The Washington University Microball

The Microball consists of 95 CsI(Tl) scintillators closely packed to cover the angular range 4.0o - 172o. The detectors are arranged in 9 rings with increasing forward segmentation. The device is optimally designed to be used in conjunction with Gammasphere. The scintillator light is collected by silicon photodiodes that provide high quantum efficiency and minimal mass. The signals are processed through a charge sensitive preamplifier followed by a slow shaper. Particle identification for 1,2,3H, 3,4He, Li, Be and B ions is accomplished by pulse shape discrimination.

Characteristics of the Microball

The features of the Microball are summarized below:

  1. Good charged-particle identification (PID). Each detector of the device provides charged-particle identification, which is essential for selecting reaction channels.
  2. Large solid angle coverage (~97% of 4p ). This becomes very important when a weak channel such as (HI, p2n) needs to be selected in the presence of much stronger channels such as (HI,3p). This large coverage was achieved geometrically, but other factors may reduce the actual detection efficiency. For forward angles excellent particle identification is achieved. However, at large angles for symmetric reactions and/or lighter compound systems the lowest energy alpha particles and protons are not be completely distinguished, and this decreases somewhat the identification efficiency. The importance of large efficiency is apparent, for example, when channels like (HI, 3p) or (HI,4p) need to be measured.
  3. Small total mass. This requirement is essential to minimize degradation of the peak to total ratio of the Gammasphere Ge detectors. It is imperative for spectroscopic studies, but not as crucial for reaction mechanism studies.
  4. Adequate segmentation. This allows for nearly equally the counting rate among the detectors for many reaction asymmetries in the entrance channel. It helps keeping up with the high event rates of Gammasphere. Decreasing the solid angle of each detector with decreasing angle relative to the beam allows angular distributions of light charged particles to be measured with nearly equal statistical quality for all angles.
  5. Reasonably good energy resolution. This requirement permits measurement of particle evaporation spectra with good energy definition below and above the emission barrier.
  6. Excellent gain stability with counting rate, temperature and time. This is very important for obtaining good quality data. Often counting rate dependent shifts may not be possible to correct in the offline analysis, or it may require very time consuming gain shift corrections in order to retain the PID.
  7. The device is small enough to fit inside the Gammasphere scattering chamber

The Geometric Parameters of the Microball

Here are some overview pictures and sketches of the Microball.

There are 9 rings of detectors spanning the angular range between 4.0o and 171o relative to the beam as seen in Fig. 1.1.


An overview of the Gammasphere with the Microball in place is shown in Fig. 1.2.

A close-up photograph of the Microball in the Gammasphere is shown in Fig. 1.3.

The parameters of the Microball are summarized in Table 1.1. The number of the detectors and their distance from the target in each ring are given in the second and third rows, respectively. The fourth and fifth rows give the polar angles, q, at the center of each ring and the corresponding half angle. A spherical polar coordinate system is used with the beam along the z axis. The azimuthal angle f = 0 is at the top and increases clockwise when going with the beam. Columns six and seven give the solid angle for one detector at each ring in milli-steradians and the normalized solid angle relative to a detector in the first ring. The next column gives the light guide thickness in mm. The last 6 rows give the average CsI(Tl) thicknesses in each ring for the two devices, as well as the maximum energies of protons and alpha particles that stop in the detectors.

Table 1.1: Geometric Parameters of the MICROBALL

# of Detectors6101212141412106
Distance (mm)1108060505050454750
Theta, q9.
Half-Theta, Dq/
DW(q) (msr)28.254.485.3113.2144.7154.1192.1182.9154.5
DW(q) / DW(9o)1.01.933.
Light guide (mm)
mBall-1, CsI (mm)
p range (MeV)24.522.821.719.917.917.317.315.814.3
a range (MeV)97.090.485.678.771.068.368.362.756.6
mBall-2, CsI (mm)
p range (MeV)50.143.540.639.137.636.031.329.028.6
a range (MeV)198.6172.3161.0155.1149.1142.7124.1115.0113.2

Read More about the Microball

The details of the design and the performance characteristics of the Microball are available in a publication in Nuclear Instruments and Methods, A381, 418 (1996). For reprints in color contact D.G. Sarantites at

To read a PDF file (3.0 MB) of the Microball NIM paper click here Microball-NIM. To see the Microball Pictures in color just go to Microball Images in the Front Page or look at some of them just above in this page.

There is also a NIM paper describing simulations of the Gammasphere performance through the Microball. There you will also find what the (H,k) response of the Gammasphere is with or without the Hevimet Shields in place. For the simulations paper (780 kB) click here Gammasphere Simulations NIM paper.

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