COMPASS

COMPASS

T6

Muon sweeping in the M2 muon beam line for the COMPASS experiment

with a view to its potential for CLIC

Lau Gatignon / EN-MEF

EN

Muon sweeping in M2 beam line 2

A potential problem for CLIC: Muons from beam haloBeam tails are scraped away by a collimation system in the BDS.Below we show simulated profiles of the beam at the BDS entrance (core of beam in red)

From I.Agapov et al,2009, to be published

From these simulations one estimates that a fraction 2 10-4 hit the collimators,i.e. about 2.4 108 particles per train assuming a total flux of 1.24 1012 per train.Preliminary estimates indicate that out of those ~ 2 105 would reach the detectors.

The final rates remain to be studied with BDSIM using the final and detailed geometryLG, MDI meeting 19-02-2010

Courtesy: H.Burkhardt

Muon sweeping in M2 beam line 3LG, MDI meeting 19-02-2010

‘Typical’ muons at CIC have momenta of a few hundred GeV/c

H.Burkhardt, CLIC09


Deepa Angal-Kalinin

Dipoles???


Basic approach to reduce muon halo: Toroidal fields in Iron

Magnetised Iron has three main effects on muons:

Energy loss: dE/dx ~ 1.5 GeV/m Multiple Coulomb scattering:

Magnetic deflection:

scatt[mrad]14

p[GeV /c]L /Xo

14p 0.0176

L 106p

L

defl[mrad]300

p[GeV /c]BL[Tm]3001.5

pL

450pL

For a useful effect, require defl >> scatt , i.e. 450 L >> 106 √L or L >> 0.05 m

e.g. for defl > 10 scatt one requires L ≥ 5 m

In the M2 beam we use two types of toroids:

1. SCRAPERS: adjustable gap, no vacuum (?), smaller outer coverage, expensive2. MIBS: fixed gap, large coverage, cheap, vacuum tube can go through


SCRAPERS(Magnetic Collimators)

B

Magnetic force

Have to get polarity right….LG, MDI meeting 19-02-2010


4 Motors: 2 upstream, 2 downstreamallows to follow beam divergence

Have to stop current before moving jaws !!!LG, MDI meeting 19-02-2010

MIB = Magnetised Iron Block


Note:

Both SCRAPERS and MIBS are easy to powerTypically operated with 100 to 200 AmpsResistance of one MIB coil (for L=1.6 m) is ~ 100 mOhmTherefore 10 m of MIB consumes only ~150 A x 100 V ~ 15 kW

There is not much reason to push the currentThis would lead to strong saturation of the field in the Ironand quick increase of the stray field on the main beam(but we use the quadrupolar stray field to measure the polarity)

There is (in principle) no effect of the field on the main beamwhereas dipole fields would require compensation

The requirements on precision of machining are very modestSee e.g. the picture of the MIB in he M2 beamJust make sure there is good magnetic contact between blocks

The deflection of a 500 GeV/c muon by a 5 m long toroid is ~ 5 mradAfter 2 km drift, it is deflected away by 10 m (well outside tunnel & detector)


‘Good’ muons

Halo after p absorber

Halo after m momentumselectionNote: Halo / ‘Good’ m ≈ 6

P (GeV/c)

Flux

(arb

itrar

y un

its)

The size of the Halo problem

Simulations based on theHALO program by C.Iselin

(1974)


X (mm) X (mm)

Y (m

m)

Without muon sweeping With muon sweeping

The effect of muon sweeping in the M2 muon beam for COMPASS

LG, MDI meeting 19-02-2010


X (mm) Y (mm)

Muo

n flu

x (a

rb.u

nits

)

No sweepingPassive ironMagnetic collimation



P (GeV/c) P (GeV/c)

Muo

n flu

x (a

rb.u

nits

)

Within 10 cm Outside 10 cm


P (GeV/c) P (GeV/c)

Muo

n flu

x (a

rb.u

nits

)Muons within 10 cm

Muons with r [10,200] cmMuons vs Halo

i.e. Halo (R=2 m) / Muons (R = 10 cm) ~ 5%,i.e. Halo/Muons in same surface ~ 1.3 10-4

LG, MDI meeting 19-02-2010 18Muon sweeping in M2 beam line

Excellent agreementWith data!

Gain > factor 100

Alternative possibility, based on MBPL dipole magnet, used in K12 beam:

Fill gap with weak Iron

40 mm diameterhole for beam passage

Run at 20 Amps(for 75 GeV/c main beam)

Put 3 magnets of 2 m lengthin series

Correctors needed !

Advantages:

Do not sweep muons upward, into the fields In the limited length available (about 100 metres), in the presence of

both positive and negative muons, avoid the problem of “refocussing” muonswith the second toroid, which were swept away by the first one

Disadvantages:

Covers a smaller surface than e.g. a MIB Needs high-precision engineering and machining, hence more costly Stray field on beam axis needs to be compensated by two correction magnets It takes about 15 minutes for the field to stabilise after a current change.

During that time the beam steering shifts continuously


Conclusions The M2 high-energy high-intensity muon beam provides a main beam of

4 108 muons of 160 GeV/c per SPS cycle to the COMPASS experiment. These “good” muons are accompanied by a “halo”of about 3 109 muons from

pion decays along the 600 metres long decay volume. With a system of 7 magnetic collimators”(5 m long each) and a total of some

15 metres of magnetised iron blocks this halo can be reduced to 5% of the beamflux over a surface of about 4x4 metres (the size of the COMPASS chambers).Both types of devices are magnetic toroids.

The halo momentum distribution is comparable to the one of the muons producedin the CLIC BDS collimation system, but in the latter case the lever arm is significantlylonger. Also in the CLIC BDS the main beam are electrons rather than muons, therefore their likelihood to become high-energy muons is reduced w.r.t. M2.

The MIBs combined with some magnetic collimators could therefore be aneconomical solution to reduce the muon background in the CLIC detectors.

This approach merits follow-up in the BDSIM calculations.


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