First application of ranging-out method and hybrid 3Hen counter at ISAC-1: measurement of absolute beta-delayed neutron rates (Iβn)

for Ga and Ge isotopes around N=50

collaboration of ORNL Physics - TRIUMF - Guelph - Simon Frasier

UTK - ORAU - ORNL Reactor ScienceLSU - Mississippi - Warsaw

Verification the absolute βn branching ratios Iβn for N=48 79Ga, N=49 80Ga, N=50 81Ga, and measure for the first time Iβn value for N=51 83Ge.

Proposed experiment represents a step towards the understanding and improving of ISAC discovery potential for decay studies of new nuclei produced in fission. (before the high resolution mass separator will be operating at TRIUMF).

3Hen at TRIUMFK. P. Rykaczewski

230 ms

480 ms

50

28Ni74 Ni75 Ni76 Ni77 Ni78

Cu77Cu76Cu75 Cu79Cu78 Cu80

Zn76 Zn80Zn79Zn78Zn77 Zn81 Zn82 Zn83

Ga77 Ga78 Ga79 Ga80 Ga81 Ga82 Ga83 Ga84 Ga85 Ga86

Ge86Ge85Ge84Ge83Ge82Ge81Ge80Ge79Ge78

As79 As80 As81 As82 As83 As84 As85 As86 As87

Se80 Se81 Se82 Se83 Se84 Se85 Se86 Se87 Se88 Se89 Se90 Se91

Br81 Br82 Br83 Br84 Br85 Br86 Br87 Br88 Br89 Br90 Br91 Br92 Br93 Br94

n 0.9%n 0.09%

n 0.1%

n 12%

1.22 s

0.9 s

5.7 s 2.08 s 1.47 s

13 s 5.09 s 2.84 s 1.68 s

344 ms 238 ms 128 ms 110 ms

637 ms 481 ms 338 ms 290 ms 170 ms

995 ms 537 ms 304 ms 228 ms 117 ms

1.21 s 599 ms 308 ms 85 ms 93 ms ~40 ms

88 m 18.98 s 29.5 s 7.6 s 4.55 s 1.85 s 946 ms 494 ms

861 ms2.02 s4.2 s13.4 s19.1 s33.3 s15.2 s9.01 m

18.45 m 22.3 m 3.3 m 32.9 s 14.3 s 5.50 s 1.53 s 410 ms 270 ms

2.40 h35.28 h 31.80 m 2.90 m 55.65 s55.1 s 16.36 s 4.40 s 1.91 s 541 ms 343 ms 152 ms 70 ms

HRIBF β--n-TAS-En

results marked in red

βn branching ratios around N=50,for 79Ga,80Ga,81Ga and 83Ge precursors,

to be measured at TRIUMF-ISAC

484 ms

221 ms

Decay studies with post-accelerated fission products

Range outexperiment

+/-40 keV+/-160 keV

HRIBF~ 0.5 %

overall efficiency

Mass separatorM/ΔM ~ 1000

54 MeV protons12- 18 A

2-3 MeV/u

HRIBFcharge exchange

cell~ 5% efficiency

Positiveions

Tandem accelerator(negative ions only)

~ 10% efficiency

ISAC ~ 0.1 % overall

efficiency

IRIS-1

ORIC : 238U

fission fragments

~1011/s

gas cell

beam kicker

En

erg

y l

os

sTotal ion energy

76Cu

76Ga

76Ge

no 76Zn !!!

Range out expgas cell spectra

C.J.Gross et al., EPJ A25,115,2005

TRIUMF 500 MeV ~ 10 A

6 g25 g

~70% n ~44%~4%

Laser ion source at TRIUMF

Isobar separator M/M ~ 10000

Why the decay spectroscopy of fission products is difficult at ISAC ?

1.beams of interesting ions come with usually much stronger contamination of higher-Z fission products (this problem is not ISAC specific).

But here the selective laser ion source combined with ranging-out technique can help to purify Ga and Ge ion beams.

2.ISAC post-acceleration scheme requiring a Charge State Booster creates a lot of stable ion contamination in the post-accelerated beams (see proposal).

But these are stable contaminants – no radiation emitted ! Higher-Z contaminants should be stopped in our Ionization Chamber.

Unfortunately, the combination of selective ionization, ranging-out and radiation detection is not (yet) an universal solution to the beam contamination problem – every effort to characterize and reduce the beam contamination at TRIUMF is extremely important

But this experiment can help to advance beam purification techniques at ISAC

3He tubes

3He tubes

Ranging-out of higher-Z components, counting and transmitting Ga ions, deposition on Ed Zganjar’s MTC followed by a transport and β-n- detection

This experimental setup is able to remove the radioactive isobaric contaminants.However, stable ions can overload the ionization chamber (tested up to 300 kHz ion rate)making the event-by-event identification and detection difficult to impossible.The proposed solution: we reduce the beam intensity to the limit of, e.g., 81Ga detectionin the IC and βn setup (factor 1000). We measure the absolute rate of 81Ga ions vs the β rate and optimize the gas pressure. Later we use the IC in a passive degrader mode and obtain the rate of implanted 81Ga ions from β counts. MCP can count reliably up to 106 pps, but it counts full ion beam intensity before IC.

300 kHz

250 kHz

225 kHz

150 kHz

76Ge76Se

76Ge76Se

76Ge76Se

76Ge76Se

The advantages of a small transmission ion chamber

Short cathode to anode distance (~3 cm) •drift time minimized over 1.5 cm of beam-anode distanceCF4 gas•a fast gas (drift time ~100 ns/cm)Total path length in gas (~7 cm)•compact to minimize beam blow upSegmented anode (6 electrically separated)•measured energy loss dependent on total energy •a measure of range (cm scale)•different combinations of anodes can select and isolate events of interestSmall windows with metal support wires•mylar windows of 0.9 μm and 2.4 μm•0.9 μm withstands excess of 200 Torr•modular design•easy to replace window quickly•openings 16 mm diameterRecent addition (untested)•central wire “guard ring” to smooth electric field gradient near window to improve anode 1 & 6 performance

Background current observed from the CSB during 2011 development runs. The vertical lines show the transmitted region that corresponds to the A/q for 79Ga12+. The 46Ti7+, 79Br12+, 86Kr12+, 92Mo14+, 131Xe20+, and 132Xe20+

ions are responsible for this current. Worst case scenario: 100 pA of 46Ti7+ means 9•107 ions/s.With factor 103 down (slits closed), we have 104 -105 pps in IC.

79Ga+12

Projected acceptance of the ISAC-I RFQ. The region labeled “B” maximizes A=79 transmission, and reduces possible 86Kr and 131Xe by an order ofmagnitude. Fine-tuning of the phase of the RFQ, depending on the exact stable-beam composition, can significantly reduce the background.

79

The proposed experiment represents an ideal commissioning experiment for the Charge State Booster,

especially when considering that we can give instantaneous online feedback of the beam composition,

and provided that the stable-beam rates are not in excess of ~ 109 pps the measurement can be successful.

The presented project has an obvious continuation pathtowards new exotic beta-delayed neutron emitters,

like 86Ga and even 87Ga, after demonstrating successful study of activities produced at higher rates.

(let’s keep 3Hen at TRIUMF, if we can get new results there)

K. Rykaczewski, TRIUMF, 12th July 2012

Beam time accepted:

Iβn for 79Ga, 80Ga and 81Ga, for each ion:one 8-hour shift of tuning plus one 8-hour shift for measurement amount to four 12-hours shifts

85Ga rate (towards new nuclei): two 12-hour shifts

Iβn for 83Ge: one 8-hour shift tuning plus six 8-hour shifts for measurement amount to about five 12-hour shifts

Total of eleven 12-hour shifts

Detectors for beta decay studies 3Hen array after “ranging-out”

hybrid 3Hen-β- array at LeRIBSS

εn~80%

εn~44%

K. Rykaczewski, TRIUMF, 12th July 2012

Ed Zganjar mounting 3Hen array at LeRIBSS