阅读说明：本技术 一种超快中子脉冲能谱探测系统及方法 (Ultrafast neutron pulse energy spectrum detection system and method ) 是由 田进寿 刘毅恒 何凯 汪韬 闫欣 高贵龙 姚东 尹飞 李亚晖 岳猛猛 李知兵 于 2021-06-16 设计创作，主要内容包括：本发明涉及一种超快中子脉冲能谱探测系统及方法,以解决现有中子能谱仪无法满足超快中子脉冲皮秒级时间分辨的能谱探测问题。该系统包括同步触发单元、线偏振光发生单元、延迟模块、分束器、参考光单元、探测光单元以及光谱仪。同步触发单元同步触发中子脉冲源和线偏振光发生单元；线偏振光发生单元产生线偏振光L,线偏振光L依次经延迟模块和分束器后,分为第一参考光L1和第一探测光L2；第一参考光L1经参考光单元相位延时π后形成第二参考光L1’；探测光单元包括光纤环形器以及设置在DIM腔室内的普克尔斯晶体和反射镜,第一探测光L2入射至普克尔斯晶体形成偏振状态改变的第二探测光L2’,再与第二参考光L1’发生干涉；光谱仪接收干涉图像。(The invention relates to an ultrafast neutron pulse energy spectrum detection system and method, and aims to solve the problem that an existing neutron energy spectrum instrument cannot meet the requirement of energy spectrum detection of ultrafast neutron pulse picosecond time resolution. The system comprises a synchronous trigger unit, a linearly polarized light generation unit, a delay module, a beam splitter, a reference light unit, a detection light unit and a spectrometer. The synchronous triggering unit synchronously triggers the neutron pulse source and the linearly polarized light generating unit; the linearly polarized light generating unit generates linearly polarized light L, and the linearly polarized light L is divided into first reference light L1 and first detection light L2 after passing through the delay module and the beam splitter in sequence; the first reference light L1 forms second reference light L1' after the phase delay of the reference light unit is pi; the detection light unit comprises a fiber circulator, a Pockels crystal and a reflecting mirror which are arranged in the DIM cavity, and the first detection light L2 enters the Pockels crystal to form second detection light L2 'with a changed polarization state and then interferes with the second reference light L1'; the spectrometer receives the interference image.)

1. An ultrafast neutron pulse energy spectrum detection system, characterized in that:

the device comprises a synchronous trigger unit (1), a linearly polarized light generating unit, a delay module (5), a beam splitter (6), a reference light unit, a detection light unit and a spectrometer (13);

the synchronous triggering unit (1) is used for synchronously triggering the neutron pulse source and the linearly polarized light generating unit;

the linearly polarized light generating unit is used for generating linearly polarized light L;

the linearly polarized light L is divided into first reference light L1 and first detection light L2 after sequentially passing through a delay module (5) and a beam splitter (6);

after the phase delay of the first reference light L1 by the reference light unit is pi, second reference light L1' is formed;

the detection light unit comprises a fiber circulator (8), a Pockels crystal (11) and a reflector (12) which are arranged in a DIM chamber (9); the reflecting surface of the reflector (12) is tightly attached to one side surface of the pockels crystal (11), and the side surface is opposite to the neutron pulse source;

the first detection light L2 is transmitted to the DIM chamber (9) through the fiber circulator (8), and is incident on the other side surface of the Pockels crystal (11), so as to form second detection light L2 ' with changed polarization state, and then the second detection light L2 ' is reflected back to the fiber circulator (8) by the reflector (12) and is output, and interferes with the second reference light L1 ';

the spectrometer (13) is used for receiving an interference image of the second reference light L1 'and the second detection light L2'.

2. The ultrafast neutron pulse spectrum detection system of claim 1, wherein:

the linearly polarized light generating unit comprises a femtosecond laser (2), a chirped grating (3) and a polaroid sheet (4) which are sequentially arranged along a light path.

3. The ultrafast neutron pulse spectrum detection system of claim 1 or 2, wherein:

the reference light unit includes a phase delay crystal (7).

4. The ultrafast neutron pulse spectrum detection system of claim 3, wherein:

one interface of the optical fiber circulator (8) is connected with an optical fiber adapter (10) of the DIM chamber (9).

5. An ultrafast neutron pulse spectrum detection method using the ultrafast neutron pulse spectrum detection system of claim 1, comprising the steps of:

1) triggering

1.1) synchronously triggering a neutron pulse source and a linearly polarized light generating unit by using a synchronous triggering unit (1), wherein the neutron pulse source emits ultrafast neutron pulses, and the linearly polarized light generating unit generates linearly polarized light L with a spectral range of lambda and a time width of T;

1.2) adjusting a delay module (5) to synchronize the linearly polarized light L with the ultrafast neutron pulse;

2) electro-optical sampling

Irradiating the pockels crystal (11) by using the ultrafast neutron pulse in the step 1), and forming an electric field E (t) in the pockels crystal (11); the direction of the electric field e (t) is opposite to the direction of the incident neutrons;

meanwhile, the linearly polarized light L in the step 1) is divided into first reference light L1 and first detection light L2 through a beam splitter (6); delaying the phase of the first reference light L1 by pi to form second reference light L1'; sending the first detection light L2 into the DIM chamber (9) through the fiber circulator (8) and irradiating the surface of the Pockels crystal (11); the first detection light L2 is modulated by an electric field E (t) in the Pockels crystal (11) to form second detection light L2 'with a changed polarization state, and the second detection light L2' is transmitted through the whole Pockels crystal (11) and then reflected back to the optical fiber circulator (8) by the reflector (12) to realize electro-optical sampling;

3) interference

The second detection light L2 'output by the fiber optic circulator (8) interferes with the second reference light L1' in space, and an interference image is received by a spectrometer (13);

4) measuring

Measuring the spectral range Delta lambda and the spectral intensity I (t) of the interference image;

5) obtaining temporal widths and energy spectra

5.1) calculating the time width Δ t of the second probe light L2' according to:

in the formula, lambda is the spectral range of linearly polarized light L;

t is the time width of linearly polarized light L;

Δ λ is the spectral range of the interference image;

5.2) calculating to obtain an energy spectrum Sig (t) of the ultrafast neutron pulse in a deconvolution mode:

in the formula (I), the compound is shown in the specification,representing a deconvolution operation;

i (t) is the spectral intensity of the interference image;

EOS (t) is broadening of electro-optical sampling, and 0.3ps is taken;

e (t) is the electric field in the Pockels crystal (11).

6. The ultrafast neutron pulse spectrum detection method of claim 5, wherein:

in the step 1), the linearly polarized light L is formed by utilizing a femtosecond laser with a spectral range of lambda emitted by a femtosecond laser (2), adjusting by a chirped grating (3) to widen the time width to T, and then injecting into a polarizing plate (4) in a vertical direction.

7. The ultrafast neutron pulse spectrum detection method of claim 5 or 6, wherein:

in step 2.2), the second reference light L1' is specifically formed by delaying the phase of the first reference light L1 by pi by the phase delay crystal (7).

Technical Field

The invention relates to the field of neutron pulse ultrafast diagnosis, in particular to an ultrafast neutron pulse energy spectrum detection system and method.

Background

In the research in the field of laser Inertial Confinement Fusion (ICF), the target pellet is compressed to a high temperature and high density state for fusion, and a large amount of X-rays, high-energy electrons, high-energy neutrons and the like are released in the process. In order to accurately grasp the physical process of fusion, highly accurate diagnosis of information such as the energy spectrum and the generation time of neutrons is required. At present, a variety of neutron diagnostic techniques have been implemented in ICF facilities around the world, and the yield of laser fusion is high enough to provide spatial, temporal and spectral information through neutron measurements, which is essential to understanding the performance of ICF implosions.

Neutron spectrometers can be used to measure time-integrated neutron spectra, from which experimental information is determined for the average of areal density (ρ R), yield (Yn), and surface plasma temperature (Ti) and their asymmetry. The current neutron energy spectrometer usually has a time resolution of only tens of picoseconds (ps), for example, a detection system combining a scintillator with a stripe camera or a photomultiplier tube converts neutron pulses into visible light by using the scintillator, the light emitting time of the plastic scintillator BC422 is 10-30ps, the decay time is 2.5ns, the light emitted by the scintillator is detected by the stripe camera, and the time and energy spectrum information of the neutron pulses is obtained by a deconvolution mode. While the data measured by the neutron spectrometer described above is critical to understanding the physical process of ICF implosion, it does not provide any information on the evolution of fuel composition changes, hot spot formation, alpha heating, and fuel combustion. This is because the time width (FWHM) of fuel combustion is about 10-20ps, the detection of the whole process requires the time resolution of the neutron spectrometer to be of the order of several picoseconds, and the existing detection methods cannot meet this requirement. Therefore, it is necessary to invent a neutron spectrum detection method with simple operation and high time resolution.

Disclosure of Invention

The invention provides an ultrafast neutron pulse energy spectrum detection system and method, aiming at solving the problem that the conventional neutron energy spectrometer cannot meet the requirement of ultrafast neutron pulse picosecond time resolution energy spectrum detection.

The principle of the invention is as follows: the method comprises the steps of irradiating a Pockels crystal by using ultrafast neutron pulses to generate a transient electric field, performing electro-optical sampling by using chirped femtosecond laser pulses, acquiring spectral information of the laser pulses by using an interference imaging mode, and finally serving for energy spectrum detection of the ultrafast neutron pulses. The method effectively improves the time resolution capability of neutron spectrum detection, provides guidance for understanding and controlling the ICF implosion physical process, and has great practical prospect.

The technical scheme adopted by the invention is as follows:

an ultrafast neutron pulse energy spectrum detection system is characterized in that:

the device comprises a synchronous trigger unit, a linearly polarized light generation unit, a delay module, a beam splitter, a reference light unit, a detection light unit and a spectrometer;

the synchronous triggering unit is used for synchronously triggering the neutron pulse source and the linearly polarized light generating unit;

the linearly polarized light generating unit is used for generating linearly polarized light L;

the linearly polarized light L is divided into first reference light L1 and first detection light L2 after sequentially passing through the delay module and the beam splitter;

after the phase delay of the first reference light L1 by the reference light unit is pi, second reference light L1' is formed;

the detection light unit comprises a fiber circulator, a Pockels crystal and a reflecting mirror which are arranged in the DIM cavity; the reflecting surface of the reflector is tightly attached to one side surface of the Pockels crystal, and the side surface is opposite to the neutron pulse source;

the first detection light L2 is transmitted to the DIM chamber through the fiber circulator, and is incident on the other side surface of the Pockels crystal to form second detection light L2 ' with changed polarization state, and the second detection light L2 ' is reflected by the reflector back to the fiber circulator and is output to interfere with the second reference light L1 ';

the spectrometer is used for receiving an interference image of the second reference light L1 'and the second probe light L2'.

Further, the linearly polarized light generating unit comprises a femtosecond laser, a chirped grating and a polaroid which are sequentially arranged along the optical path.

Further, the reference light unit includes a phase delay crystal.

Further, one interface of the optical fiber circulator is connected with an optical fiber adapter interface of the DIM chamber.

The ultrafast neutron pulse energy spectrum detection method is characterized by comprising the following steps of:

1) triggering

1.1) synchronously triggering a neutron pulse source and a linearly polarized light generating unit by using a synchronous triggering unit, wherein the neutron pulse source emits ultrafast neutron pulses, and the linearly polarized light generating unit generates linearly polarized light L with the spectral range of lambda and the time width of T;

1.2) adjusting a delay module to synchronize the linearly polarized light L with the ultrafast neutron pulse;

2) electro-optical sampling

Irradiating the pockels crystal by using the ultrafast neutron pulse in the step 1) to form an electric field E (t) in the pockels crystal; the direction of the electric field e (t) is opposite to the direction of the incident neutrons;

meanwhile, the linearly polarized light L in the step 1) is divided into first reference light L1 and first detection light L2 through a beam splitter; delaying the phase of the first reference light L1 by pi to form second reference light L1'; sending the first detection light L2 into the DIM chamber through the fiber circulator and irradiating the first detection light to the surface of the Pockels crystal; the first detection light L2 is modulated by an electric field E (t) in the Pockels crystal to form second detection light L2 'with a changed polarization state, and the second detection light L2' is reflected back to the optical fiber circulator by a reflector after transmitting the whole Pockels crystal, so that electro-optical sampling is realized;

3) interference

The second probe light L2 'output by the fiber circulator interferes with the second reference light L1' in space, and an interference image is received by a spectrometer;

4) measuring

Measuring the spectral range Delta lambda and the spectral intensity I (t) of the interference image;

5) obtaining temporal widths and energy spectra

5.1) calculating the time width Δ t of the second probe light L2' according to:

in the formula, lambda is the spectral range of linearly polarized light L;

t is the time width of linearly polarized light L;

Δ λ is the spectral range of the interference image;

5.2) calculating to obtain an energy spectrum Sig (t) of the ultrafast neutron pulse in a deconvolution mode:

in the formula (I), the compound is shown in the specification,representing a deconvolution operation;

i (t) is the spectral intensity of the interference image;

EOS (t) is broadening of electro-optical sampling, and 0.3ps is taken;

e (t) is the electric field in the Pockels crystal.

Further, in step 1), the linearly polarized light L is formed by emitting femtosecond laser with a spectral range λ by a femtosecond laser, widening a time width to T by adjusting a chirped grating, and then entering a polarizer in a vertical direction.

Further, in step 2.2), the second reference light L1' is specifically formed by delaying the phase of the first reference light L1 by pi through a phase delay crystal.

Compared with the prior art, the invention has the beneficial effects that:

1. the invention provides a novel detection system and a novel detection method aiming at the generated ultrafast neutron pulse based on the physical process of laser Inertial Confinement Fusion (ICF). The ultrafast neutron pulse is used for irradiating the pockels crystal to generate a femtosecond-level transient electric field, so that the influence of factors such as long scintillator light emitting time and uncertain light attenuation time on the time resolution capability of a system in the traditional detection method is overcome, and picosecond-level time-resolved neutron spectrum detection is finally realized.

2. The invention adopts pulsed chirped femtosecond laser as detection light, the detection light is divided into two beams after polarization by a polaroid, and the change of the polarization state of the detection light caused by the Pockels effect is measured by taking one beam of delayed pi phase as reference light and interfering with the detection light pulse modulated by a transient electric field, so that the signal-to-noise ratio of a detection system is improved.

3. The method can be used for deeply understanding the ICF implosion physical process, and has great practical prospect in the field of ICF-based neutron detection.

Drawings

Fig. 1 is a schematic structural diagram of an ultrafast neutron pulse spectrum detection system of the present invention.

In the figure, 1-synchronous trigger unit, 2-femtosecond laser, 3-chirped grating, 4-polaroid, 5-delay module, 6-beam splitter, 7-phase delay crystal, 8-optical fiber circulator, 9-DIM cavity, 10-optical fiber adapter, 11-Pockels crystal, 12-reflector and 13-spectrometer.

Detailed Description

To make the objects, advantages and features of the present invention more clear, an ultrafast neutron pulse spectrum detection system and method according to the present invention will be described in detail with reference to the accompanying drawings and specific embodiments.

The ultrafast neutron pulse energy spectrum detection system provided by the invention comprises a synchronous trigger unit 1, a linearly polarized light generation unit, a delay module 5, a beam splitter 6, a reference light unit, a detection light unit and a spectrometer 13, as shown in fig. 1.

The synchronous triggering unit 1 is used for synchronously triggering the neutron pulse source and the linearly polarized light generating unit.

The linearly polarized light generating unit is used for generating linearly polarized light L and specifically comprises a femtosecond laser 2, a chirped grating 3 and a polaroid 4 which are sequentially arranged along an optical path.

The linearly polarized light L is divided into a first reference light L1 and a first probe light L2 after passing through the delay module 5 and the beam splitter 6 in sequence.

The reference light unit includes a phase retardation crystal 7, and the first reference light L1 forms a second reference light L1' after being delayed by pi in phase.

The probe light unit includes a fiber optic circulator 8, and a pockels crystal 11 and a mirror 12 disposed within the DIM chamber 9. One interface of the fiber optic circulator 8 is connected to a fiber adapter 10 of the DIM chamber 9. The reflecting surface of the mirror 12 is closely attached to one side surface of the pockels crystal 11, and the side surface faces the neutron pulse source.

The first probe light L2 is transmitted to the DIM chamber 9 through the optical fiber circulator 8, and is incident on the other side surface of the pockels crystal 11 to form a second probe light L2 'with a changed polarization state, and is reflected by the mirror 12 back to the optical fiber circulator 8 and output to interfere with the second reference light L1'.

The spectrometer 13 is used for receiving the interference image of the second reference light L1 'and the second probe light L2'.

The method for detecting the ultrafast neutron pulse energy spectrum by using the system comprises the following steps:

1) triggering

1.1) synchronously triggering a neutron pulse source and a femtosecond laser 2 by using a synchronous triggering unit 1; generating ultrafast neutron pulses using laser Inertial Confinement Fusion (ICF); meanwhile, the femtosecond laser 2 emits femtosecond laser with a spectral range of λ, and after the time width of the femtosecond laser is adjusted by the chirped grating 3 and is widened to T, the femtosecond laser enters the polarizing film 4 in the vertical direction to form linearly polarized light L with the spectral range of λ and the time width of T.

1.2) adjusting a delay module 5 to realize high-precision synchronization of the linearly polarized light L and the ultrafast neutron pulse.

2) Electro-optical sampling

Irradiating the pockels crystal 11 rich in hydrogen element with the ultrafast neutron pulse in step 1), when the neutrons interact with the thin crystal, generating and emitting a large amount of recoil protons, while the electrons remain immobile, thereby forming an electric field e (t) in the pockels crystal 11, the direction of the electric field e (t) being opposite to the direction of the incident neutrons. The electric field E (t) reflects the time and energy spectrum information of the ultrafast neutron pulse, and can be measured by using an electro-optical sampling technology.

Meanwhile, the linearly polarized light L in step 1) is split into a first reference light L1 and a first probe light L2 by the beam splitter 6. The first reference beam L1 is phase-delayed by pi by the phase delay crystal 7, and a second reference beam L1' is formed and output. The first detection light L2 is sent to the optical fiber circulator 8, coupled into the DIM chamber 9 through the optical fiber adapter 10, and transmitted to the surface of the pockels crystal 11. After the first probe light L2 is modulated by the electric field e (t) in the pockels crystal 11, the polarization state changes, and a second probe light L2 'is formed, and after the second probe light L3578' transmits through the pockels crystal 11, the second probe light is reflected back to the fiber circulator 8 by the reflector 12, so that the electro-optical sampling is realized.

3) Interference

The second probe light L2 'output through the fiber optic circulator 8 interferes in space with the second reference light L1', and the interference image is received by the spectrometer 13.

4) Measuring

The spectral range Δ λ of the interference image and the spectral intensity i (t) representing the amplitude of the pockels effect modulation are measured.

5) Obtaining temporal widths and energy spectra

5.1) calculating the time width Δ t of the second probe light L2' according to:

in the formula, lambda is the spectral range of linearly polarized light L;

t is the time width of linearly polarized light L;

Δ λ is the spectral range of the interference image.

5.2) I (t) is obtained by three-part convolution, namely: plasma-induced ultrafast neutron pulse temporal broadening sig (t), the resulting transient electric field e (t), and the broadening of the photoelectric sample eos (t).

Calculating the energy spectrum Sig (t) of the ultrafast neutron pulse by a deconvolution mode:

in the formula (I), the compound is shown in the specification,representing a deconvolution operation;

i (t) is the spectral intensity of the interference image;

EOS (t) is the broadening of the electro-optic samples, EOS (t) is currently known as 0.3 ps;

e (t) is the electric field in the Pockels crystal 11.

Theoretical analysis and physical derivation:

denoted as femtosecond laserThe variation of the pulse with time and space is used to generate the initial laser pulseDecomposed into a superposition of two orthogonal directional components of x-ray and y-ray,is the unit vector along the polarization direction of the polarizer 4,andis the unit vector of the polarization directions of the x and y light, A₁For amplitude, ω is angular frequency, t is time, λ is wavelength, and z is spatial position.

Femtosecond laser pulseThe light is divided into a first reference light L1 and a first probe light L2 by a beam splitter 6, wherein the first reference light L1 passes through a phase delay pi of a phase delay crystal 7 to form a second reference light L1 ', and L1' can be expressed as:

the first probe light L2 is used for electro-optical sampling. Due to pockels effect, the polarization state of the first probe light L2 is transiently modulated by the electric field E to form the second probe light L2 ', and the refractive indices of both the x-and y-polarization directions of the second probe light L2' are modulated to form a phase difference.

When the laser propagates in the crystal, the propagation directions x, y and z of the laser correspond to different refractive indexes of the crystal, and the spatial refractive index distribution of the crystal is ellipsoidal.

The refractive index ellipsoid equation for pockels crystal 11 when no electric field is applied is:

n_ois the magnitude of the refractive index at the intersection of the ellipsoid and the X-axis or Y-axis, n_eIs the refractive index at the intersection of the ellipsoid and the Z axis.

When the applied DC electric field E is parallel to the optical axis (z axis), the refractive index ellipsoid equation is:

γ₆₃is a non-linear refractive index coefficient, E_ZElectric field strength in the z-axis direction, E_Z＝E。

Selecting a new coordinate system (x ', y ', z '), in order to make the refractive index ellipsoid not contain cross terms, the three main axes of the refractive index ellipsoid are rotated around the z-axis by the three main axes when no DC electric field is appliedThe refractive index ellipsoid equation in the new coordinate system is obtained as follows:

n′_x、n′_y、n′_zrespectively, the refractive index components in the x ', y ' and z ' directions in the new coordinate system.

n′_y-n′_x＝n_o ³γ₆₃E

Delta represents the optical path difference, h is the thickness of the crystal along the direction of the electric field, and V is the voltage applied to two stages of the crystal.

The second probe light L2 'returns via the mirror 12 and the fiber optic circulator 8, and L2' can be expressed as:

A₂to detect the amplitude of light L2'.

The ratio of the light intensity of the second probe light L2' to the intensity of the linearly polarized light L incident on the pockels crystal can be expressed as:

θ is an angle between the direction of the crystal optical axis and the direction of the polarizing plate 4.

If the L2 is not modulated by the transient electric field, the reflected second probe light L2 'will be consistent with the originally incident linearly polarized light L, and since the phase difference between L1' and L is pi, the L1 'and L2' are in the destructive interference state, and the detector cannot detect the signal.

If the L2 is modulated by the transient electric field, the L2 'and the L1' interfere in the space, the detector can obtain an interference signal, and an interference imageCan be expressed as:

from interference imagesThe intensity information can be obtained, the phase difference delta of transient modulation can be further obtained, the voltage change V caused by neutron pulse irradiation can be obtained according to the phase difference delta, and therefore the energy spectrum of the neutron pulse can be obtained. From the above analysis, it can be seen that this method can achieve picosecond time-resolved neutron spectrum detection.