Experimental setup

The BGOOD experiment combines the advantages of an almost 4π electromagnetic calorimeter and an open-dipole forward spectrometer, allowing to detect particles close to production threshold and with small momentum transfer.

The electron beam provided by ELSA impinges on a radiator placed inside the goniometer tank and produces a real bremsstrahlung photon beam. The post-bremsstrahlung electrons are deflected by the Tagger magnet depending on their momentum and hit different scintillators of the Tagging hodoscope, allowing to calculate the respective photon energy.

The photons impinge on a target of either liquid deuterium or liquid hydrogen positioned in the centre of the BGO ball. This is an 480 channel electro-magnetic calorimeter which is ideally suited to detect photons. The target is surrounded by a scintillator barrel to veto charged particles and two multi-wire-proportional-chambers for displaced vertex reconstruction. The central detector system covers the angular range from 25° - 155° in polar angle θ. Forward going particles are detected in the Forward Spectrometer.

The Forward Spectrometer consists of two tracking detectors (MOMO, SciFi) in front of the open-dipole magnet and eight drift chambers behind it. With this setup the momentum of charged particles is measured. Three Time-of-Flight walls allow to determine the velocity of the charged particles. When combining these information the Forward Spectrometer provides excellent particle identification.

For more information read also: S. Alef et al. The BGOOD experimental setup at ELSA Eur.Phys.J.A 56 (2020) 4, 104

overview of the BGOOD experiment
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tagging system
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Tagging System

The Tagging system conists of a set of selectable radiators, the Tagger magnet and the Tagger hodoscope. The electron beam provided by ELSA hits the chosen radiator to produce a real bremsstrahlung photon beam. Using a diamond radiator enables to produce a linearly polarized beam.

The post-bremsstrahlung electrons are bent in the Tagger magnetic field such the resulting position on the subsequent Tagger hodoscope is momentum specific. The respective photon energy is calculated as the difference of the electron energy pre and post bremsstrahlung.

Target System

The Target system is comprised of the movable target cell and a cryogenic system. Two lengths of target cells are available, 6 cm and 11 cm.

The main part of the cryogenic system is a helium compressor which allows to cool down deuterium or hydrogen to temperatures around 20 K and liquefy the gas.

Alternatively to the cryognic target, a solid target such as carbon or CH2 can be inserted.

target system
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Central Detector

Overview central detector
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The Central Detector surrounds the target cell and covers the polar angle range θ from 25° to 155°. The main component is a highly segmented BGO crystal calorimeter. Inside it is complemented by a segmented scintillator barrel and two cylindrical multi-wire-proportional-chambers (MWPC) placed directly around the target.

The calorimeter is arranged in two halfs which can be opened to allow access to the target and the inner detectors.

Also shown in the figure is the intermediate detectors covering the gap between the Central Detector and the Forward Spectrometer (≅11° - 25°).  A Multi-gap Resistive Plate Chamber (MRPC) is currently under construction. It complements the Scintillating Ring detector (SciRi) to give information about the direction of charged particles.

BGO Ball

The BGO Rugby Ball is an electromagnetic calorimeter. Charged and neutral particles are stopped in the material and the deposited energy is measured. To get position information the detector is made from 480 BGO (Bismuth Germanate, Bi4(GeO4)3 ) crystals arranged in 15 sectors with 32 crystals each.

Scintillator Barrel

The cylindrical scintillator barrel is placed between the BGO and the MWPCs and is used to distinguish between charged and neutral particles. It is made of 32 plastic scintillator bars, each 5mm thick and 43cm long made from BC448 (former Bicron, now Saint-Gobain), which form a cylinder with a mean radius of 9.75 cm.

The detection efficiency for charged particles is ≅98%. In contrast, both photon and neutron detection efficiencies are < 1 % over a wide energy range.


Two cylindrical MWPCs (multi-wire-proportional-chambers) are used for charged particle tracking inside the BGO ball. Each chamber consist of an inner and outer cathode wall made from helically wound strips and wires in between them as anodes.

Intermediate Detectors

The gap between the Central Detector and the (rectangular) Forward Spectrometer is covered by the intermediate detectors. The Multi-gap Resistive Plate Chamber (MRPC) is still under construction, in the mean time the Scintillating Ring detector SciRi covers the polar angle range from 10° to 25° and gives direction information of charged particles.

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The Scintillating Ring detector consists of 96 scintillator segments arranged in three rings and read out with avalanche photo-diodes

MRPC setup
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The MRPC is designed as a Time-of-Flight detector and is currently under construction. It has an anticipated time resolution of 40ps and an angular resolution of 2°.

Forward Spectrometer

forward spectrometer overview
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The Forward Spectrometer covers the polar angular range from 1° to ≅ 12° . It consists of the two tracking detectors MoMo and SciFi in front of an open dipole and eight double layer driftchambers behind it. These detectors allow to measure the track of charged particles. From the radius of the curvature caused by passing through the magnetic field the momentum of the particles is determined.
The setup is complemented by three Time-of-Flight walls to measure the velocity of charged particles.

The combination of momentum and velocity provides very good particle identification in forwards direction with a high angular resolution.

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MOMO is a scintillating fiber vertex detector incorporating 672 channels. It consists of six identical modules arranged in three layers of 2.5mm thick circular fibers. The planes contain 224 parallel fibers each and are rotated by 60° to each other, with readouts through 16-channel Hamamatsu R4760 phototubes. The sensitive area is circular with a diameter of 44cm. In addition, a 4.5cm wide central hole allows the beam to pass through. This detector was originally built for the MOMO experiment at COSY. With a new support structure and improved magnetic shielding it is now used as a tracking detector in the Forward Spectrometer of BGOOD.

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The SciFi detector has an active area of 66cm x 51cm.It covers an angular range of ± 10° in the x-direction and ± 8° in the y-direction. A central hole (4cm x 4cm) allows the beam to pass through. The planar detector is made of 640 scintillating fibers with a diameter of 3mm, arranged in two layers at an angle of 90°.
Groups of 16 fibers are glued together in two shifted layers to cover the gaps between the round fibers. This design guarantees a minimum path length (about 2mm) for particles traversing the detector. Each module is read out by 16-channel photomultiplier (Hamamatsu H6568).

open-dipole magnet
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Open-Dipole Magnet

The central part of the spectrometer is a dipole magnet. Very fortunately, it was possible to get an MD type magnet on permanent loan basis from DESY, which has practically ideal geometry.

The size of the central gap is 84 cm (height) × 150 cm (width) × 150 cm (length) resulting in an angular acceptance of 12.1° in x direction and 8.2° in y direction. The outer dimensions of the magnet are (H × W × L) ≈ (280 × 390 × 150)cm³ and the total weight amounts to ≈ 94 tons. The nominal magnetic field strength at the centre of the magnet is 0.53 T, corresponding to an integrated field along the beam axis of approximately 0.71 Tm.

© Daniel Hammann, Diploma thesis

Drift Chambers

Eight drift chambers are mounted behind the spectrometer magnet . Each chamber contains a double layer of hexagonal drift cells, so that each particle track will hit at least two drift cells in each chamber it passes. The chambers come in four different orientations, two chambers have vertical wires and measure the x-coordinate, two chambers have horizontal wires and measure the y-coordinate, and four chambers have wires tilted by ±9° against vertical, measuring an u- respectively v-coordinate, used to disambiguate between true and false combinations of multiple hits in the x and y chambers.

tof walls
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ToF spectrometer

The spectrometer consists of 3 walls of plastic scintillators, each segmented into individual horizontally oriented bars. We used scintillator bars from the earlier GrAAL and SAPHIR experiments.

The ToF spectrometer covers a total area of 3 × 3 m². Each wall has a gap of 10 to 22 cm in the centre, for the photon beam to pass through.

flumo and gim
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Photon Flux Monitor

In the BGO-OD experiment photons, generated by bremsstrahlung from an electron beam, are used for meson photo production. To measure the absolute cross section of these photonic reactions the absolute photon flux is needed. For this the Photon Flux Monitor has been build.

The detector consists of two parts: one that measures the absolute rate (GIM) and one that measures only a relative rate (FluMo). The detector is divided in these two parts because the expected photon rates of up to 50MHz is to much to be counted by one detector. Instead the absolute number of photons is counted at low rates with the GIM and the fraction of the rate measured by the FluMo is determined. So that at high rates the real rate can be recalculated.

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