Neutron detection in LHe ( HMI run 2004) R.Golub, E. Korobkina, J. Zou M. Hayden, G. Archibold J. Boissevain, W.S.Wilburn C. Gould

Layout of the neutron beam Cross section of the full beam was 1 cm 2, intensity ≈ 10 6 n/cm 2 /s before Bi The distance from the polarizer to the light window was ≈ 120 cm Polarizer Lead shield Collimator, Li Light guide 33 cm Bi filter Lead shield Li Beam stop when needed Beam monitor Beam stop Chopper and neutron guide of SANS Li shield Guide

Layout of the cell and light collecting system OD of the cell =5 cm, ID = 4.4 cm The distance from the neutron beam to the light window, L, was L≈2 OD PMT C2 300K 77K Light guide 33 cm 4K Al wall, 2 mm thick Neutron beam Light window of the cell Acrylic cell 22 cm Cell Cross section Reflector - Al foil

Beam Stop Cryostat View looking `upstream’ (toward the reactor)

Collimation neutrons PMT light guides cell  delay line oscilloscope

Cell, as viewed through PMT window free liquid surface

What was studied We have studied at different temperatures 1.Empty cell and Neutron guide hall background 2. Cell filled with Natural He - 90 mK, 300 mK, 700 mK, 1400 mK Full beam opened and closed with Li. 3. Cell with additional He-3 (about 100 ppm) –90 mK, 300 mK, 500 mK, 650 mK, 1500 mK, Full beam opened and closed with Li. –Small beam opened, 90 mK, 120 mK, 300 mK, 500 mK, 1500 mK, 1900 mK, 90 mK with and without coincidence, 10 mV LDL and 15 mV LDL –Ultra Small beam opened, 90 mK, 1500 mK, 1900 mK Data were taken in two modes - Count Rate, where 10 ms intervals were recorded, time scale = 10 or 1 ns/point - Sequence, where only coincided events and after pulses during 20 mks were recorded, time scale = 1 ns/point We have used LeCroy oscilloscope with a time scale down to 125 ps per point

Count Rate data evaluation Our data allowed us to compare Count Rate data evaluation with and without coincidence. Without coincidence is a real count rate of the detectors that is a sum of the background from acrylic light guides and the cell’s signals. Coincidence mode is a great tool to get read of the background pulses from the light guides.

Empty cell, no coincidence First big run; cell is empty, PMT HV=2kV, time scale 1 ns/point to see small pulses. A single photon (SP) peak and gamma are well separated. Gamma peak = 3 SP.

Empty cell, coincidence

Beam On, Li in the beam PMT pulse area spectra at different temperatures when the beam is stopped by Li rubber. Left - PMT C2, 500 mK, 300 mK with He-3 and 98 mK, Natural Helium Right - PMT C4, 500 mK, 1500 mK with He-3 and 98 mK, Natural Helium

Full beam and Small beam Full beam shows too high intensity of single pulses. We made the beam smaller Count Rate over the neutron peak range ************************************************** * C4 Small 4906 C4 Li 1733 C2 Small 4632 C2 Li 1472 Neutrons  3000 sec -1 ************************************************** * C4 Full C4 Nat He C2 Full C2 Nat He Hz Neutrons  sec -1 ************************************************** *

Small and Ultra Small beams The main set of data was taken for the Small Beam. In addition, we took several runs with a beam even smaller to get more close to EDM conditions Ultra Small Beam Count Rate over the neutron peak range ********************************************* ** C4 USB 2927 C4 Li 1733 C2 USB 2714 C2 Li 1472 Neutrons  1000 sec -1 ********************************************* ** Ratio Gamma’s/Neutrons  3/2

Full Beam, temperatures PMT spectra at different temperatures. 90 mk 350 mK 630 mK 1500 mK 90 mK were measured at two slightly different vertical scales. Setup 1 was used for all Natural He runs Setup 4 - for all He-3 runs

Count Rate in coincidence, Nat.He Here count rates for channels C2 and C4 were evaluated in coincidence. Note that here the Low Discrimination Level (LDL) is below a Single Photon amplitude. Practically, here only presence of any pulse is a trigger. From 0.2 nVsec up to 1.5 nVsec count rate is: C2,C4 Nat He, Full beam ~ 2200 Hz C2,C4 Nat He Full beam Li ~ 550 Hz

Beam with Li and Full beam in Natural He Count Rate (s -1 ) integrated over the neutron peak range [0.2,1.5] nVs C2 Li 1471 C2 Nat He C4 Li 1733 C4 Nat He High energy tail is result of the beam entering the cell

He-3 in the cell, Full Beam with Li, no coincidence PMT HV=1.9kV, time scale 10 ns/point (for count rate mode) to see larger neutron’s pulses. A single photon (SP) peak and gamma are not separated. Gamma spectra show no temperature dependence. Count rate is very stable.

Neutron Peak, C2 and C4 Neutron peak estimated from the Full beam data

Count Rate in coincidence, He-3, Ultra Small beam & Full Beam, Nat.He

Neutron Peak in coincidence, He-3, Ultra Small beam - Full Beam with Li, Nat.He Count rate is close to expected one for EDM

He-3, Ultra Small beam - spectrum evolution of a single PMT

C2+C4, LDL=15 mV - the same conditions as in the sequence mode

Sequence data evaluation Sequence data were recorded with LDL = 15 mV and coincidence time  = 20 ns, during 20 mks after a trigger. The trigger position was 1.5 mks. For 2D plot (presented here) we have integrated area of the main pulse (over 100 ns) and counted after pulses during 6 mks after the main pulse. The first 1 mks and last 10 mks were used to estimate the afterpulse background. We used sum of pulse areas and numbers of afterpulses of both PMT : C2 and C4

2D temperature dependence Single Photon Peak 300 mK 1900 mK

2D Small beam & Full Beam with Li Single Photon Peak

Rough Comparison with the NIST cell Our position is at about 50% of the light collection efficiency. The far end of EDM cell of proposal is at about 30% that implies a neutron peak position at 0.6 nVs 0r  12 single photons (at HMI we have the neutron peak at  20 SP ) HMI beam position Position of a far end of the cell from the EDM proposal Light Guide

EDM cell Probably, light collection can be made from both sides. One side with higher efficiency than another If no light collection from the opposite side, at least, the light guide must be split into 2 parts

Summary Our data allowed us to compare Count Rate data evaluation with and without coincidence. We see that coincidence mode is a great tool to get read of the background pulses from the light guides. In coincidence we can see clear 400 Hz peak of neutrons with the beam  10 4 n/sec in the cell in 1D plot. 2D plot with afterpulses shows even better separation while not as good as at higher temperatures. The single PMT amplitude of the neutron beam is  20 single photons even under our not very optimized conditions(cell mis alignment! ). Our data can be used to simulate quite precisely EDM data. This is only start of our data evaluation!