Measurements of (n,α) cross section s Khryachkov Vitaly Institute for physics and power engineering (IPPE) Obninsk, Russia
Justification for the (n, α ) reaction cross section measurement • Reactor criticality ( structural material , O, N ) • Standards ( 10 B, 6 Li ) • Gas production • Astrophysics • Dosimetry
Experimental methods for (n, α ) reaction investigation Activation method; Direct measurement of α – particle yield;
Activation method Z Z 2 n A D M M 3 Z 2 Z 1 * D N , T 1/2 3 3 M M Z 1 * Z 1 N N M 3 M 3 Sample 1-st stage n 2-st stage Gamma-ray Gamma- detector
Limitations of the activation method Residual nuclear must by radioactive! Half-life time for residual nuclear must be convenient! Energy of gamma-ray must be convenient! Yield of gamma-ray must be significant! For stable residual nuclear activation measurement can not by done at all!
∆ E-E method Target Neutrons dE-detector Alpha- particle E-detector • Low energy particles can not pass through ∆ E detector! • High energy particles will not be stopped in E-detector! • Low geometrical registration efficiency! • It is needed to repeat measurements for different angles!
Classical ionisation chamber 3 o 5 6 7 1 4 HV o o 2 1) Target; 5. Frisch grid; 238 U target; 2) 6. Guard electrodes; 3) Anode; 7. Resistor. 4) Anode signal connector;
Relative method to cross section measurement Investigated target Standard target (Cr) (U) Neutrons Ф I = Ф S n N ( , ) Cr n Cr n N ff U ( f ) U n N ff Cr ( n , ) ( U ) n N U
Classical spectrometer events classification 1. Target 2. Full absorption 3. Electrodes 4. Gas a-particles 5. Protons 6. Wall effect n
Scheme of ionisation chamber with 226 Ra α - particle source b) 226 Ra
226 Ra decay scheme
Scheme of the IPPE experimental setup Anode PA TFA Grid Input 2 Stop Cathode PA SA D DLA WFD PC 238 U Anode Input 1 TFA PA – preamplifier, TFA – timing filter amplifier, D – discriminator, SA – spectroscopy amplifier, DLA – delay line amplifier, WFD – waveform digitizer, PC – personal computer.
Anode pulse shape T L T E 800 Q A 600 Q AMax 400 T G T S T 0 200 0 0 1000 2000 3000 4000 5000 Time, ns
Amplitude of anode pulse vs electron drift time 1,000 0,875 0,750 Po area 0,625 , s 0,500 window 0,375 Rn area 0,250 0,125 20 30 40 50 60 70 80 90 100 Q AMax , channel
α -particle directionality determination Anode 800 1 2 600 Q A , channel Q F Q F 400 Q B Q B 200 Cathode 2,00 2,25 2,50 2,75 3,00 Time, s
α -particle directionality determination 2 400 222 Rn High dE/dx near the cathode 300 Count 200 High dE/dx near the anode dE dQ ( t ) 100 A ( begin ) dx dt begin G dQ ( t ) dE A ( end ) dt dx 0 end 140 218 Po 120 100 80 60 40 20 0 0 20 40 60 80 100 120 140 G , a.u.
Result of α -particle selection 6000 - original spectrum - spectrum after regection 5000 Count 4000 3000 2000 1000 0 0 20 40 60 80 100 120 Energy, channel
Justification for the light isotopes measurement 6 Li, 10 B, 12 C, 14 N, 16 O, 19 F, 20 Ne standards reactivity predictions of thermal and fast reactors; calculation of helium production in fuel pins and claddings of reactors; calibration of the strength of neutron sources; astrophysics; dosimetry.
10 B. Two-dimensional response function
10 B. Anode pulse amplitude vs electron drift time 70 60 , channel 50 40 30 20 14 N(n, ) Ar p+ 10 40 50 60 70 80 90 100 Anode pulse amplitude, channel
10 B(n, α ) 7 Li. Solid target response function o )=0 cos(90 Detector gas: Ar(90%)CH 4 (10%) 7 Li+ particles from 10 B(n, ) 7 Li quasi 80 Cathode pulse amplitude (channel) 10 B(n, ) 7 Li -particles from 0 1 60 10 B(n th , ) 7 Li -particles from 40 o )=1 cos(0 7 Li from 10 B(n, ) 7 Li 20 p, and 7 Li from 10 B(n th , ) 7 Li 20 40 60 80 Anode pulse amplitude (channel)
Solid target kinematics peculiarity. (1) (2) 0.5 0.4 cos( ) limit 0.3 N-beam 0.2 - 0 - 1 Li 0.1 0.0 0 1 2 3 4 5 6 7 Neutron energy, MeV Li(1) Li(2)
10 B. Background suppression. All 10000 Target N Gas 1000 100 10 0 20 40 60 80 100 120 Anode pulse amplitude, channel
10 B. Spectra after corrections.
10 B(n, α ) 7 Li cross-section result 0,7 Data IRMM and IPPE - ENDF B VI=JEFF 3.0 - JENDL 3.3 - JEF 2.2 0,6 - Zhang - Bichsel - Davis Cross section, barn - Friesenhahn 0,5 0,4 0,3 0,2 0,1 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 En, MeV
Why is it difficult to measure light elements? It is difficult to prepare a clean target Low reaction cross section The kinematical effect – dependence of anode pulse amplitude from the emission angle. Negative Q – value. Background from (n, α ), (n,p) reaction, elastic recoil at working gas. Background of a detector. Light elements from the air (O, N) are present on the electrodes surface. Fine structure in cross section.
Problems of a solid target Number of atoms is limited by self-absorption; No radioactive isotope number of atoms determination problem. Energy losses in the target; The particle leaking effect. Background from reactions on working gas components and chamber electrodes. Problem with solid target preparation for some elements (e.g. noble gases)
Gaseous target Kr+CO 2 , Kr+BF 3 , Kr+CH 4 , Kr+N 2 …..
Gaseous target properties Number of atoms in gaseous target is limited only by chamber size and working gas pressure; For events taking place on the working gas components we can register both alpha particle and residual nuclear. Signal to noise ratio will be better. Anode pulse amplitude is proportional to sum of alpha particle and residual nuclei energies. P A ~E α +E R =E n +Q. There are no energy losses in target. There are no the particle leaking effect
Two-dimensional response function of gaseous target Identified signatures of: 16 O(n, ) 13 C, En=7.1 MeV 120 16 O(n, 0 ) 13 C signal from the detector gas Kr(97%)CO 2 (3%) Cathode pulse amplitude (channel) detector gas (1), 100 2 1 alpha particle background of the cathode (2), 80 background of protons 3 60 emitted by the cathode that 4 stopped in the detector gas (3), 40 background of protons emitted by the cathode 20 which crossed the grid (4), 5 Protons background of the 20 40 60 80 100 120 detector gas (5). Anode pulse amplitude (channel)
Scheme of the experimental setup Anode PA TFA Grid Input 2 2 R 2 h Stop R 1 1 Cathode PA SA D DLA WFD PC 238 U Anode Input 1 R 0 TFA PA – preamplifier, TFA – timing filter amplifier, D – discriminator, SA – spectroscopy amplifier, DLA – delay line amplifier, WFD – waveform digitizer, PC – personal computer. 1-monitor chamber; 2-main chamber.
Examples of signals of the main chamber and monitor chamber 400 DSP allows you to analyse: -particle 350 Td 300 Anode 1) Amplitude of anode pulse; 250 Tr 90% of amplitude 2) Amplitude of cathode pulse; 200 Cathode 3) Time when anode signal 150 Pa appeared; 100 Pc Pulse amplitude (channel) 4) Time when anode signal 50 reached the satiation; 10% of amplitude 0 5) Time when cathode signal -50 appeared; 0 100 200 300 400 500 1300 6) Time when cathode signal reached the satiation; 1200 Fission fragment 7) Ionisation distribution along 1100 Cathode the particle track. (Anode 1000 signal shape). 900 800 700 600 Anode 500 400 0 100 200 300 400 500 Time (channel)
Geometry of the gaseous target R 2 h 2 2 ( ) V R R R R 1 1 2 2 h 3 R 1 One of the most crucial points of this technique was precise determination of the radii of the truncated cone’s bases. Investigation of the neutron beam profile was needed. Indium or aluminium stripes and disks were used to measure the profile of the collimated neutron beam at energies 2.5 and 7.4 MeV, respectively. N Oxygen = 2.464*10 20 nuclei at V=5.6346 * 10 -5 m 3 N( 238 U) atoms in the monitor= 6.831*10 18 (solid target ~500 mkg/cm 2 )
Cross section of 27 Al(n, ) 24 Na reaction 0,16 27 Al(n, ) 24 Na 0,14 ENDF/B-VI 0,12 E n =7.4 MeV, =25 mb Cross Section (b) 0,10 0,08 0,06 0,04 0,02 0,00 2 4 6 8 10 12 14 16 18 20 22 Neutron Energy (MeV)
Scheme of the experiment of neutron beam profile measurement 1,2 1,0 Normalised -ray yield 0,8 measured -ray yield error function fit 0,6 R 0 =37.73+-0.11 mm 0,4 0,2 0,0 0 10 20 30 40 50 Radial distance from beam axis at collimator exit (mm)
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