Characterization of the n TOF EAR-2 neutron beam

The experimental area 2 (EAR-2) at CERNs neutron time-of-flight facility (n TOF), which is operational since 2014, is designed and built as a short-distance complement to the experimental area 1 (EAR-1). The Parallel Plate Avalanche Counter (PPAC) monitor experiment was performed to characterize the beam profile and the shape of the neutron flux at EAR-2. The prompt γ-flash which is used for calibrating the time-of-flight at EAR-1 is not seen by PPAC at EAR-2, shedding light on the physical origin of this γ-flash.


Introduction
A second experimental area (EAR-2) at CERN's neutron time-of-flight facility (n TOF), having a flight path of ∼20 m from the spallation lead target and 90 degrees respect to the incoming proton beam, has been designed and built, offering advantages compared with the former experimental area (EAR-1): 1) much higher neutron flux of about a factor 25; 2) for highly radioactive samples an additional factor 10 is obtained for the signal to noise ratio due to shorter time interval resulting from the 10 times shorter flight distance [1,2], thus fulfilling the demands of the neutron science community for a time-of-flight facility with a higher flux [3].
In this contribution we present the characterization of the EAR-2 neutron beam by means of a measurement with the Parallel Plate Avalanche Counter (PPAC).

n TOF facility at CERN
The n TOF facility at CERN is based on a spallation neutron source which can provide neutrons from thermal energy up to GeV by impinging 20 GeV/c protons onto a thick lead target.The proton beam has a typical intensity of 7×10 12 /pulse with 7 ns (RMS) pulse width and a cycle of 1.2 s or its multiple, yielding about 300 neutrons per single incident proton.The layout of the n TOF facility is depicted in Fig. 1 showing the two perpendicular neutron beam lines.
The horizontal neutron beam line sends neutrons to EAR-1 through a ∼185 m flight path which has a 10 degree angle regarding to the proton beam in the horizontal plane.While EAR-1 is in operation since 2001, the vertical ∼20 m beam line associated with EAR-2 has been constructed and is operational since 2014, performing its high neutron flux and attenuated γ-flash as a complement.Detailed technical descriptions of the n TOF facility can be found in Refs.[4,5].

PPAC monitor (PPACmon) setup at EAR-2
The PPACs used at n TOF, which were developed at Institut de Physique Nuclaire d'Orsay (IPNO) in France [6], are gaseous detectors filled with C 3 F 8 (octafluoropropane) and working at around 4 mbar typically.Each PPAC consists of a central anode used for timing measurements and two cathodes on each side of the anode for localization with a resolution of 2 mm.The active surface is 200 mm × 200 mm, and the overall assembled dimensions of a PPAC are 305 mm × 305 mm and 13 mm in thickness.Several PPAC detectors can be arranged with interleaved targets, in different experiment configurations.
A new chamber, called PPACmon, was designed and constructed by IPNO, jointly with the University of Santiago de Compostela (Spain) and CERN.As sketched in Fig. 2   backing with a thickness of ∼70 µg/cm 2 is surrounded by 2 PPACs, so that fission events can selected by the coincident detection of the fission fragments.This coincidence method is highly selective on fission reactions, rejecting radioactive emissions and more importantly other spallation reactions above tens of MeV.

Energy determination
The incident neutron energy is determined by the velocity where L is the flight path, T is the time recorded by the detector and T 0 is the starting flight time.So T − T 0 is the time-of-flight (TOF).Among these 3 parameters, only T is precisely known which is directly given by the anode of the PPAC.L and T 0 need to be determined.L is obtained by the comparison of the detected fission rate and the standard 235 U(n, f ) cross section [7] in the resonance region where T is large enough that T 0 can be neglected and we can use T instead of TOF.We adjust L for obtaining the best match with the JENDL/HE-2007 database [7] (Fig. 3).
T 0 is usually determined by the prompt γ-flash signal which is a sharp narrow peak at the beginning of the signal frame in case of the measurement at EAR-1.But this method is not applicable here since the sharp peak is not visible at EAR-2.Instead, we use the pickup (PK) signal of proton beam as the time reference to determine T 0 .Because the PK signal is delayed compared with the true T 0 , an offset is added to determine TOF.The offset is determined based both on the simulation and the first fission event of each proton pulse which could be induced by photons or other high energy particles.
The TOF spectrum with 100 bins per decade (bpd) is shown in Fig. 4 and the fission rate as a function of neutron energy with 100 bpd is shown in Fig. 5.

Beam profile reconstruction
The position distributions of the fission fragments on PPAC 1 and PPAC 2 (Det 1 and Det 2 in Fig. 2) are shown in Figs. 6 and 7.In Fig. 7 there is a distorted      The beam profile (Fig. 8), i.e. the emitting points on the target between PPAC 1 and PPAC 2, can be reconstructed according to Figs. 6 and 7 based on the back to back emission of fission fragments.

Neutron flux
The energy dependence of the neutron flux can be obtained by dividing the fission rate (Fig. 5) by the 235 U(n,f) cross section [7].For neutrons above 1 MeV, the fission fragment angular distribution (FFAD) is not isotropic any more due to the momentum transfer from the incident neutrons to the target nuclei.The anisotropy correction is applied in the calculation of the neutron flux above 1 MeV based on the FFAD of 235 U from a PPAC measurement in 2011 [8].The unnormalized energy dependence of the lethargic neutron flux is shown in Fig. 9.
The unnormalized preliminary result in Fig. 9 shows that PPACs are covering a wide energy range, extending to very high energies.A more dedicated and specific work for the evaluation of EAR-2 neutron flux by the n TOF collaboration which will be published soon.

Conclusions
We present the first PPACmon experiment, done at n TOF EAR-2, for characterizing the beam profile and neutron flux at this new facility.We also found that the prompt γflash signal (sharp narrow peak with a high amplitude) is not visible at EAR-2, contrary to EAR-1.It means that the these prompt γ-flash is not caused by the deexcitation of nuclei in the spallation target but from the angle focused in-flight decay of high velocity particles which follow the horizontal beam line and are, therefore, excluded from the vertical beam line to EAR-2.Geant4 simulation indicates that prompt γ-rays are essentially from the decay of neutral pions (π 0 ), with π 0 s coming from nucleon-nucleon collision in intranuclear cascades.Since π 0 s are highly boosted forward along the proton beam in laboratory frame, so are the γ-rays, which means they can follow the horizontal beam line to EAR-1 but only very litte can fly to EAR-2.The deexcitation of the nuclei in the lead target and neutron capture in lead and moderation layer can produce both prompt and delayed γ-rays, results in a broad γ-flash lasting up to a few hundred nanoseconds.This is quasi isotropic and visible both at EAR-1 and EAR-2.

Figure 1 .
Figure 1.Layout of the n TOF facility.

Figure 3 .
Figure 3.Comparison of the experimental fission rate and 235 U(n,f) cross section in the database.

Figure 5 .
Figure 5.The fission rate as a function of neutron energy with 100 bpd.