South Atlantic Anomaly definition

Transcription

1 South Atlantic Anomaly definition A. Contin 1), D. Grandi ) 1) University of Bologna and INFN, Bologna, Italy ) INFN, Milano Bicocca, Italy 19 June 1 1 Introduction The very high rate of particles in the South Atlantic Anomaly (SAA) region, due to the local decrease of the Earth magnetic field, saturates the AMS- data acquisition. It is therefore mandatory to exclude the region from the data analysis. A good definition of the SAA region is needed for an homogenous and coherent data analysis, taking into account that the exclusion of the SAA region subtracts to the experiment total exposure, so it has to be as limited as possible. In the following, the definition of the SAA contour is done in terms of: live time of AMS data acquisition system, as a proxy for the total trigger rate on the detector; absolute particle rate on the first plane of the TOF system; Earth magnetic field. The following analysis has been done using data collected from May 19 to December, 1 reconstructed with the B55 reconstruction program version. Experiment live time The experiment live time is computed through the ratio between two onboard scalers, one which counts the fast trigger (FT) rate when the DAQ is active (i.e., not busy), and the other which counts the total FT rate. The result is shown in Fig. 1 as a function of the geographic coordinates. A cut: defines the white region in Fig.. The total exposure time loss due to cut (1) is.%. Absolute rate on the first TOF plane live time <.65 (1) The TOF time signals and the Fast Trigger are registered on pipeline TDCs. In particular: all Low Threshold (LT) signals are registered in individual channels; the Fast Trigger (FT) signal is registered in one channel in each SFET card. The pipelines length is 655,6 channels at.ps/ch, for a total of 15,86ns. The pipelines are frozen by the LVL1 trigger. As there is a nearly fixed delay between LVL1 and FT, the FT signal is typically in the middle of the pipeline (see Fig. ). The LT signals distribution is shown in Fig.. Off-time events, i.e. events not in coincidence with a trigger, can be clearly identified as the background before and after the triggered events. A fiducial region from to, TDC channels can be defined to look for off-time events, defined as a coincidence of LTs between the two sides of a counter. The time difference between the two sides of a counter 1

2 is shown in Fig. 5a for triggered events and in Fig. 5b for off-time events. The two peaks structure is due to the different time offsets in the different layers. As the two distributions are very similar, we are confident that a coincidence of off-time LT signals in the two sides represents a real particle hitting the counter. Each analyzed event contribute to the live time of the off-time events with,. = 8ns. The total rate in each counter can thus be computed by counting the off-time events in that counter and dividing by the total live time for off-time events. Figure 6 shows the rate of hits in any of the counters in the first TOF layer as a function of the geographic coordinates. A cut: defines the white region in Fig. 7. The total exposure time loss due to cut () is.5%. Earth magnetic field rate > Hz () The Earth has a magnetic field, also known as the geomagnetic field, that extends from the Earth s inner core to where it meets the solar wind. It can be approximated with a magnetic dipole tilted at an angle of 11 o with respect to the rotational axis and shifted from the Earth center. The Earth s field changes over time and it is most probably generated by the motion of charged ions (iron) in the Earth s outer core. The International Geomagnetic Reference Field(IGRF) is a standard mathematical description of the Earth s main magnetic field widely used in studies also for the Earth s magnetosphere. In source-free regions at the Earth s surface and above, the main field, with sources internal to the Earth, is the negative gradient of a scalar potential V which can be represented by a truncated series of spherical harmonic expansion coefficients. These IGRF model coefficients are obtained using magnetic field data from satellites and from observatories and surveys around the world. The Earth magnetic field computed with the last IGRF model (IGRF-11) at an altitude of km from the surface is shown in Fig. 8 as a function of the geographic coordinates. A cut: B <.15 5 T () defines the white region in Fig. 9. We evaluated also the variation of the above mentioned region with altitude, in the range of the ISS motion (down to a minumim of km), and we found that the main differences where below. 5 T, so this essentially do not affect the magnetic definition of this region. The total exposure time loss due to cut () is.%. 5 Conclusions Figure show the comparison between cuts (1), () and () for the identification of the SAA. All three regions are completely superimposed. A simpler solution is to apply a poligonally-shaped cut, as shown in Fig. 11. The coordinates of the points A to E can be written as: A=(-8-α,-1) B=(-8+α,-1) C=(+.5α,-) D=(+α,-6) E=(-8-α,-6) with a free parameter α which determines the width of the poligon in the longitude coordinate. Figure 1 shows the data taking time loss as a function of parameter α. A value α between and seems to be a good compromise, as shown in Fig. 1, giving a total reduction in the exposure time of 11-1%.

3 In Appendix A the code for implementing the cut is given.

4 A Code to implement the cut on an event by event basis bool pnpoly(int npol, float *xp, float *yp, float x, float y){ int i, j; bool c=false; for (i =, j = npol-1; i < npol; j = i++) { if ((((yp[i] <= y) && (y < yp[j])) ((yp[j] <= y) && (y < yp[i]))) && (x < (xp[j] - xp[i]) * (y - yp[i]) / (yp[j] - yp[i]) + xp[i])) c =!c; } return c; }... AMSEventR *pev; float alpha=; float _SAA[5] = {-8.-alpha, *alpha,.+alpha,.+alpha, -8.-alpha}; float Theta_SAA[5] = {-1., -1., -., -6., -6.}; double pigr=acos(-1.);.... event loop... HeaderR* header = &(pev->fheader); float ThetaS = header->thetas*18./pigr; float S = header->s*18./pigr; if(s>18 && S<6) S = -6+S; bool in_saa = pnpoly(5, _SAA, Theta_SAA, S, ThetaS); if(!in_saa ){... event analysis... }

5 Theta Figure 1: Fraction of live time as a function of the geographic coordinates. Theta Figure : Fraction of live time as a function of the geographic coordinates. The white region corresponds to live time <.65. 5

6 events/ TDC ch FT (TDC ch.) Figure : Time distribution of the FT signals. events/ TDC ch LT (TDC ch.) Figure : Time distribution of the LT signals. 6

7 events/ ns 6 5 events/ ns a) b) LTp-LTn (ns) LTp-LTn (ns) Figure 5: Time distribution of the difference between the LT signals from the two sides of a counter for (a) triggered events and (b) off-time events. The two peaks correspond to different planes. Theta Figure 6: Particle rate (Hz) on the first TOF layer as a function of the geographic coordinates. 7

8 Theta Figure 7: Particle rate (Hz) on the first TOF layer as a function of the geographic coordinates. The white region corresponds to a rate > Hz. Theta Figure 8: Magnetic field (T) isolines as a function of the geographic coordinates. 8

9 Theta Figure 9: Magnetic field (T) isolines as a function of the geographic coordinates.the white region corresponds to B <.15 5 T. Theta Figure : SAA definition with the three methods: fraction of live time (blue line), rate (red line), magnetic field (green line). 9

10 Theta A B C - -6 E D Figure 11: SAA definition with a poligon, superimposed to the plot of Fig.. The dashed lines represent the latitude limits of the ISS orbit. exposure loss % 15% % 5% 5 15 α Figure 1: The loss in AMS- exposure time as a function of the parameter α defined in the text.

11 Theta A' A E' E B B' C D C' D' Figure 1: SAA definition with a poligon, superimposed to the plot of Fig. 6 (rate), using a value of α of (solid lne) and (dashed line), as explained in the text. 11