Page 69 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
66-67
Figure 2
A photo of the experimental set-up used to measure the asymmetry of the α-particle emission in the reaction 10B(n,α)7Li, induced by polarised neutrons. The principles of its operation are explained in the text.
preamplifier; the electrodes attached to ‘backward’ sections are connected to a separate ‘backward’ preamplifier. The two preamplifiers convert the electric currents from the ‘forward’ and ‘backward’ chambers to electric voltages that are proportional to the α-particle flux. We used gaseous insensitive layers to absorb the heavy component and provide a mean angle of the α-particle emission. The special gaseous detector is an assembly of gaseous ionising chambers placed into
an aluminium jacket with a length of 1 800 mm and a diameter of 300 mm. The detector is filled with argon at the pressure of 0.3-0.8 bars. A detector of this type can capture about 80−90 % of the neutrons. A special solenoid, reeled to the jacket, provides the neutron spin guide field inside the detector.
Measurements of the P-odd asymmetry in the reaction 10B(n,α)7Li were performed earlier on the polarised cold-neutron beam at the WWR-M reactor at PNPI (Gatchina, Russia). The integral neutron intensity was about (1-3)·1010 s-1and the mean neutron polarisation P ~ 0.8. A second experiment was carried out on
the horizontal polarised neutron beam PF1B at the ILL [3]. The mean neutron wavelength was λ =4.7 Å, the
→→ Therefore, the observation of the P-odd asymmetry (σ p )
10 7np of an α-particle emission in the reaction B(n,α) Li is of
great interest; hence, we performed such an experiment.
→→→
Here, σn, pn, pp are the spin and momentum of the neutron,
and the momentum of the α-particle, respectively. To
eliminate the influence of a false effect associated with the
so-called left-right asymmetry on the results of measurements
of the P-odd asymmetry, we had to ensure that the vectors
→→→
σ , p , p were parallel to each other. To achieve maximum
10 -1 integral flux of polarised neutrons (4-5)·10 s
asymmetry coefficient of the α-particle emission equal to nnp -8
accuracy, we used a method based on two detectors and a special measuring procedure. This method assumes simultaneous measurement of the same process in both detector channels. However, the signs of the P-odd effects are opposite, while those of the effect of synchronous fluctuations are the same. Hence, the P-odd effects were combined and the fluctuation noise cancelled.
A photo of the experimental set-up is shown in figure 2. The detector of the α-particles emitted in the reaction 10B(n,α)7Li consists of 24 ionising double chambers installed along the beam axis. The targets made of 10B are placed in the centre of each chamber. One side
of each double chamber detects α-particles emitted in
the neutron beam direction (‘forward’), while the other
side detects α-particles in the opposite, backward
direction. Because of the different orientation of
-(11.2±3.4)·10 .
α-particle momenta →p with respect to the neutron spin p
The uncertainty of this result is compatible with uncertainty concerning the P-odd asymmetry in the reaction 6Li(n,α)3H. This is the first time in the world that this non-zero result has been obtained. It is only the second light nucleus, in addition to 6Li, for which a non-zero P-odd effect has been found. This work was awarded 1st prize in two very challenging scientific competitions in 2018 (at PNPI and FLNP JINR).
After decades of development, we have been able to increase the sensitivity of our experiments to the level needed to reliably measure such small asymmetries. Simultaneously, theoretical models such as EFT (Effective Field Theory) have begun to calculate such nuclear reactions reliably. Another new theoretical approach [4] consists of calculating parity-violating NN force in the 1/Nc expansion (where Nc is number of colours); this may be of particular interest to the present study as it naturally explains the smallness of the fπ coefficient compared with the hρ0 observed experimentally.
This situation offers exciting opportunities and promises new results.
σn, asymmetries measured in forward and backward directions have opposite signs. The electrodes attached to all ‘forward’ sections of the detector’s double chambers are connected to a common, ‘forward’
and the neutron polarisation P = 0.92 ± 0.02. The results
of both experiments, corrected for the corresponding
neutron beam polarisation P and the mean cosine of
the α-particle emission angle, gave a value of the P-odd
www.ill.eu