Background from the NIST test The pencil neutron beam (1 mm2) with intensity about 7000 n/sec The beam was completely absorbed in the beam stop with the production of the 477 keV prompt gamma’s, yield 100% per capture . To enter the helium volume,neutrons pass through 3 teflon windows and 2 thin Beryllium windows, the transmissions are, probably more than 90%. Then to enter the detection region, the neutrons pass through the 1/16" acrylic front window of the EDM cell Count rate of every PMT without coincidence was 7-8 KHz Count rate in coincidence, the threshold on 2+ p.e. in every PMT: • Count rate with the beam closed by Li-rubber - 40Hz (mostly due to muons, no veto) • T= 4K, empty cell, total count rate 80 Hz • T= 1K, Cell filled with Ultra Pure 4He, total 120 Hz Therefore, the background structure was • 40Hz ( muons + ambient gamma’s) + acrylic 40 Hz + helium 40 Hz • The effect of surrounding instrument is on the order of 10 Hz • When we first let the neutrons into the cell, we noticed that large number of events are due to gamma rays from the monochromator, we added 2" of lead to block the gamma, it turns out the transmission of the lead for neutrons is 60%.
Combined design. Signal& Background • The NIST test with a pencil beam showed that the detection system is rather insensitive to the soft gamma’s from boron (n,G) • The detection system is quite sensitive to the prompt gamma’s from the monochromator - therefore, it should not be any neutron guide inside the cryostat and the external one must be shielded by the shutter. • The background without F-activation is small compare with the expected He-3 capture rate 500 -200 Hz. Moreover, the capture rate still can be increased by increasing the He-3 concentration. • At NISt the signal is Aexp(-t/) with A= 2 Hz, the time independent background was 10-12 Hz and the time dependent from the Fluorine activation 200exp(-t/150) Hz decaying to below 1 Hz also exponentially . Therefore, they have to fit signal/noise << 1 and they still have a quite good result for the given experimental conditions. • In EDM the signal is A(t)sin(t)+B on the time independent or slow dependent background B, with A=A0exp(-t/), << 1/. • My naïve guess: At a neutron interferometer with the same type of a signal the contrast=A/(A+B)= 30% is a good routine value, that corresponds B=2A. It means 500 Hz of background is still good if the polarization is >90%. • In EDM there is no Fluorine in the cell. There will be a background from electrodes that depends on the material
Combined design. Possible Activation of HV electrodes • Neutron activation calculator: http://www.antenna.nl/wise/uranium/rnac.html • 4(layers)x15x60x10e-4=0.36 cc that is approximatel. 3.2 g if density is 9 g/cc • 1 mu layers • Neutron flux = 103 per cm2s (i assume ballistic collimated beam and voluntary put 10^3 for the flux on the walls) • Irradiation = 1000 h; (practically, a saturation of the concentrations) • Delay = 100 s • 3 g Copper: • 2.055 g Cu-63 (n,G) -> 883.0 Bq Cu-64 (12.70 h) • 3 g Aluminum: • 3.000 g Al-27 (n,G) -> 92.33 Bq Al-28 (2.240 m) • 3 g Platinum: 292.1 µg Pt-190 (n,G) -> 1.4 Bq Pt-191 (2.710 d) • 23.32 mg Pt-192 (n,G) -> 11.57 mBq Pt-193 (50.00 a) • 762.4 mg Pt-196 (n,G) -> 16.85 Bq Pt-197 (18.30 h) Probably, Platinum can be used as an electrode coating????
Platinum, natural abundance • Pt-190 +n->Pt-191 Electron capture to Ir-191 ◦Decay energy: 1.019 MeV, Gamma’s 1.6% -268 keV • Pt-192 +n->Pt-193 Electron capture to Ir-193 ◦Decay energy: 0.057 MeV • Pt-194 stable • Pt-194 +n -> Pt-195 stable, Pt-194, 97% enrichment www.isotope.com, no activation!!!!! • Pt-195 +n -> Pt-196 stable • Pt-196 +n->Pt-197 Beta to Au-197 • Beta Decay Energy: 718.919 +- 0.594 keV, Gamma’s 71% 279 keV • Pt-198 +n->Pt-191 Beta to Au-199 ◦Decay energy: 1.702 MeV
Typical Gamma spectrum of activation 511 keV • Left - from 100 to 800 keV Right from 800 to 1600 keV • Al-28 has the slightly higher energy. • Basically, the activation spectra are of low energies because are emitted by the de-exited nuclei. The annihilation peak is very small. • The main danger of activation is a high energy beta’s. The intensity is moderated by the life time exponent.
Combined design. Summary • Neutron beam polarized and ballistic focused into the cell • During measurements a shutter shields the cryostat from the gamma’s of the beam splitter/polarizer • Neutrons come through Be windows, coated with teflon • The cryostat is shielded by the neutron absorbing material (like at NIST) along the way to the cell • The electrodes are made from acrylic coated with Pt. • The cell has thin (deuterated?) plastic windows on the entrance side and • It can continue as a light guide on the other side with an insert of B2O transparent beam stopper (like at NIST). • Neutron storage time =500 sec, max UCN density 0.4x500 UCN /cm3 (from proposal, depending on the efficiency of the beam splitter can be less), acceptable activation background 500 Hz without the particle identification technique