帮我解答这道题---**Question:** 6. (15 分)
**Problem Description:**
如图所示，在 xOy 平面内存在有界匀强磁场，磁场的边界是半径为 R 的圆，圆心 C 点的坐标为 (0,R)，磁场方向垂直 xOy 平面向外. 第Ⅱ象限内垂直 x 轴放置线状粒子源，粒子源的一端在 x 轴上，长度为 2R, 沿 +x 方向均匀发射速度大小为 v₀ 的相同粒子，所有粒子经磁场偏转后从坐标原点 O 处射出. 第Ⅲ象限内垂直 x 轴放置一荧光屏 S，荧光屏的一端在 x 轴上，长为 √3 R, 到 y 轴的距离为 R. 已知粒子的质量为 m, 电荷量为 +q, 不计粒子的重力及粒子间的相互作用.
**Sub-questions:**
(1) 求磁感应强度大小 B;
(2) 求能打在屏上粒子的数目占粒子源发出粒子总数的百分比 k;
(3) 若在第 Ⅲ、Ⅳ 象限内加沿 -x 方向的匀强电场 (图中未画出)，使所有粒子都能打在屏上，求电场强度的最小值 E.
**Diagram Description:**
* **Type:** Schematic diagram illustrating particle trajectories in magnetic and possibly electric fields within a coordinate system.
* **Coordinate Axes:** Standard Cartesian x and y axes intersecting at the origin O.
* **Magnetic Field Region:** A circular region centered at point C with coordinates (0, R) and radius R. The magnetic field is uniform and perpendicular to the xOy plane, pointing outward (indicated by dots inside the circle).
* **Particle Source:** A vertical line segment located in the second quadrant (第Ⅱ象限). It is perpendicular to the x-axis. One end is on the x-axis. Its length is labeled as 2R. Particles are emitted uniformly along this source in the +x direction with velocity v₀.
* **Particle Trajectories:** Arrows originating from the particle source indicate initial velocity v₀ in the +x direction. A curved arrow within the magnetic field region shows a typical particle trajectory. It is stated that all particles exiting the magnetic field do so from the origin O.
* **Fluorescent Screen S:** A vertical line segment located in the third quadrant (第Ⅲ象限). It is perpendicular to the x-axis. One end is on the x-axis. Its length is labeled as √3 R. The distance from the y-axis to the screen is labeled as R.
* **Points:**
* O: Origin (0, 0). Particles exit the magnetic field from here.
* C: Center of the magnetic field circle, coordinates (0, R).
* B: A point on the circle boundary, also labeled with a radius vector R originating from C.
* **Labels:**
* 粒子源 (Particle Source)
* v₀: Initial velocity magnitude
* 2R: Length of the particle source
* R: Radius of the magnetic field circle, distance from the y-axis to the screen
* √3 R: Length of the screen S
* S: Label for the fluorescent screen
* **Other Annotations:**
* Dots inside the circle represent the outward magnetic field.
* "第 16 题图" (Figure for Question 16) appears below the diagram. Note: The question is numbered 6, but the figure label says 16. Assuming the question number 6 is correct.