the temporal resolution is (10 ² +50 ² +20 ² +100 ² ) ^0.5 ~ 115 ps.

The calculated impulse response (Fig. 4) of the collection of light quanta of Cherenkov radiation in the plane of a multi-channel photodetector hν for the design of the Cherenkov detector with a radiator length of 100 cm and a diameter of ~ 10 cm confirms the achieved positive effect: the detector’s temporal resolution is ~ 0.1 ns and a significant increase in sensitivity ~ (50-60) times due to the collection of Cherenkov optical quanta from practically the entire radiator volume.

Thus, the proposed Cherenkov detector for detecting pulses of ionizing radiation has a sensitivity exceeding the sensitivity of the prototype, ~ 50 times while ensuring high temporal resolution.

Literature

1. J. M. Mack, R. R. Berggren, S. E. Caldwell, S. C. Evans, J. R. Faulkner Jr., R. A. Lerche, J. A. Oertel, C. S. Young. Observation of high-energy deuterium-tritium fusion Gamma rays using gas Cherenkov detectors, NIM, A 513 (2003), 566-572.

2. Albikov Z.A., Danilenko K.H., Kazachkov Yu.P., Melnik O.B “Threshold gas Cherenkov detector”, RF Patent No. 2263331 dated 06/22/2004 - prototype.

3. P.Fonte. "Applications and New Developments in RPS". IEEE Trans. On nucl. Science, Vol. 49, No. June 3, 2002.

4. A. Biland, I. Britvitch et al. "First detection of Cherenkov light from cosmic-particle induced air showers by Geiger-mode avalanche photodioes." Nucl. Instr. and Meth. In phys. Res. Vol. 581, iss. 1-2, pp. 143-146, 2007.

Claims (2)

1. Cherenkov detector for detecting gamma-ray pulses of nano- and subnanosecond duration, containing (γ, e) a converter, a gas radiator, a rotary mirror and a photodetector, the outputs of which are connected through a coaxial cable to a multi-channel time analyzer, characterized in that the avalanche type photodetector with a time resolution of ~ 10 ^-10 s, independent of its photosensitive area, is installed after the rotary mirror, and the photosensitive area S _{fl of the} avalanche-type photodetector corresponds to the river cross section of the radiator S _rad , i.e. S _fl ~ S _glad . 2. The Cherenkov detector according to claim 1, characterized in that the avalanche type photodetector is made in the form of a gas-filled spark counter containing a gas mixture in the gas volume (C ₂ H ₂ F ₄ + 10% SF ₆ ), from the side of the light source a translucent grid , a glass dielectric plate on which a photocathode (CsJ) is applied, and a metal anode.

Images