Update on HF PMT magnetic shielding

Ianos SchmidtApril 2, 2012

2

Existing readout box PMT shielding

“Plate 2”(Garolite)

“Plate 1”(Garolite)

A box contains 24 steel and mu-metal PMT shields in addition to four “dummy” steel shields without mu-metal and PMT’s.

Steel tubes are 41.275 mm outside diameter, with a 3.2mm wall thickness.

Tubes are 121mm long.

Kapton insulator

Hamamatsu R7525

Shielding for new PMT’s

Plate2

Shield

Light guide tube, with mirror(protrudes into box from HF. Not part of the box).

Plate1

Base board

Sleeve

Light guide tube

Opening in Plate1

Shield

SHIELD LENGTH 46mm –Prototype box (54mm – Hybrid box)(14.4)

(48.4)

Photo-cathode minimum area

23.6

To P

C

Thickness 2.1mm

Relative response vs CMS magnet field

Relative response of Old PMT’s

Relative response8 “hybrid box” R7600U PMT’s

Response of HF PMTs during ramp-up of CMS magnet data taken by HCAL Operations DOCs:Brian Dorney, Kadri Ozdemir, analysis by Dima Vishnevskiy presented at HCAL Operations Meeting, on March 12, 2012

Relative response24 “PPP- box” R7600U PMT’s

Hybrid box • Installed on HF Fall 2010. • Contains 16 original R7525 PMT’s• 8 new R7600U PMT’s with 54mm

long shields

Shields

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Analysis by Slava Klyukhin• Full information was provided to Slava Klyukhin for evaluation, including PMT locations and layout (X,Y,Z) in

CMS, Details of shielding for all three configurations (old, “hybrid”, and new), and Dima’s presentation on measurements taken during magnet ramp.

His initial evaluation (or both small and large YE4 disk): “I have analysed the stray field in the HF RBX region in two CMS magnet models at 3.8 T:

cms_110322_13l_18164a_v3_1-ye5_40_30 - with small YE4 of 5 m diameter;cms_110322_13l_18164a_v3_1_ye5_40_30 - with large YE4 of 14 m diameter.

The large disk gives the reduction of the stray field in the RBX region by 1 mT only both foraxial Bz and radial Br (Bx in the tables) components.

In the model the HF outer shield is 8 cm thicker (it is from R=1.52 to 1.7 m instead of R=1.6to 1.7 m). This can make the stray field a bit less.

The stray fields are calculated in horizontal plane for the plot between Z=-13.97 to -13.77 mand R=1.03 to 1.43 m.

The maximum Bz is 19 mT (small disk), the maximum Br (Bx) is 13 mT (small disk), Bphi (By) isvanished.

The thickness of the PMT steel shield of 2.1 mm is enough to cancel the magnetic flux of2.2e-05 Wb in the inner square of 33.9 x 33.9 mm2. Really, at 19 mT the induced magnetic fluxdensity in steel is about 1.88 T, that gives a magnetic flux of 56.9e-04 Wb in the shieldcross section.

Thus, new design of the PMT shield has the only one parameter that is not sufficient forefficient shielding: the length of the shield box.

All the results you get from the test installations you can apply to the upgraded CMS yokewith larger YE4.”

Work still in progress.

Field in ROB region

Z-component

X -component

Why was shield length reduced?The limiting factor is generally the distance from the edge of the shielding to the

PMT sensitive area.• We chose long sockets for the PMT's to get the PMT's as deep into the shielding

as possible (~23mm). This was believed to be our limiting dimension.• Had encouraging results from the hybrid box.

Chose to improve aspects of the optical coupling by reducing the shield length• Reduce losses in optical coupling (in HF photons are precious).• Improve sleeve to PMT alignment• Improve robustness

– Expected light losses to be at about same level (few %) as effects from magnet.

23mm dimension at back side of shield does not appear to dominate the shielding performance.

- Due to Metal envelope of PMT ?

What is maximum shield length we can install.

• 54mm (length used in slice box) is the longest without suffering light loss beyond 5%

• Sleeves are not long enough or robust enough to extend much farther.• Depending on the distance, sleeves would likely need to be re-engineered,

tested, produced, and replace on the detector.– a large amount of effort we worked hard to avoid– effort would not likely to improve optical coupling

54mm shields respect the general rule that the sensitive area be as far into the shieldingas its size, which is not respected by the short shields. This appears to be demonstrated bythe hybrid box performance (although it is not an ideal test).

Slava's analysis results should give us a better understanding of how we should proceed.• Shield production is currently stopped until conclusion of analysis and a decision on how to proceed.

what is an acceptable level that we should achieve?