X7 BEAM COMPOSITION MEASUREMENTS For tertiary beams mainly

INTRODUCTION

The tertiary beam operation of X7 allows to run either hadron beams or electron beams.
The hadron beams are normally produced by using a Copper target. The beam contains in general a not quite negligible fraction of electrons or positrons from pi0 decays. The fraction of electrons (or positrons) may be reduced by putting 5 or 10 mm lead absorbers into the beam.
Depending on the H3 momentum, electron beams (or positron beams) are produced either by

1. Bremsstrahlung in a 4 mm thick Lead target. This way is preferred if the H3 momentum is lower than say 140 or 150 GeV/c.
2. Conversion of photons emerging from a Beryllium target in a 4 or 6 mm thick Lead converter, located just downstream of a sweeping magnet (TRIM-1), following the secondary target. This is the only way for H3 momenta above 150 GeV/c (electrons in the H3 are lost due to synchrotron radiation) and possibly the better way for positive H3 momenta (where the electron content is diluted by the presence of a significant fraction of protons in the beam).

In this note we describe how the beam composition of the X7 can be measured simply from your beam terminal. The first step is to measure the muon content of the beam. The rest of the beam can be separated in hadron and electron components, either by studing the effect of different absorber thicknesses on the beam or by measuring directly using the threshold Cerenkov counters in the beam.
The procedure is illustrated with measurements done during a summer student workshop on the 16th of September 1997. At that time the H3 was running at +250 GeV/c and the X7 at -25 GeV/c.

MUON COMPONENT

The beam flux is a linear function of the gap of the momentum slit, COLL-1. In most cases the measured beam fluxes as a function of COLL-1 slit lie on a straight line, the extrapolation of which crosses the zero-gap line at a positive value of flux. This is the muon flux.
Alternatively, one may close COLL-1 and COLL-2 to a 2 mm gap around e.g. +40 mm. In that case all hadrons and electrons are removed from the beam and one measures the muon flux directly.

As the muon flux (depending on the ratio of X7 and H3 momenta) may be weak, it is strongly recommended to measure these fluxes with the coincidence of at least 2 counters, either a suitable experimental scaler or (as in the example below) the coincidence of TRIGS 4 and 5.

USING ABSORBERS

As explained in the introduction a hadron beam is produced from a 40 cm Copper target, whereas the electron beam is produced either by Bremsstrahlung in a Lead target or by conversion of photons from pi0 decay. Let us denote the total fluxes of these beams, after subtraction of the muon component, by H0 and E0, respectively. H5 and H10 denote the fractions of the hadron beam that survive the presence of 5, respectively 10 mm of Lead absorber. E5 and E10 denote the corresponding quantities in the electron beam. The effect of the absorber is two-fold:

1. It removes a large fraction of the electron component in the beam due tu Bremsstrahlung and the consequent energy loss in the absorber,
2. It scatters the remaining hadrons in the beam and some of those may then be lost on apertures.

H0  =    hh +    eh 

H5  = A  hh + B  eh 

H10 = A2 hh + B2 eh 

E0  =    he +    ee 

E5  = A  he + B  ee 

E10 = A2 he + B2 ee 

where h_h and e_h denote the hadron and electron components in the hadron beam and h_e and e_e the corresponding components in the electron beam. Now we may assume that B is small (for momenta of 20-50 GeV/c, B is typically 3-4%) and therefore B² is negligible. Also the electron components in the hadron beam are typically small enough (10-20% or less) to make Be_h negligible. In that case some of the equations reduce to:

      H5  = A  hh          (1)
      H10 = A2 hh          (2)  

Substitution of this into (2) yields H10 = (H10/H5)² h_h, hence

By neglecting the B²-term in the expression for E10, one finds that E10 ~ A²h_e and thus

After some algebra, finally:

With this knowledge one may refine the calculations by iteration. In particular one may correct for Be_h in the expression for H5 and find a more precise value for A and thus h_h and e_h.

Example: In the summer student workshop we measured for +250 -> -25 GeV/c the following relative fluxes (the muon component was negligible):

Using the above formulae, one finds:

which justifies the approximation B²=0. Then:

I.e. the hadron beam without absorber contains about 13% of electrons. Adding 5 mm of absorber reduces this by a factor B to 0.5% and 10 mm would reduce it to 0.2 permille. The refinement in the expression for H5 would reduce the value for A h_h to A h_h = H5 - Be_h = 0.68 - 0.005 = 0.675 and thus h_h = 0.675² / 0.53 = 0.86 and e_h = 0.14, indeed a small correction only.

USING THRESHOLD CERENKOV COUNTERS

Charged particles that travel through a medium with a speed v/c = p/E larger than the velocity of light in that medium (v/c = 1/n where n denotes the refractive index), emit Cerenkov light. In a gaseous medium, the refractive index n is a function of the gas pressure. As electrons (very light) travel faster than (heavier) hadrons of the same momentum, electrons may emit light whereas pions and other hadrons don't, for suitably chosen gas types and pressures. From a given threshold pressure onward, the probability to detect a signal increases slowly from 0 at threshold to almost 1 at high pressures. Taking advantage of these features, threshold Cerenkov counters allow to measure the composition of a beam in terms of different particle types.

A good estimate of electron and pion (hadron) components can be obtained by comparing pressure scans in the hadron and electron beams without absorber. From the pressure scan in the hadron beam one obtains:

1. The pion threshold pressure P_thr,
2. The signal probability S_e for electrons just below the pion threshold.

The fraction of electrons in the hadron beam e_h is equal to

From the scans done during the summer student workshop we found S_e = 12.5% and E_e = 89%. This yields e_h=14%, in very nice agreement with the absorber method!