${}^{22}\mathrm{Na}$ activation level measurements of fused silica rods in the LHC target absorber for neutrals compared to simulations

The Target Absorbers for Neutrals (TANs) are located in a high intensity radiation environment inside the tunnel of the Large Hadron Collider (LHC). TANs are positioned about 140 m downstream from the beam interaction points. Seven 40-cm long fused silica rods with different dopant specifications were irradiated in the TAN by the Beam RAte of Neutrals (BRAN) detector group during $p+p$ data taking from 2016 to 2018 at the LHC. The peak dose delivered to the fused silica rods was 18 MGy. We report measurements of the ${}^{22}\mathrm{Na}$ activation of the fused silica rods carried out at the University of Illinois at Urbana-Champaign and Argonne National Laboratory. At the end of the irradiation campaign, the maximum ${}^{22}\mathrm{Na}$ activity observed was $A=21\mathrm{kBq}/\mathrm{c}{\mathrm{m}}^3$ corresponding to a density, $\rho =2.5\times {10}^{12}/\mathrm{c}{\mathrm{m}}^3$, of ${}^{22}\mathrm{Na}$ nuclei. fluka Monte Carlo simulations have been performed by the CERN fluka team to estimate ${}^{22}\mathrm{Na}$ activities for the irradiated BRAN rod samples. The simulations reproduce the ${}^{22}\mathrm{Na}$ activity profile measured along the rods, with a 35% underestimation of the experimental measurement results.

Figure 1

TAN setup during LHC Run 2 $p+p$ running (2016–2018). The dashed lines identify the slot used to install the zero degree calorimeter during the Heavy Ion runs. The right inset shows a top view of the BRAN prototype. The numbers identify the position of the fused silica rods. Detailed information on the materials of the rods and maximum irradiation reached for each of them are reported in Table 1.

Figure 2

BRAN prototype as installed in 2016 in the TAN on Arm 8-1 of IP1.

Figure 3

Dose distribution in the BRAN prototype for the 2016 (left) and 2017 (right) $p+p$ runs, normalized to an integrated luminosity of 38.5 and $50{\mathrm{fb}}^{-1}$, respectively. The horizontal axis gives the distance from IP1, namely the $Z$-coordinate along the ATLAS detector axis. The vertical axis gives the distance from the beam height, namely the $Y$-coordinate along the axis opposite to gravity. Dose values are averaged over a 9-mm interval in the missing third dimension, corresponding to the horizontal orthogonal $x$ axis pointing outside the LHC ring. The vertical shift in the maximum exposure position is due to the crossing angle change.

Figure 4

Top panel: Beam loss monitor (BLM) pattern along the ATLAS insertion as measured over a 2018 13 TeV center-of-mass $p+p$ run period yielding $20.3{\mathrm{fb}}^{-1}$ (red crosses) and calculated by fluka (black circles). Mid panel: Ratio between simulation values and data. Vertical bars correspond to a 20% uncertainty on data. Bottom panel: Machine layout.

Figure 5

${}^{\mathrm{nat}}\mathrm{Si}(\mathrm{p},\star ){}^{22}\mathrm{Na}$ cross section. Symbols are different experimental data sets from EXFOR [18], the gray curve results from the fluka interaction model.

Figure 6

${}^{\mathrm{nat}}\mathrm{Si}(\mathrm{n},\star ){}^{22}\mathrm{Na}$ cross section. Symbols are different experimental data sets from EXFOR [18], the gray curve results from the fluka interaction model.

Figure 7

Integrated luminosity per day delivered to (green) and recorded by (yellow) ATLAS during stable beams for $p+p$ collisions at 13 TeV center-of-mass energy in 2016. All the details about the time profile of luminosity delivered to ATLAS in 2016, 2017, and 2018 runs can be found in [19].

Figure 8

Experimental setup of HPGe detector used at the University of Illinois Urbana-Champaign. The transparent plates represent the tungsten shielding. The blue cylinder is the HPGe detector and the red cylinder indicates the location of the sample during measurements.

Figure 9

Comparison of Rod 1 activity measurements performed at UIUC, using an HPGe detector (Sec. 4c), and ANL, using a well-type detector (Sec. 4d). The origin of the horizontal axis ($y=0$) corresponds to the bottom edge of the rods in Fig. 3, located 3.7 cm below the LHC machine axis. The errors on the abscissa represent the half-length of the samples. The bottom panel shows the ratio between the two measurements. A dashed green line that corresponds to unity is drawn for direct comparison. Please note that there is an additional 3.3% correlated uncertainty on the calibration source activity that is not included in the ratio uncertainties. Results have been normalized to a reference date of December 14, 2019.

Figure 10

Comparison of Rod 3b activity measurements using an HPGe and a well-type detector. Refer to the caption of Fig. 9 for details related to the plotting.

Figure 11

Comparison of Rod 6 activity measurements using a HPGe and a well-type detector. Refer to the caption of Fig. 9 for details related to the plotting.

Figure 12

Comparison of activity measurements at ANL, using a well-type detector, and fluka simulations for Rod 1. Results have been normalized to a reference date of December 14, 2019. The dotted line represents the fit of the ratio between data and simulations.

Figure 13

Comparison of activity measurements and fluka simulations for Rod 3b. Refer to the caption of Fig. 12 for details related to the plotting.

Figure 14

Comparison of activity measurements and fluka simulations for Rod 6. Refer to the caption of Fig. 12 for details related to the plotting.