Detectors for Heavy Fragments : Design


[1] Overview


Position detectors

Beam is momentum-tagged by measureing the position at the momentum dispersive focal plane (F5).  Beam phase space is measured by a set of position detectors befort the target.  From the required momentum resolution and the bending power of the magnet, tracking detectors are designed to have an angular resolution of about , assuming that the beam positions are measured by extrapolating the beam trajectory, and that positions and angles are measured downstream of the magnet.  In addition, another chamber is installed between the target and the magnet to measure the scatteering angle.  In addition, This requires a low-mass chamber, much less than , with a good position resolution. 


Detectors for Particle Identification

  Particle identification of heavy fragments requires velocity measurement or total-energy measurement in addition to the rigidity and the charge measurement.  In order to have 5s separation () at A=100, velocity resolution of  or total energy resolution of  is necessary.  When the TOF method is used for velocity measurement, necessary time resolution is  for 10 meters of flight path.  Considering the necessity of a thin start detector for TOF, this method seems to be marginal.

 We have considered two techniques for velocity and total-energy measurements: a Cherenkov detector operated at the total internal reflection (TIRC) for velocity measurement and pure CsI detector for total-energy measurement.


Design policy

    Since all the detailed design of these detectors (i.e. all the contracts) had to be made  in FY2008 due to the nature of the construction budget, design of these detectors are , more or less, conventional.


Detector Summary

   There are four kinds of detector groups for heavy ion measurements.

* Position measurement

* Beam Proportional Chamber (BPC):                 beam rigidity tagging at F5

* Beam Drift Chamber 1, 2 (BDC1,BDC2) :            beam phase space

* Forward Drift Chamber 1 (FDC1) :                  scattering angle of fragments

* Forward Drift Chamber 2 (FDC2) :                  rigidity analysis  for fragments

* Proton Drift Chamber 1,2 (PDC1,2) :                        momentum analysis for protons

* Charge measurement

* Ion Chamber for Beam (ICB) :                     beam charge

* Ion Chamber for Fragments (ICF) :                 fragment charge

* Velocity (& charge) measurement

* Hodoscope for Fragment ( HODF) :                 velocity & charge for fragments

* Hodoscope for Protons (HODP) :                   velocity & charge for protons

* Total Internal Reflection Cherenkov (TIRC) :          velocity for fragments

* Total energy measurement

* Total Energy Detector (TED) :                     total energy


* Readout electronics for position detectors

  As a readout circuits of position detectors, anode signals are converted into LVDS logic signals using Amp-Shaper-Discriminator boards (ASD, 16ch/board, gnd, GNA210) mounted directly on the detectors.  ASD Power Supply modules are used to supply ±3V, threshold voltage, and test pulses : one ASD PS handles 10 ASD boards.  Logic signals are further processed by multihit TDC’s (AMSC, 64ch AMT-VME TDC module) with 0.8 nsec/ch precision.


* Gas mixture for position detectors

  When detectors are operated at 1 atm, He+60%CH4 is used.  This mixture comes from the compromise among multiple scattering, position resolution and running cost.  At low pressure, BPC, BDC, and FDC1 are operated using pure i-C4H10.


[2]  Beam Proportional Chamber (BPC)

* Design

     BPC is used to tag the rigidity of secondary beams at the momentum dispersive focal plane F5 by measuring the horizontal position.  It is a 4mm-spacing multiwire proportional chamber with 2 anode planes.  It is housed in detector box, which is placed in the F5 vacuum chamber.  Since the momentum dispersion at F5 is ~33mm/%, 4mm-spacing provides momentum resolution of less than 0.1% (rms).

BPC is shown in Fig. 1 and summarized in the table.

  Amp-shaper-discriminators (ASD) are mounted directly on the detector box in the F5 vacuum chamber.  LVDS signals are timed , through the vacuum feed through, by TDC’s.  Isobutane gas is used at about 20 torr for Kr, and about 200 torr for protons.


20μmφ Au-W/Re

Anode spacing

2mm (2 wires are or-ed  for readout)

Anode – cathode gap

5mm (5mm-thick G10)

Cathode, window

12μm-thick Al-Mylar

Effective area

240mm (H) x 150mm (V)

Anode configuration

x1 - x2

#readout channels

64 anodes /plane x 2 planes = 128anodes

Window of detector box

80μm-thick Kapton x2

Operation gas

i-C4H10 at 20 to 200 torr




ASD x8,  ASD PS x1,  TDC x2

Fig. 1 : BPC assembly


 [3] Beam Drift Chambers (BDC1, BDC2)

* Design

  Two sets of BDC’s are used to measure the phase space of the incident secondary beams on the reaction target.   It is a Walenta-type drift chamber with 2.5mm drift distance for high beam rates.   BDC is shown in Fig. 2, and summarized in the table.

Anode wire

16μmφ Au-W/Re

Potential wire

80μmφ Au-Al

anode – potential (drift) distance


anode – cathode gap

2.5mm (combination of 2.4mm & 2.6mm-thick G10)


8μm-thick Al-Kapton, x 9

gas window

4μm-thick Aramid, x2

effective area

80mm x 80mm

anode configuration


#anode / plane x #planes

16 wires/plane x 8 planes = 128 wires/detector

Operation gas

He+60%CH4 at 1 atm, i-C4H10 below 200 torr


cathode, potential

Readout / 2sets

ASD x16,  ASD PS x2,  TDC x4

Fig. 2 : BDC assembly


 [4]  Forward Drift Chamber 1 (FDC1)

* Design

  FDC1 is placed between the target and SAMURAL magnet in order to measure the emission angle of the projectile fragments.  It also has wide opening in order not to interfere with projectile-rapidity neutrons at zero degrees.   It is a Walenta-type drift chamber with 5mm drift distance in order to handle relatively high beam intensity right after the target.

anode wire

20μmφ Au-W/Re

potential wire

80μmφ Au-Al

anode – potential (drift) distance


anode – cathode gap

5mm (5mm-thick G10)


8μm-thick Al-Kapton, x 15

gas window

8μm-thick Al-Kapton, x2

effective area


open area for neutrons

620mm x 340mm

anode configuration

xx’uu’vv’xx’uu’vv’xx’, (±30° for u/v)

#anode / plane x #planes

32 wires/plane x 14 planes = 448 wires

operation gas

He+60%CH4 at 1 atm, i-C4H10 at low pressure


cathode, potential


ASD x28,  ASD PS x3,  TDC x7

1 VME crate (with BDC1,2)

Fig. 3 : FDC1 assembly


 [5]  Forward Drift Chamber 2 (FDC2)

* Design

   FDC2 is placed after SAMURAI magnet for rigidity analysis of projectile fragments.  The cell structure is hexagonal with 10mm drift length.  Two staggered planes named super layer, such as xx’, are separated by 100mm pitch with shield planes in between.

Fig 4 : FDC2 cell structure

   Although it was originally planned to put FDC2 in the detector box for low-pressure operation, FDC2 will be operated at 1 atom for the time being due to technical difficulties.

Anode wire

40μmφ Au-W/Re, 20mm pitch

Field & shield wire

80μmφ Au-Al, 20mm pitch

Cell structure

hexagonal, 10mm drift length


s-xx’-s-uu’-s-vv’-s-xx’-s-uu’-s-vv’-s-xx’-s  (±30°for u/v)


2296mm x 836mm

#anode wires (dummy)

224(4) anodes/super layer x 7 super layer = 1568 (28) anodes

#field / shield wires

4788 (field), 328 (shield)

Operating gas

He+60%CH4 at 1 atm ( i-C4H10 below 100 torr)


field wires, shield wires


ASD x98,  ASD PS x11,  TDC x25, 2 VME crates

Fig. 5 : FDC2 Assembly


[6]  Proton Drift Chamber 1, 2 (PDC1, PDC2)

* Design

  PDC1 and PDC2 are placed downstream of SAMURAI magnet, and used to measure the momentum of projectile-rapidity protons.  Since counting rate is expected to be low and in order to reduce number of planes, position information is obtained using the cathode readout method.  For the anode plane, Walenta-type drift chamber, with 8mm drift length, is adopted in order to reduce the number of anode wires.  Three kinds of cathode orientation are used to detect multi particles.

  Parameters for the cathode readout are as follows.

anode wire

30μmφ Au-W/Re, 16mm pitch (8mm drift length)

potential wire

80μmφ Au-Al, 16mm pitch

anode – cathode gap


cathode wire

80μmφ Au-Al, 3mm pitch

cathode strip width

12mm (4 cathode wires are or-ed for one strip)

   Present design of PDC is as follows.  Anode wires are or-ed and positive HV is applied.  Anode wires are not readout.  Slight negative HV is applied to potential wires.   Cathode strips are directly connected to the readout without decoupling capacitors.

  Present design of the PDC is shown in Fig 6.



wire angle

X(0°), U(+45°), V(-45°)

Effective area

1700mm x 800mm

anode wire (U,V)

106 anodes/plane x 2cplanes = 212 anodes

potential wire (U,V)

107 potentials/plane x 2 planes = 214 potentials

cathode wire (U,V,X)

544 wires/plane,  136 cathode strips (4 wires are or-ed)/plane


Anode(+), potential(-)

Operating gas

Ar+25% i-C4H10 or Ar+50%C2H6

Fig 6 : PDC assembly

*Readout (status)

    We have tested the charge division readout for cathode signals in order to reduce the readout channel.  Cathode strips are daisy-chained by resistors, and cathode charges are read out via charge sensitive pre amp in every 8 strips.  Using a prototype detector (600mm x 480mm effective area) with roughly the same geometry, position resolution of 1mm (rms) were obtained for x-rays.  With this method, about 110ch of charge sensitive preamp, shaping amp, and peak sensitive ADC’s are necessary to read 2 PDC’s.

     Since 2 proton events can not be handled properly by this method, we are developing a new readout circuit: every cathode signals are connected to charge sensitive preamp, shaper, sample & hold, and digitized in the front-end board (FEB, 16ch/board), and digital data are sent to the VME memory.   This method also improve the position resolution by ~factor of 5.  About 810 ch of circuits  (8-9 FEB’s/plane x 6 planes) are necessary.


[7]  Ion Chamber for Beam (ICB)

* Design

    ICB is a multi-layer ion chamber, and placed upstream of the target.  It is used to measure the charge of  incident secondary beams.


12μm-thick Al-Mylar , 10 anode & 11 cathode planes

Anode-cathode gap



16μm-thick Aramid

effective area

140mm x 140mm x 420mm (deep)


P10 @1atm




10ch, preamp ( Mesytec-MPR16 with 10μs decay time), shaping amp (MSCF-16LE, active BLR, unipolar output, 0.25μs shaping time), & peak sensitive ADC (MADC32)

Fig 7 : ICB assembly


 [8]  Ion Chamber for Fragment  (ICF)

* Design

    ICF is a multi layer ion chamber, and placed after the SAMURAI magnet.  It is used to measure the charge of projectile fragments.

    Due to several technical difficulties (available size of double-sides Al-Mylar foils, method to make double-sided segmented anodes, etc.), effective area of the present ICF is much smaller than that of FDC2.  We should have used the wire cathode as well to have larger  effective area.


12 anode & 13 cathode planes

anode-cathode gap


effective area

750mm (H) x 400mm (V) x 480mm (deep)


80μmφAu-Al, 5mm pitch, 18 wires are or-ed to make a 90mm-wide strip,

2 strips are or-ed for readout (4ch/plane)

cathode, window

12μm-thick Al-Mylar


P10 @1atm


cathode (-), anode is at ground potential


4ch/plane x 12 planes = 48ch, preamp ( Mesytec-MPR16 x3 with 10μs decay time), shaping amp (MSCF-16LE x3, active BLR, unipolar output, 0.25μs shaping time), & peak sensitive ADC (MADC32 x2)

Fig 8 : ICF assembly


[9]  Hodoscope for Fragment/Proton (HODF, HODP)

* Design

    HODF and HODP are conventional scintillator hodoscopes.  They are placed after FDC2 and ICF to measure TOF and charge of particles.

plastic scintillator

BC408, 1200mm(V) x 100mm(H) x 10mm(T)


Plastic is coupled to HPK H7195 PMT (with a booster connector) via 100mm-long fishtail light guide.

Effective area

16 slats/hodoscope, 1600mm(H) x 1200mm(V)


32ch/hodoscope x 2 = 64ch, with additional HV’s for booster,

CAEN A1733N x6(+4)


16ch CAMAC discriminator (Phillips 7106) x4,

500nsec logic & analog cable delay x 64,

CAEN 32ch ADC V792AC x2

CAEN 32ch TDC V775AC x2

Fig 9 : HODF/HODP Assembly


[10]  Total Internal Reflection Cherenkov (TIRC)

* Design

    TIRC is a Cherenkov detector operated at the total internal reflection (TIR) threshold.  Since photon numbers increase sharply at the TIR threshold, good velocity resolution is expected.   Refractive index of the radiator chosen is n~1.9 so that the TIR threshold is around 250 MeV/A.


TAFD30(n~1.92),  65mm x 240mm x 2mmt  (max. size available)


HPK H6559 (3”φ) with booster connector, radiator is viewed by 2 PMT’s.  10 PMT’s available

effective area

632(317)mm x 240mm(V), covered by 10(5) elements


20(10) ch,  CAEN A1733N x2(1)


20(10) x 500nsec cable delay, CAEN 32ch ADC V792AC x1

Fig 10 : TIRC assembly


[11] Total Energy Detector (TED)

* Design


Pure CsI, 100mm x 100mm x 50mm-thick


HPK – R6233HA(3”φ),    with non-UV window,  booster connector


tapered, with high breeder current (3mA @1kV), 

effective area

800mm(H) x 400mm(V), with 8 x 4 (32) crystals


32 ch,  CAEN A1733N x3


32 x 500nsec cable delay, CAEN 32ch ADC V792AC x1

* R&D for TED

    As a total energy detector, we have tested  (1) NaI(Tl) coupled to PMT, (2) HP-Ge crystal, (3) CsI(Tl) coupled to photodiode, using Ar & Kr & secondary beams between 200 – 400 MeV/A.  Energy resolution of 0.3 – 0.4 % (rms)  for total energy of 25 – 30 GeV was obtained only at low counting rate (below 1kHz). 

    Since above detectors are relatively slow, we have also tested pure CsI crystal coupled to PMT.  It has smaller light output compared to CsI(Tl), but has faster decay time and believed to be strong against the radiation damage.  After high-current tapered breeder was designed, energy resolution of 0.2 to 0.4% was observed for Kr beams at 400 MeV/A.  By comparing the light output between Ar and Kr beams, large saturation effect was observed.  Since energy resolution between PMT with UV window and with non-UV window has no noticeable difference, PMT with non-UV window was selected.  Light output was stable for beam rates up to 10-20kHz.  It was also observed that the pulse shape for heavy ion is different from those for γ-rays, electrons, and possibly protons.

Fig 14 : TED assembly



Updated 2-Sep-2011