STUDY OF MAGNETIC FIELD INFLUENCE ON HADRON CALORIMETER RESPONSE

V.Abramov^1^, N.Akchurin^2^, P.Baillon^3^, A.Ball^4^, P.de Barbaro^5^,
V.Barnes^6^, G.Bayatian^7^, G.Bencze^8^, A.Bodek^5^, V.Bolotov^9^,
H.Budd^5^, L.Dimitrov^10^, A.Dyshkant^1^, I.Emelianchik^11^, D.Fong^4^,
J.Freeman^12^, V.Genchev^10^, I.Golutvin^13^, P.Goncharov^1^,
A.Gorin^1^, D.Green^12^, A.Gurzhiev^1^, E.Guschin^9^, V.Hagopian^14^,
P.Iaydjiev^10^, V.Klimenko^9^, Yu.Korneev^1^, A.Krinitsyn^1^,
V.Kryshkin^1^, S.Kunori^4^, A.Kurilin^11^, D.Lazic^14^, L.Levchuk^15^,
L.Litov^16^, V.Marin^9^, K.Michaud^5^, Yu.Musienko^9^, A.Nemashkalo^15^,
Y.Onel^2^, P.Petev^16^, Yu.Petukhov^13^, S.Piperov^10^, V.Podstavkov^1^,
V.Popov^15^, A.Proskurjakov^9^, A.Ronzhin^12^, A.Rubashkin^15^,
I.Semenyuk^9^, Q.Shen^6^, V.Shmatkov^9^, A.Sirunian^7^, F.Szoncso^17^,
A.Skuja^4^, P.Sorokin^15^, S.Tereschenko^1^, V.Tyukov^13^, T..Virdee^3,^
N.Vlasov^13^, Volod\'ko^13^, A.Zaichenko^1^, A.Zarubin^13^,
A.Zatzerklyany^15^

^1^ IHEP, Protvino, RUSSIA

^2^ University of Iowa, Iowa City, IA, USA

^3^ CERN, Geneva, SWITZERLAND

^4^ University of Maryland, College Park, MD, USA

^5^ University of Rochester, Rochester, NY, USA

^6^ Purdue University, West Lafayette, IN, USA

^7^ ErPI, Yerevan, ARMENIA

^8^ KFKI Research Institute for Particle and Nuclear Physics, Budapest,
HUNGARY

^9^ INR, Moscow, RUSSIA

^10^ INRNE, Sofia, BULGARIA

^11^ NCPHEP, Minsk, BELARUS

^12^ FNAL, Batavia, USA

^13^ JINR, Dubna, RUSSIA

^14^ Florida State University, FL, USA

^15^ KhFTI, Kharkov, UKRAINE

^16^ SU, Sofia, BULGARIA

^17^ Institute fur Hochenergiephysik, Wien, AUSTRIA

**Abstract**

The response of tile/fibre calorimeter to incident muons, electrons, and
pions in transverse and longitudinal magnetic field up to 3 T has been
studied. The light yield increase with magnetic field depends on the
field orientation and particle species. Within the errors the energy
resolution and transverse and longitudinal shower profiles are not
affected by magnetic field.

**1. Introduction**

The CMS hadron calorimeter based on plastic scintillator read out with
wavelength-shifting (WLS) fibre embedded in the scintillator plate \[1\]
will operate in strong magnetic field (4 Tesla). Magnetic field changes
light yield of scintillator \[2\] and affects the shower development
\[3\] that in principle can depend on the field orientation. For typical
collider geometry the magnetic field is parallel (transverse) to a
calorimeter plates for the central part and perpendicular (longitudinal)
to the end cap. The primary objective of the study was the calorimeter
performance in dependence on magnetic field direction up to 3 T with
muon, electron, and pion beams, to identify practical combinations of
thickness and sampling fractions which yield acceptable resolution for
CMS, to test possible candidates of photodetectors capable to work in
magnetic field, to tune up various hadronic cascade Monte Carlo
programs. In section 2 the prototype calorimeter is described. The
experimental set-up is presented in section 3. The results are discussed
in section 4.

**2. Configuration of the calorimeter**

A \"reconfigurable-stack calorimeter\" with brass plates (59 % Cu, 39 %
Zn, 1 % Fe, 1 % Mn) as absorber has been built. It can be reconfigured
to vary sampling fractions and longitudinal division and, as shown in
fig. 1, consists of 3 light tight boxes. Two of them contain stainless
steel support frame on which the absorber plates and scintillator plates
are hanging. The dimensions of the plates (66 cm x 66 cm) are determined
by the size of the hole in the magnet RD5. The third box contains
photodetectors.

The active medium is 4 mm thick scintillator, produced in Kharkov, with
dimensions 22 cm x 22 cm. A WLS fibre is routed through a key-hole
shaped groove milled in the tile. One end of the fibre is machined by
flying diamond cutter and aluminised. The 3x3 tiles wrapped in
reflective material and fixed between aluminium plates 2 and 1 mm thick
form a megatile (fig. 2).

Two configuration of the calorimeter and the active elements were used.

**2.1 Transverse magnetic field**

To a 1 mm diameter WLS fibre (doped with K27, produced in Tver, Russia)
a 4 m long PMMA clear fibre is glued in tube to transport light to a
photodetector. The WLS fibre turned out to be very fragile and cracked
along the bending radius in a month time resulting in drop of light
yield. The tile with the fibre is wrapped with aluminised mylar. The
fibres from each tower are bundled together, glued in a tube and go into
separate holes in a photodetector box which contained 25 mm diameter
proximity focused Hybrid Photo Diodes \[4\]. The HPD outputs are
directly connected to preamplifiers and than by 60 m cables to
amplifiers to equalise the HPD gain. The HPD HV tension is set to 8.08
kV, the preamplifier gain is 0.125 V /10^6^ e. The gate length is 150 ns
corresponding to the pulse length from preamplifiers. Only 12 HPD are
available (instead of 36: 9 towers x 4 longitudinal divisions), so the
horizontal towers are combined into one.

The calorimeter is divided longitudinally into four compartments with
the following sampling: the first two consists of 10 4 cm thick plates,
the third 5 8 cm thick absorber plates, and the forth - 2 8 cm thick
plates (136 cm of brass).

The light tight box with HPD is placed at 110 cm from the centre of the
magnet where the magnetic field is about 20 % lower than in its maximum.
The HPD axes are aligned along the magnetic field with precision of
several degrees. Between the spills the HPD gain is controlled by single
LED, the light is fanned out by fibres. A PIN diode measures the LED
signal to study the magnetic field dependence on LED light output. The
pedestal position is checked by using random triggers for each
measurements.

**2.2 Longitudinal magnetic field**

To the WLS fibre 0.83 mm diameter (Y11, produced by Kuraray) a clear
fibre is spliced, the other end of the clear fibre is glued into optical
disconnect. The tile with the fibre is wrapped with Tyvek. A 10 m long
optical cable with disconnects on both ends connects the megatile to a
decoder box containing 18 photo-multiplier (Philips XP 2081 with
extended green photocathode) where fibres are rearranged from
layer-to-layer cables to tower-to-tower bundles. The photo-multipliers
have magnetic shielding. The calorimeter is divided longitudinally into
two read-outs. The sampling is: the first 9 plates 5 cm thick and the
last 11 plates 10 cm thick.

**3.** **Experimental set-up**

The study of the HCAL performance was carried out on H2 beam at CERN.
Beam defining scintillator has dimensions 2 cm x 2 cm. Particle
trajectories were measured by drift chambers. The following particles
were used: electrons (100 GeV), muons (100 and 300 GeV), and pions (50 -
300 GeV). The magnetic field varied from 0 to 3 T. The deflection of the
incident particle by the transverse magnetic field was negligible even
for 50 GeV particles. For each measurements about 2x10^4^ triggers were
collected.

The calorimeter was installed in the centre of the superconducting RD5
magnet. The magnetic field has nonuniform distribution shown in fig. 3
by thin line.

In the run with longitudinal magnetic field (see fig. 3b) the magnetic
field affected the phototube gain though the distance between the
decoder box and the magnet was about 7 m. To control stability of PM
operation a LED illuminated all PM through a bundle of fibres. Besides
there was a wire radioactive source \[5\]. To minimise temperature
dependence of the LED for each particle type magnetic field was changed
separately to make it as quickly as possible.

**4.** **Data analysis and results**

With rise of magnetic field from 0 to 3 T the gain variation of
different HPD was in the range 0-3 %. This dependence was taken into
account during data analysis. Position of the LED amplitude
distributions was measured for each spill for each channel and the data
were corrected.

Neither the calorimeter nor the beam could be moved in horizontal or
vertical directions. Therefore only the central tower was illuminated.
The calibration constants for towers and longitudinal sections were
determined by minimisation of energy resolution.

The magnetic field has a high non-uniformity but it is not clear how to
correct the shower profile. The value of the magnetic field cited below
corresponds to its meaning at the center of the magnet.

In transverse magnetic field the light yield per scintillator was too
small (about 0.2 p.e. per scintillator for minimum ionising particle
(m.i.p.)) to reliably measure muon pulse height distribution and as a
consequence these dependence was not measured. In the longitudinal
magnetic field the WLS fibres were replaced and the light yield was
about 1 p.e. per tile for m.i.p. Fig. 4 shows pulse height distribution
for muons obtained for the first compartment (9 tiles).

The calorimeter, as was mentioned above, was roughly divided in
transverse and longitudinal directions. Comparing normalised
distributions we conclude that transverse and longitudinal shower
development does not depend on magnetic field for both orientations
within the measurement errors and longitudinal division (4 and 2
longitudinal compartments for transverse and longitudinal field
correspondingly). The energy resolution also does not depend on magnetic
field. Fig. 5 shows the normalized responses vs. transverse magnetic
field for 100 GeV electrons and different energy pions. There is almost
linear rise of the calorimeter response on magnetic field and dependence
on pion energy.

Figs. 6 and 7 show this dependence for longitudinal magnetic field for
electrons and muons and pions. In this case the behaviour is very close
to the scintillator light yield dependence on magnetic field obtained
with radioactive source and does not depend on particle species and pion
energy.

**5. Summary and conclusions**

The study of the hadron calorimeter response vs. magnetic field shows
that the light yield for muons, electrons, and pions increases up 3 T
and for showering particles depends on the field orientation and pion
energy. Within the errors there is no appreciable change of transverse
and longitudinal shower profile.

There are two effects which lead to calorimeter light yield dependence
on magnetic field: light emission of scintillators and shower energy
absorption in scintillator. Simulations of the calorimeter response due
to only the second effect give the results presented in the table.

> Table. Mote Carlo simulation of calorimeter respons.

+---------------------+------------------------+-----------------------+
|                     | K~T~=A(4 T)/A(0 T)     | K~L~ = A(4 T)/A(0 T)  |
+---------------------+------------------------+-----------------------+
| > 100 GeV electrons | 1.14                   | 1.06                  |
+---------------------+------------------------+-----------------------+
| 100 GeV pions       | 1.05                   | 0.98                  |
+---------------------+------------------------+-----------------------+
| 225 GeV pions       | 1.08                   | 0.99                  |
+---------------------+------------------------+-----------------------+

There K~T~ is the ratio of energy absorption in scintillator for 4 T
transverse magnetic field over energy absorption in scintillator without
magnetic field and K~L~ is the ratio for the longitudinal magnetic
field. If the scintillator light yield dependence on magnetic field is
added (about 5 % independent on magnetic field orientation) there is a
qualitative agreement of the experimental results and the calculation.

For muons if one neglects the electromagnetic interaction at these
energies the light yield dependence is mainly determined by the first
effect and must not depend on magnetic field orientation.

In conclusion we summarise the main results:

> \- a calorimeter response vs. magnetic field depends on field
> orientation and particle species and energy (for transverse field);
>
> -the e/h ratio depends on magnetic field and its orientation;
>
> -a radioactive source calibration to transfer coefficients obtained
> with extracted beams and calibration with muons will be more
> complicated at high magnetic field so calibration in situ using
> physical processes \[6\] becomes very important.

We gratefully acknowlege the support of E.Radermacher.

**Figure captions**

Fig. 1. Schematic view of the \"reconfigurable-stack calorimeter\".

Fig. 2. The plate with scintillating tile/fibre assembly (megatile).

Fig. 3. Set up of measurements with two field orientation and the
magnetic field distribution (thin lines).

Fig. 4. Muon pulse height distribution, the first compartment (9
scintillators).

Fig. 5. The relative change of light yield vs. transverse magnetic field
for 100 GeV electrons and 100, 200, and 300 GeV pions.

Fig. 6. The relative change of light yield vs. longitudinal magnetic
field for electrons and muons.

Fig. 7. The relative change of light yield vs. longitudinal magnetic
field for 50, 100, and 300 GeV pions.

**References**

> \[1\] The Compact Muon Solenoid. Letter of Intent. CERN/LHCC 92-3,
> LHCC/I 1, 1 October 1992.
>
> \[2\] S.Bertolucci et al., NIM A254 (1987) 561.
>
> D.Blomker et al., IEEE Transactions on Nuclear Science, Vol. 37/2(
> 1990) 220.
>
> J.P.Cumalat et al., Nucl. Instr. And Meth. A 293 (1990) 606.
>
> \[3\] V. Abramov, Nucl. Instr. And Meth. A 374 (1996) 34.
>
> \[4\] H.Arnauden, P.Benetti, L.Boskma et al., CERN-LAA/HC/93-16.
>
> \[5\] P.de Barbaro et al., Univ. of Rochester preprint UR-1360 (July
> 1994).
>
> \[6\] J.Freeman and W.Wu. FNAL-TM-1984 (August 1996).
>
> ![](media/image1.emf)
>
> Fig. 1
>
> ![](media/image2.emf)
>
> Fig. 2

![](media/image3.emf)

> Fig. 3
>
> ![](media/image4.emf)
>
> Fig. 4

![](media/image5.emf)

Fig. 5

> ![](media/image6.emf)

Fig. 6

![](media/image7.emf)

Fig. 7