Search for Strangelets with Solid State Nuclear Track Detectors

ATANU MAULIK

Center for Astroparticle Physics and Space Science Bose InstituteKolkata, INDIA

What is Strange Quark Matter (SQM) ?

SQM is a proposed state of hadronic matter containing roughly equal numbers of up, down and strange quarks.

It has been suggested by various authors [Witten(1984) etc.] that SQM and not normal nuclear matter is the true ground state of hadronic matter.

The stability of SQM cannot yet be determined from first principles within QCD but has been addressed in various phenomenological models like the MIT bag model.

Such calculations[Farhi(1984),Madsen(1993) etc.] show that for significant part of the reasonable parameter spaces in those models SQM can be stable for baryon number greater than some minimum value Amin ~ 50-1000 although shell effects may cause islands of stability for A values smaller than Amin

What are Strangelets ?

Small stable lumps of SQM is what is being referred to as strangelets.

Such strangelets will have a small positive charge, thus an unique experimental signature of strangelets is an unusually low charge to mass ratio (Z/A<<1/2) compared to ordinary nuclei.

The decay of normal nuclear matter into strange matter

will require about a third of the up and down quarks to transform simultaneously into strange quarks via weak interactions, making it prohibitively unlikely.

Where can SQM be found and how can they reach earth ?

SQM, if stable is almost unavoidable in the core of dense steller objects like neutron stars.

Strangelets may be produced when two such stars in a binary system collide.

A strange star – black hole collision may also release lumps of quark matter.

Certain calculations suggest a galactic production rate equivalent to 10 -10 M⊙ yr -1

Strangelets in many ways behave like ordinary cosmic ray nuclei and are accelerated by similar mechanisms e.g. Fermi acceleration in supernova shocks.

Theoretical studies [ Madsen (2005)] point to significant measurable strangelet flux in our part of the galaxy.

Inside the earth’s atmosphere

According to one model of strangelet propagation [S. Banerjee et.al (2000)] an initially small strangelet passing through the earth’s atmosphere, would pick up mass by preferentially absorbing neutrons over protons from the nuclei of atmospheric atoms, protons being Coulomb repelled. As a consequence Z/A ratio gets even more skewed.

At the same time the strangelet will lose energy through ionisation of the surrounding media which effectively puts a lower limit to the altitude at which it can be detected.

For example a strangelet with initial mass ~ 64 amu , charge ~ 2 and β ~ 0.6 will evolve to mass ~ 340 amu and Z~10 -20 at an altitude of 3.6 Km above sea level and will have β ~ 0.01. So it will have a Z/β ~ 1000-2000.

The flux of strangelets at that altitude is estimated to be 5-10/100sq.m./yr.

Reports of events with unusual Z/A ratio

There are several observations of exotic nuclear fragments with unusual charge to mass ratio in different cosmic ray experiments.

In 1990, Saito et al. analyzed the data of 1981 balloon borne experiment and claimed to have identified two events which were consistent with A ~ 370 and Z ~ 14 and was explained in the scheme of strange quark matter.

In 1993 Ichimura et al. reported an event with unusually long m.f.p. called the ‘exotic track’ event with Z ~ 20 and A ~ 460. The report was based on an analysis of a 1989 balloon borne experiment using solid state nuclear track detector.

In a paper in 2001 Fujii et. al. reported detection of a possible SQM candidate, an anomalous massive nuclei of charge Z~14 and M~370 amu in a hybdrid system combining active (Cherenkov and scintillation )and passive detectors (CR-39) in a 19 hr balloon flight.

Analysis of data from AMS-01, which flew to space on board Space Shuttle Discovery, has given hints of some interesting events, such as one with Z=8, A=54 [Aguilar et.al.(2002)]

How polymer track detectors work ?

Heavy charged particles on their passage through materials loose energy according to the Bethe formula :

- dE/dx=(4πe4z2/m0v2)NZ[ln(2m0v2/I)-ln(1-v2/c2)-v2/c2]

The ionization and excitation produced by the charged particles along their path causes molecular bonds to break, thereby producing narrow(30-100Å) damage trails. Naturally the damaged portions become chemically more reactive to suitable chemical reagents (etchants) (e.g. ionic compounds like NaOH) compared to the undamaged bulk material.

During etching, the material along the damage trails are etched out at a much higher rate VT (Track etch rate) compared to the rate of etching VB also called VG (Bulk etch rate) of the undamaged surface. The etch pits so formed can be closely approximated by geometrical cones with the damage trail along its axis and is observable under an optical microscope.

Etching Exposed detectors are cut

into small pieces and are dipped into the etchant once the desired temperature is reached.

During etching the solution is stirred continually. It helps in maintaining a uniform temperature and concentration and prevents the build up of etchants.

Leica DM 4000 M

Determination of VT / VB

The ratio VT/VB = f(dE/dx) and hence (Z/β)2 of the incident ion is called the charge response of the detector.

It can be determined by studying the geometry of etch pit openings and is given by the formula:

VT/VB=Zobs/[VB×t×cos(i)]+1/cos(i)

Where i is the incident angle, cos(i)=Db/Da, where Da and Db are the

major and minor axis of etch pit openings.

Zobs is the depth of the tip of the etch cone from the surface.

t is the duration of the etching process.

The ratio VT/VB as well as the range R of the particle inside the detector are the two most important parameters which help us in identifying the particle forming the track.

Method of analysis First the detector has to be

calibrated. For that known ions with well

defined energies are impinged on them.

The VT/VB values corresponding to those ions are determined from etch pit geometry.

The dE/dx values corresponding to the incident energies are then determined using the standard computer code SRIM-2003 [Zeigler et. al.( 2003)]

Thus the VT/VB vs. dE/dx calibration curve for the detector is obtained.

It enables us to determine the dE/dx values corresponding to some unknown track once the VT/VB values for them is determined.

Method of analysis (Contd..) Using that same computer code

SRIM-2003 one can get the range (R) of various ions for different energies.

By combining these two sets of data dE/dx vs. R plots for various values of Z. Each point on this plot corresponds to a particular energy.

Then by plotting the dE/dx and R values for an unknown ion on this graph we can estimate its Z value

Again since R=(M/Z)[f(β)] we

can also estimate M for the unknown particle once Z is determined.

Choice of NTD for strangelet search Widely used polymer detectors like CR-39 and Lexan are unsuitable for

rare event search, because their very low Z/β detection threshold [for CR-39: (Z/β)thres> 6, for Lexan:(Z/β)thres>57] compared to the predicted Z/β values of strangelets of 1000 – 2000 at mountain altitudes will result in their recording of a huge low-Z background

A particular brand of overhead projector (OHP) transparencies was found to be very suitable as a NTD. Elemental analysis revealed that the plastic was Polyethylene Terephthalate (PET) with chemical formula (C10H8O4)n . This was reconfirmed with IR spectroscopy.

Crucially, it was found that PET has a much higher detection threshold [(Z/β)thres > 120] compared to other commercially available NTDs and as such is significantly better at background suppression.

Another advantage of PET is that it is substantially cheaper compared to detectors like CR-39.

Determination of ideal etching condition The etch pit geometry depends on the

temperature and the concentration of the etchant and also the duration of etching.

The goal was to maximize the charge response VT/VB and to see whether the track quality is good.

An exposed PET detector was etched in NaOH soln. at three different concentrations (5.0N,6.25 N,7.5N) and at three different temperatures (45°C,55°C,70°C).

It can be seen that VT/VB is getting maximized at the concentration of 6.25 N and at the temp. of 55°C. Track quality was also good.

So we have used 6.25 N NaOH soln. at temp 55° ± 0.1°C for subsequent etching processes.

Charge response study of PET

In order to study charge response, PET films were exposed to various ion beams from accelerators.

Ion and their energies were chosen so as to cover a wide range of Z/β values.

Incident ion energies were so chosen that they were higher than the Bragg peak energies for those ions on PET. Because at lower energies the Bethe equation starts to break down due to charge neutralization.

Beam currents and exposure durations were chosen so that the number of ions impinging on the detector lies in the range (104 – 105)/cm2. This is to prevent overlapping of tracks and detector burnout and also to optimize the data analysis process.

Experimental arrangement for 16O exposures at IUAC, New Delhi

Calibration of PET with 16O beam

The charge response of the PET detector was studied using 53.6 MeV 16O – ions at IUAC, New Delhi.

To determine the charge response of the detectors at different energies, they were exposed at varying distances from the flange, with the air gaps between the flange of the beam pipe and the detectors acting as the energy degrader.

Eight separate runs were used and the incident energies were allowed to vary between 16.2 MeV and 24.4 MeV.

Calibration curve for 16O - ions The adjoining image

shows the charge response parameter VT/VB of PET plotted against corresponding Z/β values.

If the response curve is extrapolated towards lower values of Z/β, we could see that the threshold will be obtained at around Z/β ~ 120, which supports the claim that PET has a higher detection threshold compared to CR-39 or Lexan.

Charge response studies using Iron (56Fe) and Sulphur (32S) beams at IUAC

Charge response of PET was also studied using Iron (56 Fe) & Sulphur (32S) ion beams from the Pelletron accelerator at IUAC.

This time the GPSC facility was used for the purpose of irradiating the detectors.

Experimental setup Small (5 cm x 5 cm) pieces of

PET were mounted on aluminum holders and were placed on top of two movable arms inside the GPSC.

By moving those two arms it was possible to vary the energies with which the ions will be impinging on the PET detectors after being scattered by a 250 µg/cm2

gold foil used as a scatterer.

Experimental setup (contd.) For one set of those

aluminium holders placed on one of the arms, the frames holding the films were turned by an angle of 30° so that the incidence angle of the impinging ions was 30° .

This was done so that one can get a better view of the conical etch pits after etching thereby enabling a more precise determination of the depth of the tip of the etch cone (Zobs).

Experiment with Iron beam Beam energy : 150 MeV (~ 2.7 MeV/A) Beam current: 1 pnA Three separate runs were used. The two arms inside the GPSC with the

detectors mounted on top of them were placed at angles between 20 – 145 degrees with respect to the beam direction. So the energies of the scattered ions impinging on the detectors were between 140 – 50 MeV.

Fe – tracks on PET Tracks after 3 hr

etching, with the microscope focused on the surface. (Incident ion energy 130 MeV)

The conical profile of the tracks could be seen with the microscope focused at a depth of 9μm.

Calibration curve for 56Fe

Experiment with Sulphur beam

Beam energy : 125 MeV (~ 3.9 MeV/A)

Beam current : 1 pnA Two separate runs were used. Detectors were placed making angles

between 20-160 degrees with respect to the beam direction. The incident energies were between 120 - 65 MeV.

S – tracks on PET Tracks after 3 hr

etching. (Energy of incident ions ~ 118 MeV)

Conical profile could be seen with the microscope focused at a depth of 5μm

Calibration curve for 32S

Calibration with 238U-ions at GSI The charge response of PET to

heavier ions was also studied using 11.1 MeV/n 238U – ion beam from GSI, Darmstadt, Germany.

This time Aluminium foils of various thicknesses were used to degrade the energy to different values.(774.5 MeV, 1724.5 MeV, 2641.8 MeV)

The value of VT/VG increases with Z/β in the range 600-750 but then it decreases.

It so happens because the data point corresponding to the lowest energy (774.5 MeV) lies on the left side of the Bragg peak where dE/dx decreases with decreasing energy, i.e. with increasing Z/β. Hence VT/VG which is proportional to dE/dx also decreases with increasing Z/β.

Calibration curve for PET with O, S, Fe, U data

Experiment at CERNExperiment utilized 20° beam line of the

REX-ISOLDE facility at CERN

2.82 MeV/u 129Xe and 78Kr beams were produced in the ISOLDE GPS target and accelerated with the REX-ISOLDE linear accelerator.

The 78Kr beam contained an admixture of 49Ti ions which provided an additional data point for calibration.

ISOLDE layout

Beam line for irradiations

The ISOLDE facility.

Interior of the target chamber used for the irradiations.

PET films were cut into rectangular strips (8 cm x 3.5 cm) and mounted onto the downstream side of the target ladder. This ensured that exposed areas were well defined.

Faraday cup was used for monitoring beam current.

Faraday cup

Target ladder

Kr and Ti tracks on PET

The different dimensions for 78Kr and 49Ti tracks can be seen with the microscope focused (a) on the surface and (b) at a depth of 8.2 micron from the surface.

Diameters and lengths of Xe tracks on PET

Adjoining figures show the variation of the diameters and track lengths for the 129Xe tracks on PET with the duration of etching.

The monotonic rise with etching duration implies a consistency in the rate of etching of PET.

Comparison of track lengths and diameters The adjoining figures

compare the track diameters and lengths for Ti, Kr and Xe ions, all with energies of 2.82 MeV/u after 2 hr etching.

The same energy per nucleon implies the same value of β.

Thus the differences in diameter and length reflect differences in Z for the three ions.

Comparison of PET from different batches

Calibration curve for PET

Test of the particle identification scheme

The identification scheme applied to tracks of 2.82 MeV/u 49Ti ions.

Calibration curve without REX-ISOLDE data used.

Charge Z could be ascertained with an accuracy of ± 1

Corresponding incident energy is 2.61 ±0.24

Detectors exposed to cosmic rays.

Darjeeling (2200 m) Ooty (2200 m)

Kolkata Hanle (4500 m)

Detectors at Ooty (2200m)

Charged particle flux recorded with CR-39 detectors

4.7 x 10-5/(cm2.s.sr)

Thorough scanning of over 10cm2 of PET failed to reveal any charged particle track.

Upper limit of flux on PET 2.2 x 10-9/(cm2.s.sr)

Detectors at Hanle (4500m)

Charged particle flux recorded with CR-39 detectors

4.0 x 10-4/(cm2.s.sr)

Thorough scanning of over 10cm2 of PET failed to reveal any charged particle track.

Upper limit of flux on PET 2.2 x 10-9/(cm2.s.sr)

Exposures at Darjeeling (2200 m) Steady charged particle

flux~1.5x 10-5/(cm2.s.sr) recorded on PET over different sets of exposures of various durations.

Flux recorded on CR-39 ~ 1.0 x 10-3/(cm2.s.sr) which is ~ 70 times greater than the flux recorded on PET

Analysis of tracks on PET found at Darjeeling

VT/VB=2.94±0.42 corresponding to these tracks.

Tracks very short ranged (~10

μm) making particle identification difficult.

Origin of these charged particle tracks on PET not very clear. Characterization will require further studies of local radiation environment of Darjeeling.

References

1. Witten, E., Phys. Rev. D30 (1984) 272.2. Farhi, E. and Jaffe, R.L., Phys. Rev. D30 (1984) 2379.3. Madsen, J., Phys. Rev.Lett.70 (1993) 391.4. Madsen, J., Phys. Rev.D71(2005) 014026.5. Banerjee, S., Ghosh, S.K., Raha, S., Syam, D., Phys. Rev. Lett.

85 (2000) 1384.6. Saito,T., Rev. Lett. 65 (1990) 2094.7. Ichimura, M., et. al., Nuovo Cimento 106A (1993) 843.8. Aguilar, M., et. al.,Phys.Rep. 366 (2002) 331.9. Fuji,M., et.al.,Rad.Meas. 34 (2001) 255.10. Ziegler,J.F.,Biersack, J.P., 2003. The Stopping and Range of

Ions in Matter (SRIM Computer Code), Version:2003.26 .

Publications and patent

Charge response of polyethyelene terephthalate polymers (PET) to light and heavy nuclei _ B. Basu, S. Dey, B. Fischer, A. Maulik, A. Mazumdar, S. Raha, S. Saha, Swapan K. Saha , D. Syam.

Radiation Measurements 43 (2008) S95 – S97

Polyethylene terephthalate polymers at mountain altitude as cosmic ray heavy particle detector _ B. Basu, S. Biswas, S. Dey, A. Maulik, A. Mazumdar, S. Raha, S. Saha, Swapan K. Saha, D. Syam.

Radiation Measurements 43 (2008) S262 – S265

Determination of the depth of an etch pit through studies of diffraction rings _ B.Basu, S. Dey, A. Maulik, S. Raha, S. Saha, Swapan K. Saha, D. Syam . Radiation Measurements 44 (2009) 359-362 

Characterization and calibration of a SSNTD for heavy-ion detection and strangelet search in cosmic rays _ D. Bhowmik, S. Dey, A. Maulik, Sibaji Raha, S. Saha, Swapan K. Saha, D. Syam.

Nuclear Instrum. and Meth. B 269 (2011) 197-201 Calibration of a solid state nuclear track detector (SSNTD) with high detection threshold to search for rare events in cosmic rays _ S. Dey, D. Gupta, A. Maulik, Sibaji Raha, Swapan K.

Saha, D. Syam, J. Pakarinen, D. Voulot, F. Wenander. Astroparticle Physics 34 (2011) 805 -808 Indian patent application [No. 1473/KOL/2011] titled “A novel solid state

nuclear track detector for charged particles” filed on 18 Nov. 2011

Thank you!