I have made a priliminary estimation of neutron flux in WAH based on existing geometry for WAH (Jason), and geant/gcalor version with low kinematical cuts (electons, gamma = 10 keV,

neutron = 1e-13 GeV, charged hadrons = 1MeV). The fluxes have been calculated for pp 510 GeV and BBC rate ~1.2 MHz.

The goals for this study are the following:

1. To make an estimation CPU time to process 1 event with the above cuts and for whole STAR WAH.
2. To make an estimation neutron, gamma, electron and charged hadron fluxes.
3. To make an estimation of rate for a typical neutron He3 counter.
4. To see 1/R dependence of charged hadrons and electron flux in TPC to see how well the present model for space charged density corresponds to thes calculation

• The existing WAH geometry is shown below as X- and Y-cuts. It include some elments of tunnel but not all of them. The geometry has to be revisited.  

• With present geometry and the above cuts CPU/event is ~1 minute.
• Neutron kinetic energy and fluxes for all neutrons and  neutrons with kinetic energy below 250 meV are presented below.

The flux of neutrons in WAH is 100-1000 Hz, and for termal neutrons ~10 Hz.

To estimate couter rate  for Centronic 31He3/304/25e counter 

• flux = 10 (Hz/cm^2)
• Tempereture (T) = 300 (K)
• Thermal neutron kinetic energy (kT) = 25.8(meV)
• Thermal neutron verlocity v0 = 7.4e-06 (C)
• Thermal neutron cross section (n + He3 -> p + H3) = 5400 * v0/v (barn)
• Centronic 31He3/304/25e counter 
• Pressure = 4 (ATM)
• Volume = 152.171 (cm^3)
• Total neutron path length in counter = l = V * flux = 1521.71 (cm * Hz)
• No. interaction per atom = N  = l * xsection0 = 8.2e-18 (cm^3 * Hz)
• A = 3  density = A/22.4(10e3 cm^3) = 0.000133929 (g/cm^3)
• No. of atoms in 1 cm^3 = Avogadro/22.4(l) = 1.44531e+20 (1/cm^3)
• counts =120  per neutron flux 1 (Hz/cm^2)
• rate @  flux =10 (Hz/cm^2) = 1.2 (kHz)

Fit by 1/R^n dependense in TPC (|Z|<200 cm) gives n = 1.88.

Fit by 1/R^n dependense in TPC (|Z|<200 cm) gives n = 1.36 and flux from electrons (~5 kHz/cm^2) is factor of 5 higer than flux from charged hadrons (~0.8 kHz/cm^2).

Conclusions.

1. The flux of neutrons in WAH ~1 kHz/cm^2 (for pp 510 GeV with BBC rate ~1.2 MHz). The thermal neutron flux is ~10 Hz/cm^2.

2. A typical rate of He3 counter is 120 Hz @ flux of thermal neutron 1 Hz/cm^2.

3. The flux from charged particles clearly shows two components : one from charged hadrons and another from electrons.

4. CPU processing time for pp 510 GeV @ STAR is ~ 1 minute / event. This is a factor of  60 less that it was for LHC calculation 15 years ago.

I guess that this reflects the first of all a factor 30 in reduction of  total energy (510 GeV and 14 TeV).