Results of the beam monitoring test Dec 2015

First pictures taken with the setup shown in gure 3.15 were used to determine the best camera settings. The most suitable exposure time for the measurements was a tradeo between an increase in the background level and the quantization error due to the 8-Bit limitation of the camera and the readout noise. The exposure time was nally been xed at 100 sec with a master gain amplication of≈12, with the resulting pixel intensities at the centre of the beam spot using∼2/3 of the dynamic range, which ensures values in the linear range of the CCD ADC. The linearity of the ADC is shown in gure 4.1[103]. One of the rst pictures taken with the 28MeV/c surface muon beam impinging on the scintillator foil is shown in gure 3.21(a). The heatplots are generated from the intensity levels of the pixels. The muon beam spot is clearly visible. However the camera shows a considerable dark noise level that is also non-uniform, showing a bright spot in the top left region. This can be clearly a dark current phenomenon, conrmed by taking a background frame with the beam blocker closed. The righthand picture in the same gure shows the background image with the same accumulation of hotpixels in the top left region. Subtracting the background image (subgure 3.21(b)) from the beam image (subgure 3.21(a)) solves this issue. The result (subgure 3.21(d)) is an image that shows intensities that are proportional to the scintillation light originating from the muon beam spot. Image subtraction in this context means a pixel per pixel subtraction of the individual ADC values. The muon distribution is clearly seen in the image of the scintillation light. A closer look at the background subtracted image shows highlighted lines which on investigation proved to be surface scratches. This implies that the handling of the scintillator has to be improved in order to avoid scratches on the scintillator surface. Nevertheless since the "scratches" seem to be ∼uniformly distributed across the surface (see gure 3.22) and the fraction of the areas with "scratches" is small the overall inuence on the evaluation of the beam proles can be regarded as small. As described above the image now has to be transformed in order to correct for the perpective distortion thereby dening a Field-of-view (FOV) that is used for the further evaluation of the beam proles. No further lters are applied to the the background subtracted and transformed image. The so derived intensity distribution is tted with a 2D Gaussian distribution:

f(x, y) =A₀+ ˆA·e⁻ with the coordinates in the FOV being x and y. The beam prole parameters that are derived from the t are Aˆ the intensity amplitude, the horizontal/vertical centre of the t x¯/y¯, the corresponding standard deviations σ_x / σ_y and the Pearson correlation para-meterρxy. The oset term A0 takes into account the residual homogeneous background illumination from for example stray light. The t result is illustrated as a 1-σ outline and shown together with the background subtracted and transformed image in gure 3.23. For comparison a beam prole raster scan was taken at approximately the same position along the beam axis with the XY-scanner, described in section 2.5.2, which measures the prole perpendicular to the beam axis. The result is shown in gure 3.24. A comparison of the t results derived from the scintillation image and the pill raster scan discriminating on low threshold (muons and positrons are counted) and discriminating on high threshold (only muons counted) is given in table 3.3. Due to the rough alignment the values for the centre

(a) BB open. (b) BB closed.

(c) = (a) - (b)

Figure 3.21.: Shown the image processing sequence. Image (a) was captured with BB open at

∼nominal beam intensity (beam picture). The beam spot is clearly visible - however several con-tinuous areas / lines, that show an enhanced LY, can be attributed to defects on the scintillator surface. Image (b) was taken with BB closed i.e. without muon stops in the target (background picture). From (b) two types of noise-related artefacts can be identied, namely isolated hot pixels and areas of high excitation (e.g. glow of "light" on the top left). Subtracting the ADC values of the background image from the beam image for each pixel individually is called background correction and shown in (c).

Table 3.3.: Beam Prole Comparison between XY beam scanner and scintillation light distribution.

σ_x(mm) σ_y(mm) ρ_xy ∆σ_x (mm) ∆σ_y (mm)

Scintillator Image 20.9 24.8 -0.01 -

-Pill High Threshold 18.3 19.8 -0.04 +2.6 +5.0

Pill Low Threshold 19.4 20.8 -0.04 +1.5 +4.0

G4BL Phase Space propagation 26.6 24.4 - -5.7 +0.4

Figure 3.22.: The close-up image shows inhomogenities and scratches in the scintillator surface that can be identied in the beam picture 3.21.

Figure 3.23.: The background subtracted and transformed image is shown together with the 1-σ outline of the tted distribution. The picture already shows qualitatively that x and y are essentially uncorrelated, consistent with the small xy-coupling throughout theπE5 beam line.

Figure 3.24.: The plot shows a pill raster scan carried out with the scanner system that has been described in section 2.5.2 discriminating at low threshold. The blue dashed lines indicate the 1-, 2-and 3-σoutline respectively, (note the scales are not the same).

¯

x/y¯of the 2D Gauss distributions are not compared. The scintillator image is considered to show a beam size slightly broader than the pure muon distribution due to scattering of emitted light and excitation by Michel positrons. The previously qualitatively observed negligable coupling between x and y is conrmed by the small correlation coecientsρ_xy. However the comparison between the two methods needs more renement. The pill scan-ner measurement being perpendicular to the incoming beam can be used to simulate the beam at the 45° slant angle scintillator with its centre∼15 cm downstream of the scanner measurement plane.

The more rened method is based on phase space measurements at the entrance to QSK43, upstream of the scintillation target. The phase space is calculated in a similar way as in section 2 of the Compact Beam Line setup. Figure 3.25(a) shows the TRANSPORT en-velopes for a multi-envelope t constrained by the pill scanner measurements carried out for 10 dierent quadrupole settings. The measured and the simulated beam size based on the reconstructed phasespace are compared in gure 3.25(b). Based on these parameters a beam is tracked in TURTLE [52] and extracted 40 cm upstream the centre of the scin-tillator. The beam le is then converted to the G4BL compatible G4BLTrackle format.

The geometry in the G4BL simulation encompasses a simplied Mylar vacuum window, the air between the Mylar and the aluminum window of the light-tight box and the air up to the scintillator screen - see gure 3.26. The muon distribution on a virtualdetector representing the slanted scintillator is then projected onto the x-y plane. The projected beam size is then compared to the scintillator results in table 3.3. The vertical direction is in good agreement, whereas the horizontal prole is found to be 5.7 mm larger than measured. Possible reasons for this deviation can be found in the dicult alignment in the horizontal direction. Furthermore, the scratches are mostly in the horizontal direction and therefore aect the horizontal phase space more. The phase space for the simulation was derived in a paraxial approximation rst order matrix description. The proles measured with the pill scanner show a strong dependence ∼ 1.6 ^mm_A on the current applied to the preceeding quadrupole (QSK43) as shown in plot 3.27. Thus one can conclude that the muon beam passes the last quadrupole of Triplet II o axis and propagates with an angle to the reference orbit which also predominantly aects the horizontal direction.