Target and Hadron Absorber

The target produces a large number of hadrons (pions, kaons, D and B mesons) from the 400 GeV proton beam. These hadrons then decay into SM or long-lived particles, so called "Feebly Interacting Particles" (FIPs), which are potential dark matter candidates (Technically speaking, Neutrinos are also FIPs, but the term refers to particles that are not part of the Standard Model (SM) and have not been detected yet). Separating the production target from the hadron absorber allows the target geometry to be optimized for particle production and thermal stability, while the absorber can be optimized purely for radiation containment.

The SPS beam is expected to deliver \(4 \cdot 10^{19}\) protons per year, corresponding to a total energy of \(2.6\,\text{TJ}\) per year. A significant fraction of all the energy is deposited into the target, which needs to be highly thermally resistent to withstand the radiation dose. Additional cooling is needed to prevent the metal alloys from melting. \(120\,\text{m}\) upstream the target, two "Beam Sweeping Magnets" are used to spread the beam in order to distribute the energy deposit over a wider area and to prevent overheating on the incidence point. The beam spot keeps its original size of \(1\,\text{cm}\) and is continuously moved across the target surface in a circular motion. The incidence radius grows from initially \(5\,\text{mm}\) to \(35\,\text{mm}\) with about fife circular rotations per second². Over a lifespan of 5 years a integrated exposure of about \(2 \cdot 10^{20}\) protons is expected.

Target Material Design Approach

The design approach is described very detailed within the Proposal Technical Design Report and is a combination of different high-density metal alloy blocks. The considered baseline design is a combination of:

13 TZM alloy (titanium-zirconium-doped molybdenum) in the front area.
5 pure Tungsten blocks in the later section.
Tantalum-Tungsten clad for all the blocks to prevent oxidation and corrosion.
Active water cooling between all blocks to prevent overheating.

Since SHiP has evolved during the later design iterations, the exact segmentation might have changed by now. Fig. 1 shows a drawing of the target, with the different material blocks and their arrangement. The upper part of the figure shows a heatmap of the beam incidence on the target, which is used to calculate the energy deposit and the resulting temperature distribution within the target. The total length of the target is \(\approx 12\) nuclear interaction lengths (\(\lambda\)).

**Fig. 1:** **Upper part**: The beam incidence heatmap within the target. **Lower part**: A Drawing of the target, showing the different material blocks and their arrangement¹.

Overall the target was optimized to produce a large number of candidate particles. The TZM blocks are less dense than the Tungsten blocks but have a higher thermal conductivity, a higher fracture toughness, a lower thermal shock susceptibility and therefore can better withstand the thermal stress from the beam incident. Since the large energy deposit is going to heat up the blocks rapidly, the Tungsten blocks need to be prevented from overheating and melting. The Tungsten section absorbs the hadronic showers and maximizes the charm production. Fig. 2 shows the thermal energy deposit within the target. It is clearly visible that the energy drops to less than half the peak value at the first Tungsten block.

**Fig. 2:** An analysis of the thermal energy deposit within the target³.

Overall the target is optimized for the following criteria:

Large Charm Production
Muon reduction
Hadronic Shower absorption
Thermal and mechanical stability
Radiation hardness

Shielding

To mitigate the emission of radiation, the target is located inside a "low-pressure gas environment". If it would be surrounded by air, the hadronic showers, neutrons and high energy photons could initiate a process called "air activation", which could create radioactive gases, making the environment dangerous for working personnel. Typical examples of radioactive isotopes produced by air activation are \(^{11}\text{C}\), \(^{13}\text{N}\) and \(^{15}\text{O}\). The reduced pressure limits the number of interactions and therefore the radiation exposure. The chamber is kept at a slightly lower pressure to detect leaks and prevent the escape of radioactive gases.

Furthermore, the target and gas vessel are surrounded by a \(4\,\text{m}\) thick, water cooled cast iron shielding³. For this, a total amount of \(450\,\text{m}^3\) of cast iron is used and the cooling capacity is designed to remove \(20\,\text{kW}\) of heat. Afterwards, concrete blocks reduce the remaining radiation to a safe level. A CAD model of the whole target bunker, showing the target and the surrounding shielding, is shown in Fig. 3.

**Fig. 3:** A CAD model of the whole target bunker, showing the target and the surrounding shielding³.

Hadron Absorber

The target is followed by a hadron absorber, which consists of iron blocks. The hadron absorber is used to stop the hadrons produced in the target and reduce the background for the subsequent detector. Even though, the Target material is designed to mitigate hadronic showers, secondary hadrons are still produced and can enter the decay volume of the Hidden Sector Decay Spectrometer. The goal is to strongly suppress the hadronic component while allowing weakly interacting particles and a reduced amount of muons to pass down to the HSDS.

The design has changed multiple times during the rewrites of the Technical Proposals. Therefore, only the requirements of the hadron absorber can be summarized here with certainty. The hadron absorber should have:

A high density to stop hadrons.
A good thermal conductivity to withstand the heat deposit.
A sufficient radiation hardness.
Enough mechanical stability to withstand its own weight.

Current design approaches specify a length of \(5\,\text{m}\) and using iron at a total length of \(30\lambda\). Later designs considered magnetizing part of the iron absorber with a \(1.6\,\text{T}\) magnetic field and using it as the first section of the muon shield. High-density inserts, such as Copper and Tungsten, were studied to block forward cascades and concrete is used for radioactive shielding. As for the other components of the Target sector, active water cooling is planned to dissipate heat.