Eichrom’s Resolve PTFE filter is designed to optimize performance of radon/thoron stripping algorithm in spectra recording low background alpha/beta spectrometers. This second filter product in Eichrom’s Resolve product line joins the 0.1µm polypropylene filter used for alpha spectroscopy source preparation. The PTFE filters have been demonstrated to provide results superior to either glass fiber or cellulose filters in Canberra’s family of instruments known as the iSeries® including the iSolo and iMatic counters.

The Eichrom PTFE filters are manufactured to the following specifications.

Resolve PTFE Filter Properties
Diameter 25mm, 47 mm and 50mm
Reference Pore Size 3.0 µ nominal
Thickness 0.15 –0.4 mm thick
DOP Efficiency >99.99 % 0.3 um @5.33 cm/sec
Chemical Compatibility Resistant to most chemicals and solvents
Air Permeability 6 ft3/ft2/min @ 0.5″ H2O minimum

As confirmed through Eichrom’s ISO9001 Quality Control Plan, the Resolve PTFE Filters have a low radiological background, excellent peak resolution as measured by FWHM, and has physical characteristics that make it easy to handle. The filters are labeled with a grid on the back side of the filter to facilitate correct filter orientation on the air sampling unit.

The table below compares the Resolve PTFE Filter’s alpha and beta backgrounds as measured on a gas flow proportional counter to cellulose and glass fiber filters. The Resolve filter has a background indistinguishable from zero, in contrast to the glass fiber which, as would be expected, has small background levels of alpha and beta radioactivity.

α – alpha Background
MDA=2.24 (dpm)
β – beta Background
MDA=4.20 (dpm)
Cellulose 0.78 ± 1.12 0.17 ± 1.30
Glass Fiber 2.00 ± 1.56 4.52 ± 3.44
Resolve PTFE Filter 0.24 ± 0.70 0.31 ± 3.54

As part of Eichrom’s ISO Certified quality control plan, each lot of PTFE material is tested for 233U and 241Am peak resolution using CeF3 micro-precipitation and counting on an alpha spectroscopy system. The peak resolution achievable with the PTFE filter rivals that of a 0.1 µm polypropylene. See the table below for results comparing peak full width at half maximum for Eichrom’s two Resolve Filter products.

Material Pore Size FWHM 233U FWHM 241Am
Polypropylene 0.1 µ 35.1 33.8
Laminated PTFE 3.0 µ 46.7 40.4

The real test of a product is how well it performs in its intended use. The Canberra’s iSeries line of instruments are designed with an algorithm to eliminate the bias caused by naturally occurring alpha emitting daughters of radon and thoron from a counted air filter. The quality of the alpha spectrum is an important factor in the ability of the algorithm to do its job. Eichrom’s Resolve PTFE Filter delivers a better alpha spectrum than filters of other material can. The proof is in the pudding.

In a side-by-side comparison of Resolve PTFE, cellulose and glass fiber filters, each was spiked with a combined 474 dpm 233U and 241Am spike solution. The alpha activity due to this spike was measured on an iSolo instrument before and after air containing radon was “sampled” with the filter. The Resolve PTFE filter allowed the iSolo to measure an 84% recovery, while the cellulose filter resulted in an 8% recovery and the glass fiber resulted in a 47% recovery.

Filter type Alpha spike
(dpm)
Measured before air sampling Alpha (dpm) Initial % Recovery Measured after air sampling Alpha (dpm) % Recovery
Cellulose 474 106 ± 3.4 22 % 35.9 ± 20.2 8 %
Glass 474 237 ± 5.1 50 % 225 ± 18.8 47 %
Resolve PFTE 474 415 ± 6.9 88 % 397.8 ± 14.2 84 %

The spectra below show the spectra achieved by the three filter types. The blue lines are the ‘uncompensated’ alpha spectrum and the red spectra are the ‘compensated’ alpha spectra after the removal of the effect of the alpha emitting daughters of radon and thoron.

Figure 1: Cellulose Filter

Figure 2: Glass Fiber Filter

Figure 3: Resolve PTFE Filter

The Resolve PTFE Filter is available in packages of 50. Contact Eichrom or your local distributor to place an order.

Request part numbers: RP-60-25PTFE3, RP-50-47PTFE3 or RP-50-50PTFE3. Other custom sizes available please contact us for a custom price quotation.